# -*- coding: utf-8 -*-
"""Back-port compiler for Python 3.8 assignment expressions."""

import argparse
import os
import pathlib
import re
import sys
import traceback
from typing import Dict, Generator, List, Optional, Union

import f2format
import parso.python.tree
import parso.tree
import tbtrim
from bpc_utils import (BaseContext, BPCSyntaxError, Config, TaskLock, archive_files,
                       detect_encoding, detect_files, detect_indentation, detect_linesep,
                       first_non_none, get_parso_grammar_versions, map_tasks, parse_boolean_state,
                       parse_indentation, parse_linesep, parse_positive_integer, parso_parse,
                       recover_files)
from bpc_utils import Linesep
from typing_extensions import Literal, TypedDict, final

__all__ = ['main', 'walrus', 'convert']

# version string
__version__ = '0.1.5'

###############################################################################
# Types for annotation

ScopeKeyword = Literal['global', 'nonlocal']


class WalrusConfig(Config):
    indentation = ''  # type: str
    linesep = '\n'  # type: Literal[Linesep]
    pep8 = True  # type: bool
    filename = None  # Optional[str]
    source_version = None  # Optional[str]


FunctionEntry = TypedDict('FunctionEntry', {
    'name': str,
    'uuid': str,
    'scope_keyword': ScopeKeyword,
})

LambdaEntry = TypedDict('LambdaEntry', {
    'param': str,
    'suite': str,
    'uuid': str,
})

###############################################################################
# Auxiliaries

#: Get supported source versions.
#:
#: .. seealso:: :func:`bpc_utils.get_parso_grammar_versions`
WALRUS_SOURCE_VERSIONS = get_parso_grammar_versions(minimum='3.8')

# option default values
#: Default value for the ``quiet`` option.
_default_quiet = False
#: Default value for the ``concurrency`` option.
_default_concurrency = None  # auto detect
#: Default value for the ``do_archive`` option.
_default_do_archive = True
#: Default value for the ``archive_path`` option.
_default_archive_path = 'archive'
#: Default value for the ``source_version`` option.
_default_source_version = WALRUS_SOURCE_VERSIONS[-1]
#: Default value for the ``linesep`` option.
_default_linesep = None  # auto detect
#: Default value for the ``indentation`` option.
_default_indentation = None  # auto detect
#: Default value for the ``pep8`` option.
_default_pep8 = True

# option getter utility functions
# option value precedence is: explicit value (CLI/API arguments) > environment variable > default value


def _get_quiet_option(explicit: Optional[bool] = None) -> Optional[bool]:
    """Get the value for the ``quiet`` option.

    Args:
        explicit (Optional[bool]): the value explicitly specified by user,
            :data:`None` if not specified

    Returns:
        bool: the value for the ``quiet`` option

    :Environment Variables:
        :envvar:`WALRUS_QUIET` -- the value in environment variable

    See Also:
        :data:`_default_quiet`

    """
    # We need short circuit evaluation, so first_non_none(a, b, c) does not work here
    # with PEP 505 we can simply write a ?? b ?? c
    def _option_layers() -> Generator[Optional[bool], None, None]:
        yield explicit
        yield parse_boolean_state(os.getenv('WALRUS_QUIET'))
        yield _default_quiet
    return first_non_none(_option_layers())


def _get_concurrency_option(explicit: Optional[int] = None) -> Optional[int]:
    """Get the value for the ``concurrency`` option.

    Args:
        explicit (Optional[int]): the value explicitly specified by user,
            :data:`None` if not specified

    Returns:
        Optional[int]: the value for the ``concurrency`` option;
        :data:`None` means *auto detection* at runtime

    :Environment Variables:
        :envvar:`WALRUS_CONCURRENCY` -- the value in environment variable

    See Also:
        :data:`_default_concurrency`

    """
    return parse_positive_integer(explicit or os.getenv('WALRUS_CONCURRENCY') or _default_concurrency)


def _get_do_archive_option(explicit: Optional[bool] = None) -> Optional[bool]:
    """Get the value for the ``do_archive`` option.

    Args:
        explicit (Optional[bool]): the value explicitly specified by user,
            :data:`None` if not specified

    Returns:
        bool: the value for the ``do_archive`` option

    :Environment Variables:
        :envvar:`WALRUS_DO_ARCHIVE` -- the value in environment variable

    See Also:
        :data:`_default_do_archive`

    """
    def _option_layers() -> Generator[Optional[bool], None, None]:
        yield explicit
        yield parse_boolean_state(os.getenv('WALRUS_DO_ARCHIVE'))
        yield _default_do_archive
    return first_non_none(_option_layers())


def _get_archive_path_option(explicit: Optional[str] = None) ->  str:
    """Get the value for the ``archive_path`` option.

    Args:
        explicit (Optional[str]): the value explicitly specified by user,
            :data:`None` if not specified

    Returns:
        str: the value for the ``archive_path`` option

    :Environment Variables:
        :envvar:`WALRUS_ARCHIVE_PATH` -- the value in environment variable

    See Also:
        :data:`_default_archive_path`

    """
    return explicit or os.getenv('WALRUS_ARCHIVE_PATH') or _default_archive_path


def _get_source_version_option(explicit: Optional[str] = None) -> Optional[str]:
    """Get the value for the ``source_version`` option.

    Args:
        explicit (Optional[str]): the value explicitly specified by user,
            :data:`None` if not specified

    Returns:
        str: the value for the ``source_version`` option

    :Environment Variables:
        :envvar:`WALRUS_SOURCE_VERSION` -- the value in environment variable

    See Also:
        :data:`_default_source_version`

    """
    return explicit or os.getenv('WALRUS_SOURCE_VERSION') or _default_source_version


def _get_linesep_option(explicit: Optional[str] = None) -> Optional[Linesep]:
    r"""Get the value for the ``linesep`` option.

    Args:
        explicit (Optional[str]): the value explicitly specified by user,
            :data:`None` if not specified

    Returns:
        Optional[Literal['\\n', '\\r\\n', '\\r']]: the value for the ``linesep`` option;
        :data:`None` means *auto detection* at runtime

    :Environment Variables:
        :envvar:`WALRUS_LINESEP` -- the value in environment variable

    See Also:
        :data:`_default_linesep`

    """
    return parse_linesep(explicit or os.getenv('WALRUS_LINESEP') or _default_linesep)


def _get_indentation_option(explicit: Optional[Union[str, int]] = None) -> Optional[str]:
    """Get the value for the ``indentation`` option.

    Args:
        explicit (Optional[Union[str, int]]): the value explicitly specified by user,
            :data:`None` if not specified

    Returns:
        Optional[str]: the value for the ``indentation`` option;
        :data:`None` means *auto detection* at runtime

    :Environment Variables:
        :envvar:`WALRUS_INDENTATION` -- the value in environment variable

    See Also:
        :data:`_default_indentation`

    """
    return parse_indentation(explicit or os.getenv('WALRUS_INDENTATION') or _default_indentation)


def _get_pep8_option(explicit: Optional[bool] = None) -> Optional[bool]:
    """Get the value for the ``pep8`` option.

    Args:
        explicit (Optional[bool]): the value explicitly specified by user,
            :data:`None` if not specified

    Returns:
        bool: the value for the ``pep8`` option

    :Environment Variables:
        :envvar:`WALRUS_PEP8` -- the value in environment variable

    See Also:
        :data:`_default_pep8`

    """
    def _option_layers() -> Generator[Optional[bool], None, None]:
        yield explicit
        yield parse_boolean_state(os.getenv('WALRUS_PEP8'))
        yield _default_pep8
    return first_non_none(_option_layers())


###############################################################################
# Traceback Trimming (tbtrim)

# root path
ROOT = pathlib.Path(__file__).resolve().parent


def predicate(filename: str) -> bool:
    return pathlib.Path(filename).parent == ROOT


tbtrim.set_trim_rule(predicate, strict=True, target=BPCSyntaxError)

###############################################################################
# Main Conversion Implementation

# walrus wrapper template

# NAME_TEMPLATE = '''\
# if False:
# %(indentation)s%(name_list)s = NotImplemented
# '''.splitlines()  # `str.splitlines` will remove trailing newline
# make it Cython compilable
NAME_TEMPLATE = '''\
if '%(name)s' not in dir():
%(indentation)s%(name)s = NotImplemented
'''.splitlines()  # `str.splitlines` will remove trailing newline
CALL_TEMPLATE = '_walrus_wrapper_%(name)s_%(uuid)s(%(expr)s)'
FUNC_TEMPLATE = '''\
def _walrus_wrapper_%(name)s_%(uuid)s(expr):
%(indentation)s"""Wrapper function for assignment expression."""
%(indentation)s%(scope_keyword)s %(name)s
%(indentation)s%(name)s = expr
%(indentation)sreturn %(name)s
'''.splitlines()  # `str.splitlines` will remove trailing newline

# special template for lambda
LAMBDA_CALL_TEMPLATE = '_walrus_wrapper_lambda_%(uuid)s'
LAMBDA_FUNC_TEMPLATE = '''\
def _walrus_wrapper_lambda_%(uuid)s(%(param)s):
%(indentation)s"""Wrapper function for lambda definitions."""
%(indentation)s%(suite)s
'''.splitlines()  # `str.splitlines` will remove trailing newline

# special templates for ClassVar
CLS_TEMPLATE = "(__import__('builtins').locals().__setitem__(%(name)r, %(expr)s), %(name)s)[1]"


class Context(BaseContext):
    """General conversion context.

    Args:
        node (parso.tree.NodeOrLeaf): parso AST
        config (Config): conversion configurations

    Keyword Args:
        indent_level (int): current indentation level
        scope_keyword (Optional[Literal['global', 'nonlocal']]): scope keyword for wrapper function
        context (Optional[List[str]]): global context (:term:`namespace`)
        raw (bool): raw processing flag

    Important:
        ``raw`` should be :data:`True` only if the ``node`` is in the clause of another *context*,
        where the converted wrapper functions should be inserted.

        Typically, only if ``node`` is an assignment expression (:token:`namedexpr_test`) node,
        ``raw`` will be set as :data:`True`, in consideration of nesting assignment expressions.

    For the :class:`Context` class of :mod:`walrus` module,
    it will process nodes with following methods:

    * :token:`suite`

      - :meth:`Context._process_suite_node`
      - :meth:`ClassContext._process_suite_node`

    * :token:`namedexpr_test`

      - :meth:`Context._process_namedexpr_test`
      - :meth:`ClassContext._process_namedexpr_test`

    * :token:`global_stmt`

      - :meth:`Context._process_global_stmt`
      - :meth:`ClassContext._process_global_stmt`

    * :token:`classdef`

      - :meth:`Context._process_classdef`

    * :token:`funcdef`

      - :meth:`Context._process_funcdef`

    * :token:`lambdef`

      - :meth:`Context._process_lambdef`

    * :token:`if_stmt`

      - :meth:`Context._process_if_stmt`

    * :token:`while_stmt`

      - :meth:`Context._process_while_stmt`

    * :token:`for_stmt`

      - :meth:`Context._process_for_stmt`

    * :token:`with_stmt`

      - :meth:`Context._process_with_stmt`

    * :token:`try_stmt`

      - :meth:`Context._process_try_stmt`

    * :token:`argument`

      - :meth:`Context._process_argument`

    * :token:`name`

      - :meth:`ClassContext._process_defined_name`

    * :token:`nonlocal_stmt`

      - :meth:`ClassContext._process_nonlocal_stmt`

    * :token:`stringliteral`

      * :meth:`Context._process_strings`
      * :meth:`Context._process_string_context`
      * :meth:`ClassContext._process_strings`
      * :meth:`ClassContext._process_string_context`

    * :token:`f_string`

      * :meth:`Context._process_fstring`
      * :meth:`ClassContext._process_fstring`

    """

    @final
    @property
    def lambdef(self) -> List[LambdaEntry]:
        """Lambda definitions (:attr:`self._lamb <walrus.Context._lamb>`).

        :rtype: List[LambdaEntry]

        """
        return self._lamb

    @final
    @property
    def variables(self) -> List[str]:
        """Assignment expression variable records (:attr:`self._vars <walrus.Context._vars>`).

        The variables are the *left-hand-side* variable name of the assignment expressions.

        :rtype: List[str]

        """
        return self._vars

    @final
    @property
    def functions(self) -> List[FunctionEntry]:
        """Assignment expression wrapper function records (:attr:`self._func <walrus.ontext._func>`).

        :rtype: List[FunctionEntry]

        """
        return self._func

    @final
    @property
    def global_stmt(self) -> List[str]:
        """List of variables declared in the :token:`global <global_stmt>` statements.

        If current root node (:attr:`self._root <walrus.Context._root>`) is a function definition
        (:class:`parso.python.tree.Function`), then returns an empty list; else returns
        :attr:`self._context <walrus.Context._context>`.

        :rtype: List[str]

        """
        if self._root.type == 'funcdef':
            return []
        return self._context

    def __init__(self, node: parso.tree.NodeOrLeaf, config: WalrusConfig, *,
                 indent_level: int = 0, scope_keyword: Optional[ScopeKeyword] = None,
                 context: Optional[List[str]] = None, raw: bool = False):
        if scope_keyword is None:
            scope_keyword = self.determine_scope_keyword(node)
        if context is None:
            context = []

        #: Literal['global', 'nonlocal']:
        #: The :token:`global <global_stmt>` / :token:`nonlocal <nonlocal_stmt>` keyword.
        self._scope_keyword = scope_keyword  # type: ScopeKeyword
        #: List[str]: Variable names in :token:`global <global_stmt>` statements.
        self._context = list(context)  # type: List[str]

        #: List[str]: Original *left-hand-side* variable names in assignment expressions.
        self._vars = []  # type: List[str]
        #: List[LambdaEntry]: Converted *lambda* expressions described as :class:`LambdaEntry`.
        self._lamb = []  # type: List[LambdaEntry]
        #: List[FunctionEntry]: Converted wrapper functions described as :class:`FunctionEntry`.
        self._func = []  # type: List[FunctionEntry]

        # call super init
        super().__init__(node, config, indent_level=indent_level, raw=raw)

    def _process_suite_node(self, node: parso.tree.NodeOrLeaf, func: bool = False,
                            raw: bool = False, cls_ctx: Optional[str] = None) -> None:
        """Process indented suite (:token:`suite` or others).

        Args:
            node (parso.tree.NodeOrLeaf): suite node
            func (bool): if ``node`` is suite from function definition
            raw (bool): raw processing flag
            cls_ctx (Optional[str]): class name if ``node`` is in class context

        This method first checks if ``node`` contains assignment expression.
        If not, it will not perform any processing, rather just append the
        original source code to context buffer.

        If ``node`` contains assignment expression, then it will initialise another
        :class:`Context` instance to perform the conversion process on such
        ``node``; whilst if ``cls_ctx`` is provided, then it will initialise a
        :class:`ClassContext` instance instead.

        Note:
            If ``func`` is True, when initiating the :class:`Context` instance,
            ``scope_keyword`` will be set as ``'nonlocal'``, as in the wrapper function
            it will refer the original *left-hand-side* variable from the outer
            function scope rather than global namespace.

        The method will keep *global* statements (:meth:`Context.global_stmt`)
        from the temporary :class:`Context` (or :class:`ClassContext`) instance in the
        current instance.

        And if ``raw`` is set as :data:`True`, the method will keep records of converted wrapper
        functions (:meth:`Context.functions`), converted *lambda* expressions (:meth:`Context.lambdef`)
        and *left-hand-side* variable names (:meth:`Context.variables`) into current instance as well.

        Important:
            ``raw`` should be :data:`True` only if the ``node`` is in the clause of another *context*,
            where the converted wrapper functions should be inserted.

        """
        if not self.has_expr(node):
            self += node.get_code()
            return

        indent = self._indent_level + 1
        self += self._linesep + self._indentation * indent

        if func:
            scope_keyword = 'nonlocal'  # type: ScopeKeyword
        else:
            scope_keyword = self._scope_keyword

        # process suite
        if cls_ctx is None:
            ctx = Context(node=node, config=self.config,  # type: ignore[arg-type]
                          context=self._context, indent_level=indent,
                          scope_keyword=scope_keyword, raw=raw)
        else:
            ctx = ClassContext(cls_ctx=cls_ctx,
                               node=node, config=self.config,  # type: ignore[arg-type]
                               context=self._context, indent_level=indent,
                               scope_keyword=scope_keyword, raw=raw)
        self += ctx.string.lstrip()

        # keep records
        if raw:
            self._lamb.extend(ctx.lambdef)
            self._vars.extend(ctx.variables)
            self._func.extend(ctx.functions)
        self._context.extend(ctx.global_stmt)

    def _process_string_context(self, node: parso.python.tree.PythonNode) -> None:
        """Process string contexts (:token:`stringliteral`).

        Args:
            node (parso.python.tree.PythonNode): string literals node

        This method first checks if ``node`` contains assignment expression.
        If not, it will not perform any processing, rather just append the
        original source code to context buffer. Later it will check if
        ``node`` contains *debug f-string*. If not, it will process the
        *regular* processing on each child of such ``node``.

        See Also:
            The method calls :meth:`f2format.Context.has_debug_fstring`
            to detect *debug f-strings*.

        Otherwise, it will initialise a new :class:`StringContext` instance
        to perform the conversion process on such ``node``, which will first
        use :mod:`f2format` to convert those formatted string literals.

        Important:
            When initialisation, ``raw`` parameter **must** be set to :data:`True`;
            as the converted wrapper functions should be inserted in the *outer*
            context, rather than the new :class:`StringContext` instance.

        After conversion, the method will keep records of converted wrapper
        functions (:meth:`Context.functions`), converted *lambda* expressions (:meth:`Context.lambdef`)
        and *left-hand-side* variable names (:meth:`Context.variables`) into current instance as well.

        """
        if not self.has_expr(node):
            self += node.get_code()
            return

        if not f2format.Context.has_debug_fstring(node):
            for child in node.children:
                self._process(child)
            return

        # initialise new context
        ctx = StringContext(node=node, config=self.config, context=self._context,  # type: ignore[arg-type]
                            indent_level=self._indent_level, scope_keyword=self._scope_keyword, raw=True)
        self += ctx.string

        # keep record
        self._lamb.extend(ctx.lambdef)
        self._vars.extend(ctx.variables)
        self._func.extend(ctx.functions)

        self._context.extend(ctx.global_stmt)

    def _process_namedexpr_test(self, node: parso.python.tree.PythonNode) -> None:
        """Process assignment expression (:token:`namedexpr_test`).

        Args:
            node (parso.python.tree.PythonNode): assignment expression node

        This method converts the assignment expression into wrapper function
        and extracts related records for inserting converted code.

        * The *left-hand-side* variable name will be recorded in
          :attr:`self._vars <walrus.Context._vars>`.
        * The *right-hand-side* expression will be converted using another
          :class:`Context` instance and replaced with a wrapper function call
          rendered from :data:`CALL_TEMPLATE`; information described as
          :class:`FunctionEntry` will be recorded into :attr:`self._func <walrus.Context._func>`.

        """
        # split assignment expression
        node_name, _, node_expr = node.children
        name = node_name.value
        nuid = self._uuid_gen.gen()

        # calculate expression string
        ctx = Context(node=node_expr, config=self.config,  # type: ignore[arg-type]
                      context=self._context, indent_level=self._indent_level,
                      scope_keyword=self._scope_keyword, raw=True)
        expr = ctx.string.strip()
        self._vars.extend(ctx.variables)
        self._func.extend(ctx.functions)

        # replacing code
        code = CALL_TEMPLATE % dict(name=name, uuid=nuid, expr=expr)
        prefix, suffix = self.extract_whitespaces(node.get_code())
        self += prefix + code + suffix

        self._context.extend(ctx.global_stmt)
        if name in self._context:
            scope_keyword = 'global'  # type: ScopeKeyword
        else:
            scope_keyword = self._scope_keyword

        # keep records
        self._vars.append(name)
        self._func.append(dict(name=name, uuid=nuid, scope_keyword=scope_keyword))

    def _process_global_stmt(self, node: parso.python.tree.GlobalStmt) -> None:
        """Process function definition (:token:`global_stmt`).

        Args:
            node (parso.python.tree.GlobalStmt): global statement node

        This method records all variables declared in a *global* statement
        into :attr:`self._context <walrus.Context._context>`.

        """
        children = iter(node.children)

        # <Keyword: global>
        next(children)
        # <Name: ...>
        name = next(children)
        self._context.append(name.value)

        while True:
            try:
                # <Operator: ,>
                next(children)
            except StopIteration:
                break

            # <Name: ...>
            name = next(children)
            self._context.append(name.value)

        # process code
        self += node.get_code()

    def _process_classdef(self, node: parso.python.tree.Class) -> None:
        """Process class definition (:token:`classdef`).

        Args:
            node (parso.python.tree.Class): class node

        This method converts the whole class suite context with
        :meth:`~Context._process_suite_node` through
        :class:`ClassContext` respectively.

        """
        # <Name: ...>
        name = node.name

        # <Keyword: class>
        # <Name: ...>
        # [<Operator: (>, PythonNode(arglist, [...]]), <Operator: )>]
        # <Operator: :>
        for child in node.children[:-1]:
            self._process(child)

        # PythonNode(suite, [...]) / PythonNode(simple_stmt, [...])
        suite = node.children[-1]
        self._process_suite_node(suite, cls_ctx=name.value)

    def _process_funcdef(self, node: parso.python.tree.Function) -> None:
        """Process function definition (:token:`funcdef`).

        Args:
            node (parso.python.tree.Function): function node

        This method converts the function suite with
        :meth:`~Context._process_suite_node`.

        """
        # 'def' NAME '(' PARAM ')' [ '->' NAME ] ':' SUITE
        for child in node.children[:-1]:
            self._process(child)
        self._process_suite_node(node.children[-1], func=True)

    def _process_lambdef(self, node: parso.python.tree.Lambda) -> None:
        """Process lambda definition (``lambdef``).

        Args:
            node (parso.python.tree.Lambda): lambda node

        This method first checks if ``node`` contains assignment expressions.
        If not, it will append the original source code directly to the buffer.

        For *lambda* expressions with assignment expressions, this method
        will extract the parameter list and initialise a :class:`LambdaContext`
        instance to convert the lambda suite. Such information will be recorded
        as :class:`LambdaEntry` in :attr:`self._lamb <Context._lamb>`.

        .. note:: For :class:`LambdaContext`, ``scope_keyword`` should always be ``'nonlocal'``.

        Then it will replace the original lambda expression with a wrapper function
        call rendered from :data:`LAMBDA_CALL_TEMPLATE`.

        """
        if not self.has_expr(node):
            self += node.get_code()
            return

        children = iter(node.children)

        # <Keyword: lambda>
        next(children)

        # vararglist
        para_list = []
        for child in children:
            if child.type == 'operator' and child.value == ':':
                break
            para_list.append(child)
        param = ''.join(map(lambda n: n.get_code(), para_list))

        # test_nocond | test
        indent = self._indent_level + 1
        ctx = LambdaContext(node=next(children), config=self.config,  # type: ignore[arg-type]
                            context=self._context, indent_level=indent,
                            scope_keyword='nonlocal')
        suite = ctx.string.strip()

        # keep record
        nuid = self._uuid_gen.gen()
        self._lamb.append(dict(param=param, suite=suite, uuid=nuid))

        # replacing lambda
        self += LAMBDA_CALL_TEMPLATE % dict(uuid=nuid)

    def _process_if_stmt(self, node: parso.python.tree.IfStmt) -> None:
        """Process if statement (:token:`if_stmt`).

        Args:
            node (parso.python.tree.IfStmt): if node

        This method processes each indented suite under the *if*, *elif*,
        and *else* statements.

        """
        children = iter(node.children)

        # <Keyword: if>
        self._process(next(children))
        # namedexpr_test
        self._process(next(children))
        # <Operator: :>
        self._process(next(children))
        # suite
        self._process_suite_node(next(children))

        while True:
            try:
                # <Keyword: elif | else>
                key = next(children)
            except StopIteration:
                break
            self._process(key)

            if key.value == 'elif':
                # namedexpr_test
                self._process(next(children))
                # <Operator: :>
                self._process(next(children))
                # suite
                self._process_suite_node(next(children))
                continue
            if key.value == 'else':
                # <Operator: :>
                self._process(next(children))
                # suite
                self._process_suite_node(next(children))
                continue

    def _process_while_stmt(self, node: parso.python.tree.WhileStmt) -> None:
        """Process while statement (:token:`while_stmt`).

        Args:
            node (parso.python.tree.WhileStmt): while node

        This method processes the indented suite under the *while* and optional
        *else* statements.

        """
        children = iter(node.children)

        # <Keyword: while>
        self._process(next(children))
        # namedexpr_test
        self._process(next(children))
        # <Operator: :>
        self._process(next(children))
        # suite
        self._process_suite_node(next(children))

        try:
            key = next(children)
        except StopIteration:
            return

        # <Keyword: else>
        self._process(key)
        # <Operator: :>
        self._process(next(children))
        # suite
        self._process_suite_node(next(children))

    def _process_for_stmt(self, node: parso.python.tree.ForStmt) -> None:
        """Process for statement (:token:`for_stmt`).

        Args:
            node (parso.python.tree.ForStmt): for node

        This method processes the indented suite under the *for* and optional
        *else* statements.

        """
        children = iter(node.children)

        # <Keyword: for>
        self._process(next(children))
        # exprlist
        self._process(next(children))
        # <Keyword: in>
        self._process(next(children))
        # testlist
        self._process(next(children))
        # <Operator: :>
        self._process(next(children))
        # suite
        self._process_suite_node(next(children))

        try:
            key = next(children)
        except StopIteration:
            return

        # <Keyword: else>
        self._process(key)
        # <Operator: :>
        self._process(next(children))
        # suite
        self._process_suite_node(next(children))

    def _process_with_stmt(self, node: parso.python.tree.WithStmt) -> None:
        """Process with statement (:token:`with_stmt`).

        Args:
            node (parso.python.tree.WithStmt): with node

        This method processes the indented suite under the *with* statement.

        """
        children = iter(node.children)

        # <Keyword: with>
        self._process(next(children))

        while True:
            # with_item | <Operator: ,>
            item = next(children)
            self._process(item)

            # <Operator: :>
            if item.type == 'operator' and item.value == ':':
                break

        # suite
        self._process_suite_node(next(children))

    def _process_try_stmt(self, node: parso.python.tree.TryStmt) -> None:
        """Process try statement (:token:`try_stmt`).

        Args:
            node (parso.python.tree.TryStmt): try node

        This method processes the indented suite under the *try*, *except*,
        *else*, and *finally* statements.

        """
        children = iter(node.children)

        while True:
            try:
                key = next(children)
            except StopIteration:
                break

            # <Keyword: try | else | finally> | PythonNode(except_clause, [...]
            self._process(key)
            # <Operator: :>
            self._process(next(children))
            # suite
            self._process_suite_node(next(children))

    def _process_argument(self, node: parso.python.tree.PythonNode) -> None:
        """Process function argument (:token:`argument`).

        Args:
            node (parso.python.tree.PythonNode): argument node

        This method processes arguments from function argument list.

        """
        children = iter(node.children)

        # test
        test = next(children)
        try:
            # <Operator: :=>
            op = next(children)
        except StopIteration:
            self._process(test)
            return

        if self.is_walrus(op):
            self._process_namedexpr_test(node)
            return

        # not walrus
        self._process(test)
        self._process(op)
        for child in children:
            self._process(child)

    def _process_strings(self, node: parso.python.tree.PythonNode) -> None:
        """Process concatenable strings (:token:`stringliteral`).

        Args:
            node (parso.python.tree.PythonNode): concatentable strings node

        As in Python, adjacent string literals can be concatenated in certain
        cases, as described in the `documentation`_. Such concatenable strings
        may contain formatted string literals (:term:`f-string`) within its scope.

        _documentation: https://docs.python.org/3/reference/lexical_analysis.html#string-literal-concatenation

        """
        self._process_string_context(node)

    def _process_fstring(self, node: parso.python.tree.PythonNode) -> None:
        """Process formatted strings (:token:`f_string`).

        Args:
            node (parso.python.tree.PythonNode): formatted strings node

        """
        self._process_string_context(node)

    def _concat(self) -> None:
        """Concatenate final string.

        This method tries to inserted the recorded wrapper functions and variables
        at the very location where starts to contain assignment expressions, i.e.
        between the converted code as :attr:`self._prefix <Context._prefix>` and
        :attr:`self._suffix <Context._suffix>`.

        The inserted code include variable declaration rendered from
        :data:`NAME_TEMPLATE`, wrapper function definitions rendered from
        :data:`FUNC_TEMPLATE` and extracted *lambda* expressions rendered from
        :data:`LAMBDA_FUNC_TEMPLATE`. If :attr:`self._pep8 <Context._pep8>` is
        :data:`True`, it will insert the code in compliance with :pep:`8`.

        """
        flag = any((self._vars, self._func, self._lamb))  # if have code to insert

        # strip suffix comments
        prefix, suffix = self.split_comments(self._suffix, self._linesep)
        match = re.match(r'^(?P<linesep>(%s)*)' % self._linesep, suffix, flags=re.ASCII)
        suffix_linesep = match.group('linesep') if match is not None else ''

        # first, the prefix code
        self._buffer += self._prefix + prefix + suffix_linesep
        if flag and self._pep8 and self._buffer:
            if (self._node_before_expr is not None
                    and self._node_before_expr.type in ('funcdef', 'classdef')
                    and self._indent_level == 0):
                blank = 2
            else:
                blank = 1
            self._buffer += self._linesep * self.missing_newlines(prefix=self._buffer, suffix='',
                                                                  expected=blank, linesep=self._linesep)

        # then, the variables and functions
        indent = self._indentation * self._indent_level
        if self._pep8:
            linesep = self._linesep * (1 if self._indent_level > 0 else 2)
        else:
            linesep = ''
        if self._vars:
            for name in sorted(set(self._vars)):
                self._buffer += indent + (
                    '%s%s' % (self._linesep, indent)
                ).join(NAME_TEMPLATE) % dict(indentation=self._indentation, name=name) + self._linesep
        for func in sorted(self._func, key=lambda func: func['name']):
            if self._buffer:
                self._buffer += linesep
            self._buffer += indent + (
                '%s%s' % (self._linesep, indent)
            ).join(FUNC_TEMPLATE) % dict(indentation=self._indentation, **func) + self._linesep
        for lamb in self._lamb:
            if self._buffer:
                self._buffer += linesep
            self._buffer += indent + (
                '%s%s' % (self._linesep, indent)
            ).join(LAMBDA_FUNC_TEMPLATE) % dict(indentation=self._indentation, **lamb) + self._linesep

        # finally, the suffix code
        if flag and self._pep8:
            blank = 2 if self._indent_level == 0 else 1
            self._buffer += self._linesep * self.missing_newlines(prefix=self._buffer, suffix=suffix,
                                                                  expected=blank, linesep=self._linesep)
        self._buffer += suffix.lstrip(self._linesep)

    @final
    @classmethod
    def has_expr(cls, node: parso.tree.NodeOrLeaf) -> bool:
        """Check if node has assignment expression (:token:`namedexpr_test`).

        Args:
            node (parso.tree.NodeOrLeaf): parso AST

        Returns:
            bool: if ``node`` has assignment expression

        """
        if cls.is_walrus(node):
            return True
        if hasattr(node, 'children'):
            for child in node.children:  # type: ignore[attr-defined]
                if cls.has_expr(child):
                    return True
        return False

    # backward compatibility and auxiliary alias
    has_walrus = has_expr

    @final
    @classmethod
    def determine_scope_keyword(cls, node: parso.tree.NodeOrLeaf) -> ScopeKeyword:
        """Determine scope keyword based on node position.

        Args:
            node (parso.tree.NodeOrLeaf): parso AST

        Returns:
            Literal['global', 'nonlocal']: scope keyword

        This method recursively perform the following checks on the parents
        of ``node``:

        * If current ``node`` is a module (:class:`parso.python.tree.Module`),
          or a direct child of module then returns ``'global'``.
        * If the direct parrent of current ``node`` is a function
          (:class:`parso.python.tree.Function`) and/or class
          (:class:`parso.python.tree.Class`) definition, then
          returns ``'nonlocal'``.

        """
        if isinstance(node, parso.python.tree.Module):
            return 'global'

        parent = node.parent  # type: ignore[attr-defined]
        if isinstance(parent, parso.python.tree.Module):
            return 'global'
        if parent.type in ['funcdef', 'classdef']:
            return 'nonlocal'
        return cls.determine_scope_keyword(parent)

    @final
    @staticmethod
    def is_walrus(node: parso.tree.NodeOrLeaf) -> bool:
        """Check if ``node`` is assignment expression.

        Args:
            node (parso.tree.NodeOrLeaf): parso AST

        Returns:
            bool: if ``node`` is assignment expression

        """
        if node.type == 'namedexpr_test':
            return True
        if node.type == 'operator' and node.value == ':=':  # type: ignore[attr-defined]
            return True
        return False


class StringContext(Context):
    """String (f-string) conversion context.

    This class is mainly used for converting **formatted string literals**.

    Args:
        node (parso.python.tree.PythonNode): parso AST
        config (Config): conversion configurations

    Keyword Args:
        indent_level (int): current indentation level
        scope_keyword (Literal['nonlocal']): scope keyword for wrapper function
        context (Optional[List[str]]): global context (:term:`namespace`)
        raw (Literal[True]): raw processing flag

    Note:
        * ``raw`` should always be :data:`True`.

    As the conversion in :class:`Context` changes the original expression,
    which may change the content of *debug f-string*.

    """

    def __init__(self, node: parso.python.tree.PythonNode, config: WalrusConfig, *,
                 indent_level: int = 0, scope_keyword: Optional[ScopeKeyword] = None,
                 context: Optional[List[str]] = None, raw: Literal[True] = True):
        # convert using f2format first
        prefix, suffix = self.extract_whitespaces(node.get_code())
        code = f2format.convert(node.get_code().strip())
        node = parso_parse(code, filename=config.filename, version=config.source_version)  # type: ignore[assignment]

        # call super init
        super().__init__(node, config, indent_level=indent_level,
                         scope_keyword=scope_keyword, context=context, raw=raw)
        self._buffer = prefix + self._buffer + suffix


class LambdaContext(Context):
    """Lambda (suite) conversion context.

    This class is mainly used for converting **lambda expressions**.

    Args:
        node (parso.python.tree.Lambda): parso AST
        config (Config): conversion configurations

    Keyword Args:
        indent_level (int): current indentation level
        scope_keyword (Literal['nonlocal']): scope keyword for wrapper function
        context (Optional[List[str]]): global context (:term:`namespace`)
        raw (Literal[False]): raw processing flag

    Note:
        * ``scope_keyword`` should always be ``'nonlocal'``.
        * ``raw`` should always be :data:`False`.

    """

    # pylint: disable=useless-super-delegation
    def __init__(self, node: parso.tree.NodeOrLeaf, config: WalrusConfig, *,
                 indent_level: int = 0, scope_keyword: Optional[ScopeKeyword] = None,
                 context: Optional[List[str]] = None, raw: Literal[False] = False):
        super().__init__(node, config,  indent_level=indent_level,
                         scope_keyword=scope_keyword, context=context, raw=raw)

    def _concat(self) -> None:
        """Concatenate final string.

        Since conversion of *lambda* expressions doesn't involve inserting
        points, this method first simply adds wrapper code to the buffer
        (:data:`self._buffer <Context._buffer>`); then it adds a *return*
        statement yielding the converted *lambda* suite stored in
        :data:`self._prefix <Context._prefix>` and :data:`self._suffix <Context._suffix>`.

        The wrapper code include variable declaration rendered from
        :data:`NAME_TEMPLATE`, wrapper function definitions rendered from
        :data:`FUNC_TEMPLATE` and extracted *lambda* expressions rendered from
        :data:`LAMBDA_FUNC_TEMPLATE`. If :attr:`self._pep8 <Context._pep8>` is
        :data:`True`, it will insert the code in compliance with :pep:`8`.

        """
        flag = self.has_expr(self._root)

        # first, the variables and functions
        indent = self._indentation * self._indent_level
        if self._pep8:
            linesep = self._linesep * (1 if self._indent_level > 0 else 2)
        else:
            linesep = ''
        if self._vars:
            for name in sorted(set(self._vars)):
                self._buffer += indent + (
                    '%s%s' % (self._linesep, indent)
                ).join(NAME_TEMPLATE) % dict(indentation=self._indentation, name=name) + self._linesep
        for func in sorted(self._func, key=lambda func: func['name']):
            if self._buffer:
                self._buffer += linesep
            self._buffer += indent + (
                '%s%s' % (self._linesep, indent)
            ).join(FUNC_TEMPLATE) % dict(indentation=self._indentation, **func) + self._linesep
        for lamb in self._lamb:
            if self._buffer:
                self._buffer += linesep
            self._buffer += indent + (
                '%s%s' % (self._linesep, indent)
            ).join(LAMBDA_FUNC_TEMPLATE) % dict(indentation=self._indentation, **lamb) + self._linesep
        if flag and self._pep8:
            blank = 2 if self._indent_level == 0 else 1
            self._buffer += self._linesep * self.missing_newlines(prefix=self._buffer, suffix=self._prefix,
                                                                  expected=blank, linesep=self._linesep)

        # then, the `return` statement
        self._buffer += indent + 'return'

        # finally, the source code
        self._buffer += self._prefix + self._suffix


class ClassContext(Context):
    """Class (suite) conversion context.

    This class is mainly used for converting *:term:`class variable <class-variable>`*.

    Args:
        node (parso.python.tree.Class): parso AST
        config (Config): conversion configurations

    Keyword Args:
        cls_ctx (str): class context name
        cls_var (Dict[str, str]): mapping for assignment variable and its UUID
        indent_level (int): current indentation level
        scope_keyword (Optional[Literal['global', 'nonlocal']]): scope keyword for wrapper function
        context (Optional[List[str]]): global context (:term:`namespace`)
        raw (Literal[False]): raw context processing flag
        external (Optional[Dict[str, Literal['global', 'nonlocal']]]):
            mapping of :term:`class variable <class-variable>` declared in :token:`global <global_stmt>` and/or
            :token:`nonlocal <nonlocal_stmt>` statements

    Important:
        ``raw`` should be :data:`True` only if the ``node`` is in the clause of another *context*,
        where the converted wrapper functions should be inserted.

        Typically, only if ``node`` is an assignment expression (:token:`namedexpr_test`) node,
        ``raw`` will be set as :data:`True`, in consideration of nesting assignment expressions.

    """

    @final
    @property
    def cls_var(self) -> Dict[str, str]:
        """Mapping for assignment variable and its UUID (:attr:`self._cls_var <walrus.Context._cls_var>`).

        :rtype: Dict[str, str]

        """
        return self._cls_var

    @final
    @property
    def external_variables(self) -> Dict[str, ScopeKeyword]:
        """Assignment expression variable records (:attr:`self._ext_vars <walrus.ClassContext._ext_vars>`).

        The variables are the *left-hand-side* variable name of the assignment expressions
        for :term:`class variable <class-variable>` declared in :token:`global <global_stmt>` and/or
        :token:`nonlocal <nonlocal_stmt>` statements.

        :rtype: Dict[str, Literal['global', 'nonlocal']]

        """
        return self._ext_vars

    @final
    @property
    def external_functions(self) -> List[FunctionEntry]:
        """Assignment expression wrapper function records (:attr:`self._ext_func <walrus.ClassContext._ext_func>`)
        for :term:`class variable <class-variable>` declared in :token:`global <global_stmt>` and/or
        :token:`nonlocal <nonlocal_stmt>` statements.

        :rtype: List[FunctionEntry]

        """
        return self._ext_func

    def __init__(self, node: parso.python.tree.Class, config: WalrusConfig, *,
                 cls_ctx: str, cls_var: Optional[Dict[str, str]] = None,
                 indent_level: int = 0, scope_keyword: Optional[ScopeKeyword] = None,
                 context: Optional[List[str]] = None, raw: bool = False,
                 external: Optional[Dict[str, ScopeKeyword]] = None):
        if cls_var is None:
            cls_var = {}
        if external is None:
            external = {}

        #: bool: Raw context processing flag.
        self._cls_raw = raw  # type: bool
        #: Dict[str, str]: Mapping for assignment variable and its UUID.
        self._cls_var = cls_var  # type: Dict[str, str]
        #: str: Class context name.
        self._cls_ctx = cls_ctx  # type: str

        #: Dict[str, Literal['global', 'nonlocal']]: Original
        #: *left-hand-side* variable names in assignment expressions for
        #: :term:`class variable <class-variable>` declared in
        #: :token:`global <global_stmt>` and/or :token:`nonlocal <nonlocal_stmt>`
        #: statements.
        self._ext_vars = external  # type: Dict[str, ScopeKeyword]
        #: List[FunctionEntry]: Converted wrapper functions for :term:`class variable <class-variable>`
        #: declared in :token:`global <global_stmt>` and/or :token:`nonlocal <nonlocal_stmt>`
        #: statements described as :class:`FunctionEntry`.
        self._ext_func = []  # type: List[FunctionEntry]

        super().__init__(node=node, config=config, context=context,
                         indent_level=indent_level, scope_keyword=scope_keyword, raw=raw)

    def _process_suite_node(self, node: parso.tree.NodeOrLeaf, func: bool = False,
                            raw: bool = False, cls_ctx: Optional[str] = None) -> None:
        """Process indented suite (:token:`suite` or others).

        Args:
            node (parso.tree.NodeOrLeaf): suite node
            func (bool): if ``node`` is suite from function definition
            raw (bool): raw processing flag
            cls_ctx (Optional[str]): class name if ``node`` is in class context

        This method first checks if ``node`` contains assignment expression.
        If not, it will not perform any processing, rather just append the
        original source code to context buffer.

        If ``node`` contains assignment expression, then it will initialise another
        :class:`ClassContext` instance to perform the conversion process on such
        ``node``; whilst if ``func`` is provided, then it will initialise a
        :class:`Context` instance instead.

        Note:
            If ``func`` is True, when initiating the :class:`Context` instance,
            ``scope_keyword`` will be set as ``'nonlocal'``, as in the wrapper function
            it will refer the original *left-hand-side* variable from the outer
            function scope rather than global namespace.

        The method will keep *global* statements (:meth:`Context.global_stmt`)
        from the temporary :class:`Context` (or :class:`ClassContext`) instance in the
        current instance.

        And if ``raw`` is set as :data:`True`, the method will keep records of converted wrapper
        functions (:meth:`Context.functions`), converted *lambda* expressions (:meth:`Context.lambdef`)
        and *left-hand-side* variable names (:meth:`Context.variables`), class variable
        (:meth:`ClassContext.cls_var`), external variables (:meth:`ClassContext.external_variables`),
        wrapper functions for external variables (:meth:`ClassContext.external_functions`) into current
        instance as well.

        Important:
            ``raw`` should be :data:`True` only if the ``node`` is in the clause of another *context*,
            where the converted wrapper functions should be inserted.

        """
        if not self.has_expr(node):
            self += node.get_code()
            return

        indent = self._indent_level + 1
        self += self._linesep + self._indentation * indent

        if cls_ctx is None:
            cls_ctx = self._cls_ctx
        cls_var = self._cls_var

        if func:
            scope_keyword = 'nonlocal'  # type: ScopeKeyword

            # process suite
            ctx = Context(node=node, config=self.config,  # type: ignore[arg-type]
                          context=self._context, indent_level=indent,
                          scope_keyword=scope_keyword, raw=raw)
        else:
            scope_keyword = self._scope_keyword

            # process suite
            ctx = ClassContext(node=node, config=self.config,  # type: ignore[arg-type]
                               cls_ctx=cls_ctx, cls_var=cls_var,
                               context=self._context, indent_level=indent,
                               scope_keyword=scope_keyword, raw=raw, external=self._ext_vars)
        self += ctx.string.lstrip()

        # keep record
        if raw:
            self._lamb.extend(ctx.lambdef)
            self._vars.extend(ctx.variables)
            self._func.extend(ctx.functions)

            if not func:
                self._cls_var.update(ctx.cls_var)  # type: ignore[attr-defined]
                self._ext_vars.update(ctx.external_variables)  # type: ignore[attr-defined]
                self._ext_func.extend(ctx.external_functions)  # type: ignore[attr-defined]
        self._context.extend(ctx.global_stmt)

    def _process_string_context(self, node: parso.python.tree.PythonNode) -> None:
        """Process string contexts (:token:`stringliteral`).

        Args:
            node (parso.python.tree.PythonNode): string literals node

        This method first checks if ``node`` contains assignment expression.
        If not, it will not perform any processing, rather just append the
        original source code to context buffer.

        If ``node`` contains assignment expression, then it will initialise a new
        :class:`ClassStringContext` instance to perform the conversion process
        on such ``node``, which will first use :mod:`f2format` to convert those
        formatted string literals.

        Important:
            When initialisation, ``raw`` parameter **must** be set to :data:`True`;
            as the converted wrapper functions should be inserted in the *outer*
            context, rather than the new :class:`ClassStringContext` instance.

        After conversion, the method will keep records of converted wrapper
        functions (:attr:`Context.functions`), converted *lambda* expressions (:attr:`Context.lambdef`)
        and *left-hand-side* variable names (:attr:`Context.variables`), class variable
        (:attr:`ClassContext.cls_var`), external variables (:attr:`ClassContext.external_variables`),
        wrapper functions for external variables (:attr:`ClassContext.external_functions`) into current
        instance as well.

        """
        if not self.has_expr(node):
            self += node.get_code()
            return

        # initialise new context
        ctx = ClassStringContext(node=node, config=self.config,  # type: ignore[arg-type]
                                 cls_ctx=self._cls_ctx, cls_var=self._cls_var,
                                 context=self._context, indent_level=self._indent_level,
                                 scope_keyword=self._scope_keyword, raw=True, external=self._ext_vars)
        self += ctx.string

        # keep record
        self._lamb.extend(ctx.lambdef)
        self._vars.extend(ctx.variables)
        self._func.extend(ctx.functions)

        self._cls_var.update(ctx.cls_var)
        self._ext_vars.update(ctx.external_variables)
        self._ext_func.extend(ctx.external_functions)

        self._context.extend(ctx.global_stmt)

    def _process_namedexpr_test(self, node: parso.python.tree.PythonNode) -> None:
        """Process assignment expression (:token:`namedexpr_test`).

        Args:
            node (parso.python.tree.PythonNode): assignment expression node

        This method converts the assignment expression into wrapper function
        and extracts related records for inserting converted code.

        * The *left-hand-side* variable name will be recorded in
          :attr:`self._vars <walrus.Context._vars>`; and its corresponding UUID will
          be recorded in :attr:`self._cls_var <ClassContext._cls_var>`.
        * The *right-hand-side* expression will be converted using another
          :class:`ClassContext` instance and replaced with a wrapper tuple with
          attribute setting from :data:`CLS_TEMPLATE`; information described as
          :class:`FunctionEntry` will be recorded into :attr:`self._func <walrus.Context._func>`.

        Important:
            :class:`~walrus.ClassContext` will `mangle`_ *left-hand-side* variable name
            through :meth:`self.mangle <bpc_utils.BaseContext.mangle>` when converting.

            .. _mangle: https://docs.python.org/3/reference/expressions.html?highlight=mangling#atom-identifiers

        For special :term:`class variable <class-variable>` declared in :token:`global <global_stmt>`
        and/or :token:`nonlocal <nonlocal_stmt>` statements:

        * The *left-hand-side* variable name will **NOT** be considered as *class variable*,
          thus shall **NOT** be recorded.
        * The expression will be replaced with a wrapper function call rendered from
          :data:`CALL_TEMPLATE`; information described as :class:`FunctionEntry` will be
          recorded into :attr:`self._ext_func <walrus.ClassContext._ext_func>` instead.

        """
        # split assignment expression
        node_name, _, node_expr = node.children
        name = node_name.value
        nuid = self._uuid_gen.gen()

        # calculate expression string
        ctx = ClassContext(node=node_expr, config=self.config,  # type: ignore[arg-type]
                           cls_ctx=self._cls_ctx, cls_var=self._cls_var,
                           context=self._context, indent_level=self._indent_level,
                           scope_keyword=self._scope_keyword, raw=True,
                           external=self._ext_vars)
        expr = ctx.string.strip()

        self._lamb.extend(ctx.lambdef)
        self._vars.extend(ctx.variables)
        self._func.extend(ctx.functions)
        self._context.extend(ctx.global_stmt)

        self._cls_var.update(ctx.cls_var)
        self._ext_vars.update(ctx.external_variables)
        self._ext_func.extend(ctx.external_functions)

        # if declared in global/nonlocal statements
        external = name in self._ext_vars

        # replacing code
        if external:
            code = CALL_TEMPLATE % dict(name=name, uuid=nuid, expr=expr)
        else:
            code = CLS_TEMPLATE % dict(name=self.mangle(self._cls_ctx, name), expr=expr)
        prefix, suffix = self.extract_whitespaces(node.get_code())
        self += prefix + code + suffix

        if external:
            self._ext_func.append(dict(name=name, uuid=nuid, scope_keyword=self._ext_vars[name]))
            return

        if name in self._context:
            scope_keyword = 'global'  # type: ScopeKeyword
        else:
            scope_keyword = self._scope_keyword

        # keep records
        self._vars.append(name)
        self._func.append(dict(name=name, uuid=nuid, scope_keyword=scope_keyword))
        self._cls_var[self.mangle(self._cls_ctx, name)] = nuid

    def _process_defined_name(self, node: parso.python.tree.Name) -> None:
        """Process defined name (:token:`name`).

        Args:
            node (parso.python.tree.Name): defined name node

        This method processes name of defined :term:`class variable <class-variable>`. The original
        variable name will be recorded in :attr:`self._vars <walrus.Context._vars>`;
        its corresponding UUID will be recorded in :attr:`self._cls_var <ClassContext._cls_var>`;
        information described as :class:`FunctionEntry` will be recorded into
        :attr:`self._func <walrus.Context._func>`.

        Note:
            If the *left-hand-side* variable was declared in :token:`global <global_stmt>`
            and/or :token:`nonlocal <nonlocal_stmt>`, then it shall **NOT** be
            considered as *class variable*.

        """
        name = node.value
        self += node.get_code()

        # if declared in global/nonlocal statements
        if name in self._ext_vars:
            return

        name = self.mangle(self._cls_ctx, name)
        nuid = self._uuid_gen.gen()

        self._vars.append(name)
        self._func.append(dict(name=name, uuid=nuid, scope_keyword=self._scope_keyword))
        self._cls_var[self.mangle(self._cls_ctx, name)] = nuid

    def _process_expr_stmt(self, node: parso.python.tree.ExprStmt) -> None:
        """Process variable name (:token:`expr_stmt`).

        Args:
            node (parso.python.tree.ExprStmt): expression statement

        This method processes expression statements in the class context.
        It will search for *left-hand-side* variable names of defined
        class variables and call :meth:`~ClassContext._process_defined_name`
        to process such nodes.

        """
        # right hand side expression
        rhs = node.get_rhs()

        for child in node.children:
            if child == rhs:
                self._process(child)
                continue
            if child.type == 'name':
                self._process_defined_name(child)
                continue
            self += child.get_code()

    def _process_global_stmt(self, node: parso.python.tree.GlobalStmt) -> None:
        """Process function definition (:token:`global_stmt`).

        Args:
            node (parso.python.tree.GlobalStmt): global statement node

        This method records all variables declared in a *global* statement
        into :attr:`self._context <walrus.Context._context>` and
        :attr:`self._ext_vars <walrus.ClassContext._ext_vars>`.

        """
        children = iter(node.children)

        # <Keyword: global>
        next(children)
        # <Name: ...>
        name = next(children)
        self._context.append(name.value)
        self._ext_vars[name.value] = 'global'

        while True:
            try:
                # <Operator: ,>
                next(children)
            except StopIteration:
                break

            # <Name: ...>
            name = next(children)
            self._context.append(name.value)
            self._ext_vars[name.value] = 'global'

        # process code
        self += node.get_code()

    def _process_nonlocal_stmt(self, node: parso.python.tree.KeywordStatement) -> None:
        """Process function definition (:token:`nonlocal_stmt`).

        Args:
            node (parso.python.tree.KeywordStatement): nonlocal statement node

        This method records all variables declared in a *nonlocal* statement
        into :attr:`self._ext_vars <walrus.ClassContext._ext_vars>`.

        """
        children = iter(node.children)

        # <Keyword: nonlocal>
        next(children)
        # <Name: ...>
        name = next(children)
        self._ext_vars[name.value] = 'nonlocal'

        while True:
            try:
                # <Operator: ,>
                next(children)
            except StopIteration:
                break

            # <Name: ...>
            name = next(children)
            self._ext_vars[name.value] = 'nonlocal'

        # process code
        self += node.get_code()

    def _concat(self) -> None:
        """Concatenate final string.

        This method tries to inserted the recorded wrapper functions and variables
        at the very location where starts to contain assignment expressions, i.e.
        between the converted code as :attr:`self._prefix <Context._prefix>` and
        :attr:`self._suffix <Context._suffix>`.

        For special :term:`class variable <class-variable>` declared in :token:`global <global_stmt>`
        and/or :token:`nonlocal <nonlocal_stmt>` statements, when assigning to
        to such variables, i.e. they are on *left-hand-side* of assignment
        expressions, the expressions will be assigned with a wrapper function
        rendered from :data:`FUNC_TEMPLATE`.

        """
        flag = self.has_expr(self._root)

        # strip suffix comments
        prefix, suffix = self.split_comments(self._suffix, self._linesep)
        match = re.match(r'^(?P<linesep>(%s)*)' % self._linesep, suffix, flags=re.ASCII)
        suffix_linesep = match.group('linesep') if match is not None else ''

        # first, the prefix code
        self._buffer += self._prefix + prefix + suffix_linesep

        # then, the class and functions
        indent = self._indentation * self._indent_level
        linesep = self._linesep
        if flag and self._pep8:
            if (self._node_before_expr is not None
                    and self._node_before_expr.type in ('funcdef', 'classdef')
                    and self._indent_level == 0):
                blank = 2
            else:
                blank = 1
            self._buffer += self._linesep * self.missing_newlines(prefix=self._buffer, suffix='',
                                                                  expected=blank, linesep=self._linesep)

        for index, func in enumerate(sorted(self._ext_func, key=lambda func: func['name'])):
            if index > 0:
                self._buffer += linesep
            self._buffer += indent + (
                '%s%s' % (self._linesep, indent)
            ).join(FUNC_TEMPLATE) % dict(indentation=self._indentation, cls=self._cls_ctx, **func) + linesep
        for lamb in self._lamb:
            if self._buffer:
                self._buffer += linesep
            self._buffer += indent + (
                '%s%s' % (self._linesep, indent)
            ).join(LAMBDA_FUNC_TEMPLATE) % dict(indentation=self._indentation, **lamb) + linesep

        # finally, the suffix code
        if flag and self._pep8:
            blank = 2 if self._indent_level == 0 else 1
            self._buffer += self._linesep * self.missing_newlines(prefix=self._buffer, suffix=suffix,
                                                                  expected=blank, linesep=self._linesep)
        self._buffer += suffix.lstrip(self._linesep)


class ClassStringContext(ClassContext):
    """String (f-string) conversion context.

    This class is mainly used for converting **formatted string literals**
    inside a class suite (:term:`ClassVar <class-variable>`).

    Args:
        node (parso.python.tree.PythonNode): parso AST
        config (Config): conversion configurations

    Keyword Args:
        cls_ctx (str): class context name
        cls_var (Dict[str, str]): mapping for assignment variable and its UUID
        indent_level (int): current indentation level
        scope_keyword (Optional[Literal['global', 'nonlocal']]): scope keyword for wrapper function
        context (Optional[List[str]]): global context (:term:`namespace`)
        raw (Literal[True]): raw context processing flag
        external (Optional[Dict[str, Literal['global', 'nonlocal']]]):
            mapping of :term:`class variable <class-variable>` declared in :token:`global <global_stmt>` and/or
            :token:`nonlocal <nonlocal_stmt>` statements

    Note:
        ``raw`` should always be :data:`True`.

    As the conversion in :class:`ClassContext` introduced quotes (``'``)
    into the converted code, it may cause conflicts on the string parsing
    if the assignment expression was inside a formatted string literal.

    Therefore, we will use :mod:`f2format` ahead to convert such formatted
    string literals into normal :func:`str.format` calls then convert any assignment
    expressions it may contain.

    """

    def __init__(self, node: parso.python.tree.PythonNode, config: WalrusConfig, *,
                 cls_ctx: str, cls_var: Optional[Dict[str, str]] = None,
                 indent_level: int = 0, scope_keyword: Optional[ScopeKeyword] = None,
                 context: Optional[List[str]] = None, raw: Literal[True] = True,
                 external: Optional[Dict[str, ScopeKeyword]] = None):
        # convert using f2format first
        prefix, suffix = self.extract_whitespaces(node.get_code())
        code = f2format.convert(node.get_code().strip())
        node = parso_parse(code, filename=config.filename, version=config.source_version)  # type: ignore[assignment]

        # call super init
        super().__init__(node=node, config=config, cls_ctx=cls_ctx, cls_var=cls_var,  # type: ignore[arg-type]
                         context=context, indent_level=indent_level, scope_keyword=scope_keyword,
                         raw=raw, external=external)
        self._buffer = prefix + self._buffer + suffix


###############################################################################
# Public Interface

def convert(code: Union[str, bytes], filename: Optional[str] = None, *,
            source_version: Optional[str] = None, linesep: Optional[Linesep] = None,
            indentation: Optional[Union[int, str]] = None, pep8: Optional[bool] = None) -> str:
    """Convert the given Python source code string.

    Args:
        code (Union[str, bytes]): the source code to be converted
        filename (Optional[str]): an optional source file name to provide a context in case of error

    Keyword Args:
        source_version (Optional[str]): parse the code as this Python version (uses the latest version by default)
        linesep (Optional[:data:`~bpc_utils.Linesep`]): line separator of code (``LF``, ``CRLF``, ``CR``)
            (auto detect by default)
        indentation (Optional[Union[int, str]]): code indentation style, specify an integer for the number of spaces,
            or ``'t'``/``'tab'`` for tabs (auto detect by default)
        pep8 (Optional[bool]): whether to make code insertion :pep:`8` compliant

    :Environment Variables:
     - :envvar:`WALRUS_SOURCE_VERSION` -- same as the ``source_version`` argument and the ``--source-version`` option
        in CLI
     - :envvar:`WALRUS_LINESEP` -- same as the `linesep` `argument` and the ``--linesep`` option in CLI
     - :envvar:`WALRUS_INDENTATION` -- same as the ``indentation`` argument and the ``--indentation`` option in CLI
     - :envvar:`WALRUS_PEP8` -- same as the ``pep8`` argument and the ``--no-pep8`` option in CLI (logical negation)

    Returns:
        str: converted source code

    """
    # parse source string
    source_version = _get_source_version_option(source_version)
    module = parso_parse(code, filename=filename, version=source_version)

    # get linesep, indentation and pep8 options
    linesep = _get_linesep_option(linesep)
    indentation = _get_indentation_option(indentation)
    if linesep is None:
        linesep = detect_linesep(code)
    if indentation is None:
        indentation = detect_indentation(code)
    pep8 = _get_pep8_option(pep8)

    # pack conversion configuration
    config = Config(linesep=linesep, indentation=indentation, pep8=pep8,
                    filename=filename, source_version=source_version)

    # convert source string
    result = Context(module, config).string  # type: ignore[arg-type]

    # return conversion result
    return result


def walrus(filename: str, *, source_version: Optional[str] = None, linesep: Optional[Linesep] = None,
           indentation: Optional[Union[int, str]] = None, pep8: Optional[bool] = None,
           quiet: Optional[bool] = None, dry_run: bool = False) -> None:
    """Convert the given Python source code file. The file will be overwritten.

    Args:
        filename (str): the file to convert

    Keyword Args:
        source_version (Optional[str]): parse the code as this Python version (uses the latest version by default)
        linesep (Optional[:data:`~bpc_utils.Linesep`]): line separator of code (``LF``, ``CRLF``, ``CR``)
            (auto detect by default)
        indentation (Optional[Union[int, str]]): code indentation style, specify an integer for the number of spaces,
            or ``'t'``/``'tab'`` for tabs (auto detect by default)
        pep8 (Optional[bool]): whether to make code insertion :pep:`8` compliant
        quiet (Optional[bool]): whether to run in quiet mode
        dry_run (bool): if :data:`True`, only print the name of the file to convert but do not perform any conversion

    :Environment Variables:
     - :envvar:`WALRUS_SOURCE_VERSION` -- same as the ``source-version`` argument and the ``--source-version`` option
        in CLI
     - :envvar:`WALRUS_LINESEP` -- same as the ``linesep`` argument and the ``--linesep`` option in CLI
     - :envvar:`WALRUS_INDENTATION` -- same as the ``indentation`` argument and the ``--indentation`` option in CLI
     - :envvar:`WALRUS_PEP8` -- same as the ``pep8`` argument and the ``--no-pep8`` option in CLI (logical negation)
     - :envvar:`WALRUS_QUIET` -- same as the ``quiet`` argument and the ``--quiet`` option in CLI

    """
    quiet = _get_quiet_option(quiet)
    if not quiet:
        with TaskLock():
            print('Now converting: %r' % filename, file=sys.stderr)
    if dry_run:
        return

    # read file content
    with open(filename, 'rb') as file:
        content = file.read()

    # detect source code encoding
    try:
        encoding = detect_encoding(content)
    except SyntaxError as e:
        raise BPCSyntaxError('failed to detect encoding for source file %r: %s' % (filename, e)) from None

    # do the dirty things
    result = convert(content, filename=filename, source_version=source_version,
                     linesep=linesep, indentation=indentation, pep8=pep8)

    # overwrite the file with conversion result
    with open(filename, 'w', encoding=encoding, newline='') as file:
        file.write(result)


###############################################################################
# CLI & Entry Point

# option values display
# these values are only intended for argparse help messages
# this shows default values by default, environment variables may override them
__cwd__ = os.getcwd()
__walrus_quiet__ = 'quiet mode' if _get_quiet_option() else 'non-quiet mode'
__walrus_concurrency__ = _get_concurrency_option() or 'auto detect'
__walrus_do_archive__ = 'will do archive' if _get_do_archive_option() else 'will not do archive'
__walrus_archive_path__ = os.path.join(__cwd__, _get_archive_path_option())
__walrus_source_version__ = _get_source_version_option()
__walrus_linesep__ = {
    '\n': 'LF',
    '\r\n': 'CRLF',
    '\r': 'CR',
    None: 'auto detect'
}[_get_linesep_option()]
__walrus_indentation__ = _get_indentation_option()
if __walrus_indentation__ is None:
    __walrus_indentation__ = 'auto detect'
elif __walrus_indentation__ == '\t':
    __walrus_indentation__ = 'tab'
else:
    __walrus_indentation__ = '%d spaces' % len(__walrus_indentation__)
__walrus_pep8__ = 'will conform to PEP 8' if _get_pep8_option() else 'will not conform to PEP 8'


def get_parser() -> argparse.ArgumentParser:
    """Generate CLI parser.

    Returns:
        argparse.ArgumentParser: CLI parser for walrus

    """
    parser = argparse.ArgumentParser(prog='walrus',
                                     usage='walrus [options] <Python source files and directories...>',
                                     description='Back-port compiler for Python 3.8 assignment expressions.')
    parser.add_argument('-V', '--version', action='version', version=__version__)
    parser.add_argument('-q', '--quiet', action='store_true', default=None,
                        help='run in quiet mode (current: %s)' % __walrus_quiet__)
    parser.add_argument('-C', '--concurrency', action='store', type=int, metavar='N',
                        help='the number of concurrent processes for conversion (current: %s)' % __walrus_concurrency__)
    parser.add_argument('--dry-run', action='store_true',
                        help='list the files to be converted without actually performing conversion and archiving')
    parser.add_argument('-s', '--simple', action='store', nargs='?', dest='simple_args', const='', metavar='FILE',
                        help='this option tells the program to operate in "simple mode"; '
                             'if a file name is provided, the program will convert the file but print conversion '
                             'result to standard output instead of overwriting the file; '
                             'if no file names are provided, read code for conversion from standard input and print '
                             'conversion result to standard output; '
                             'in "simple mode", no file names shall be provided via positional arguments')

    archive_group = parser.add_argument_group(title='archive options',
                                              description="backup original files in case there're any issues")
    archive_group.add_argument('-na', '--no-archive', action='store_false', dest='do_archive', default=None,
                               help='do not archive original files (current: %s)' % __walrus_do_archive__)
    archive_group.add_argument('-k', '--archive-path', action='store', default=__walrus_archive_path__, metavar='PATH',
                               help='path to archive original files (current: %(default)s)')
    archive_group.add_argument('-r', '--recover', action='store', dest='recover_file', metavar='ARCHIVE_FILE',
                               help='recover files from a given archive file')
    archive_group.add_argument('-r2', action='store_true', help='remove the archive file after recovery')
    archive_group.add_argument('-r3', action='store_true', help='remove the archive file after recovery, '
                                                                'and remove the archive directory if it becomes empty')

    convert_group = parser.add_argument_group(title='convert options', description='conversion configuration')
    convert_group.add_argument('-vs', '-vf', '--source-version', '--from-version', action='store', metavar='VERSION',
                               default=__walrus_source_version__, choices=WALRUS_SOURCE_VERSIONS,
                               help='parse source code as this Python version (current: %(default)s)')
    convert_group.add_argument('-l', '--linesep', action='store',
                               help='line separator (LF, CRLF, CR) to read '
                                    'source files (current: %s)' % __walrus_linesep__)
    convert_group.add_argument('-t', '--indentation', action='store', metavar='INDENT',
                               help='code indentation style, specify an integer for the number of spaces, '
                                    "or 't'/'tab' for tabs (current: %s)" % __walrus_indentation__)
    convert_group.add_argument('-n8', '--no-pep8', action='store_false', dest='pep8', default=None,
                               help='do not make code insertion PEP 8 compliant (current: %s)' % __walrus_pep8__)

    parser.add_argument('files', action='store', nargs='*', metavar='<Python source files and directories...>',
                        help='Python source files and directories to be converted')

    return parser


def do_walrus(filename: str, **kwargs: object) -> None:
    """Wrapper function to catch exceptions."""
    try:
        walrus(filename, **kwargs)  # type: ignore[arg-type]
    except Exception:  # pylint: disable=broad-except
        with TaskLock():
            print('Failed to convert file: %r' % filename, file=sys.stderr)
            traceback.print_exc()


def main(argv: Optional[List[str]] = None) -> int:
    """Entry point for walrus.

    Args:
        argv (Optional[List[str]]): CLI arguments

    Returns:
        int: program exit code

    :Environment Variables:
     - :envvar:`WALRUS_QUIET` -- same as the ``--quiet`` option in CLI
     - :envvar:`WALRUS_CONCURRENCY` -- same as the ``--concurrency`` option in CLI
     - :envvar:`WALRUS_DO_ARCHIVE` -- same as the ``--no-archive`` option in CLI (logical negation)
     - :envvar:`WALRUS_ARCHIVE_PATH` -- same as the ``--archive-path`` option in CLI
     - :envvar:`WALRUS_SOURCE_VERSION` -- same as the ``--source-version`` option in CLI
     - :envvar:`WALRUS_LINESEP` -- same as the ``--linesep`` option in CLI
     - :envvar:`WALRUS_INDENTATION` -- same as the ``--indentation`` option in CLI
     - :envvar:`WALRUS_PEP8` -- same as the ``--no-pep8`` option in CLI (logical negation)

    """
    parser = get_parser()
    args = parser.parse_args(argv)

    options = {
        'source_version': args.source_version,
        'linesep': args.linesep,
        'indentation': args.indentation,
        'pep8': args.pep8,
    }

    # check if running in simple mode
    if args.simple_args is not None:
        if args.files:
            parser.error('no Python source files or directories shall be given as positional arguments in simple mode')
        if not args.simple_args:  # read from stdin
            code = sys.stdin.read()
        else:  # read from file
            filename = args.simple_args
            options['filename'] = filename
            with open(filename, 'rb') as file:
                code = file.read()
        sys.stdout.write(convert(code, **options))  # print conversion result to stdout
        return 0

    # get options
    quiet = _get_quiet_option(args.quiet)
    processes = _get_concurrency_option(args.concurrency)
    do_archive = _get_do_archive_option(args.do_archive)
    archive_path = _get_archive_path_option(args.archive_path)

    # check if doing recovery
    if args.recover_file:
        recover_files(args.recover_file)
        if not args.quiet:
            print('Recovered files from archive: %r' % args.recover_file, file=sys.stderr)
        # TODO: maybe implement deletion in bpc-utils?
        if args.r2 or args.r3:
            os.remove(args.recover_file)
            if args.r3:
                archive_dir = os.path.dirname(os.path.realpath(args.recover_file))
                if not os.listdir(archive_dir):
                    os.rmdir(archive_dir)
        return 0

    # fetch file list
    if not args.files:
        parser.error('no Python source files or directories are given')
    filelist = sorted(detect_files(args.files))

    # terminate if no valid Python source files detected
    if not filelist:
        if not args.quiet:
            # TODO: maybe use parser.error?
            print('Warning: no valid Python source files found in %r' % args.files, file=sys.stderr)
        return 1

    # make archive
    if do_archive and not args.dry_run:
        archive_files(filelist, archive_path)

    # process files
    options.update({
        'quiet': quiet,
        'dry_run': args.dry_run,
    })
    map_tasks(do_walrus, filelist, kwargs=options, processes=processes)

    return 0


if __name__ == '__main__':
    sys.exit(main())
