# Copyright (c) 2012-2013 Mitch Garnaat http://garnaat.org/
# Copyright 2012-2014 Amazon.com, Inc. or its affiliates. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License"). You
# may not use this file except in compliance with the License. A copy of
# the License is located at
#
# http://aws.amazon.com/apache2.0/
#
# or in the "license" file accompanying this file. This file is
# distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
# ANY KIND, either express or implied. See the License for the specific
# language governing permissions and limitations under the License.
"""
This module contains the main interface to the botocore package, the
Session object.
"""

import copy
import logging
import os
import platform

from botocore import __version__
import botocore.configloader
import botocore.credentials
import botocore.client
from botocore.exceptions import ConfigNotFound, ProfileNotFound
from botocore.exceptions import UnknownServiceError, PartialCredentialsError
from botocore.errorfactory import ClientExceptionsFactory
from botocore import handlers
from botocore.hooks import HierarchicalEmitter, first_non_none_response
from botocore.loaders import create_loader
from botocore.parsers import ResponseParserFactory
from botocore.regions import EndpointResolver
from botocore.model import ServiceModel
from botocore import paginate
from botocore import waiter
from botocore import retryhandler, translate


logger = logging.getLogger(__name__)


class Session(object):
    """
    The Session object collects together useful functionality
    from `botocore` as well as important data such as configuration
    information and credentials into a single, easy-to-use object.

    :ivar available_profiles: A list of profiles defined in the config
        file associated with this session.
    :ivar profile: The current profile.
    """

    #: A default dictionary that maps the logical names for session variables
    #: to the specific environment variables and configuration file names
    #: that contain the values for these variables.
    #: When creating a new Session object, you can pass in your own dictionary
    #: to remap the logical names or to add new logical names.  You can then
    #: get the current value for these variables by using the
    #: ``get_config_variable`` method of the :class:`botocore.session.Session`
    #: class.
    #: These form the keys of the dictionary.  The values in the dictionary
    #: are tuples of (<config_name>, <environment variable>, <default value>,
    #: <conversion func>).
    #: The conversion func is a function that takes the configuration value
    #: as an argument and returns the converted value.  If this value is
    #: None, then the configuration value is returned unmodified.  This
    #: conversion function can be used to type convert config values to
    #: values other than the default values of strings.
    #: The ``profile`` and ``config_file`` variables should always have a
    #: None value for the first entry in the tuple because it doesn't make
    #: sense to look inside the config file for the location of the config
    #: file or for the default profile to use.
    #: The ``config_name`` is the name to look for in the configuration file,
    #: the ``env var`` is the OS environment variable (``os.environ``) to
    #: use, and ``default_value`` is the value to use if no value is otherwise
    #: found.
    SESSION_VARIABLES = {
        # logical:  config_file, env_var,        default_value, conversion_func
        'profile': (None, ['AWS_DEFAULT_PROFILE', 'AWS_PROFILE'], None, None),
        'region': ('region', 'AWS_DEFAULT_REGION', None, None),
        'data_path': ('data_path', 'AWS_DATA_PATH', None, None),
        'config_file': (None, 'AWS_CONFIG_FILE', '~/.aws/config', None),
        'ca_bundle': ('ca_bundle', 'AWS_CA_BUNDLE', None, None),
        'api_versions': ('api_versions', None, {}, None),

        # This is the shared credentials file amongst sdks.
        'credentials_file': (None, 'AWS_SHARED_CREDENTIALS_FILE',
                             '~/.aws/credentials', None),

        # These variables only exist in the config file.

        # This is the number of seconds until we time out a request to
        # the instance metadata service.
        'metadata_service_timeout': (
            'metadata_service_timeout',
            'AWS_METADATA_SERVICE_TIMEOUT', 1, int),
        # This is the number of request attempts we make until we give
        # up trying to retrieve data from the instance metadata service.
        'metadata_service_num_attempts': (
            'metadata_service_num_attempts',
            'AWS_METADATA_SERVICE_NUM_ATTEMPTS', 1, int),
        'parameter_validation': ('parameter_validation', None, True, None),
    }

    #: The default format string to use when configuring the botocore logger.
    LOG_FORMAT = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'

    def __init__(self, session_vars=None, event_hooks=None,
                 include_builtin_handlers=True, profile=None):
        """
        Create a new Session object.

        :type session_vars: dict
        :param session_vars: A dictionary that is used to override some or all
            of the environment variables associated with this session.  The
            key/value pairs defined in this dictionary will override the
            corresponding variables defined in ``SESSION_VARIABLES``.

        :type event_hooks: BaseEventHooks
        :param event_hooks: The event hooks object to use. If one is not
            provided, an event hooks object will be automatically created
            for you.

        :type include_builtin_handlers: bool
        :param include_builtin_handlers: Indicates whether or not to
            automatically register builtin handlers.

        :type profile: str
        :param profile: The name of the profile to use for this
            session.  Note that the profile can only be set when
            the session is created.

        """
        self.session_var_map = copy.copy(self.SESSION_VARIABLES)
        if session_vars:
            self.session_var_map.update(session_vars)
        if event_hooks is None:
            self._events = HierarchicalEmitter()
        else:
            self._events = event_hooks
        if include_builtin_handlers:
            self._register_builtin_handlers(self._events)
        self.user_agent_name = 'Botocore'
        self.user_agent_version = __version__
        self.user_agent_extra = ''
        # The _profile attribute is just used to cache the value
        # of the current profile to avoid going through the normal
        # config lookup process each access time.
        self._profile = None
        self._config = None
        self._credentials = None
        self._profile_map = None
        # This is a dict that stores per session specific config variable
        # overrides via set_config_variable().
        self._session_instance_vars = {}
        if profile is not None:
            self._session_instance_vars['profile'] = profile
        self._client_config = None
        self._components = ComponentLocator()
        self._register_components()

    def _register_components(self):
        self._register_credential_provider()
        self._register_data_loader()
        self._register_endpoint_resolver()
        self._register_event_emitter()
        self._register_response_parser_factory()
        self._register_exceptions_factory()

    def _register_event_emitter(self):
        self._components.register_component('event_emitter', self._events)

    def _register_credential_provider(self):
        self._components.lazy_register_component(
            'credential_provider',
            lambda:  botocore.credentials.create_credential_resolver(self))

    def _register_data_loader(self):
        self._components.lazy_register_component(
            'data_loader',
            lambda:  create_loader(self.get_config_variable('data_path')))

    def _register_endpoint_resolver(self):
        def create_default_resolver():
            loader = self.get_component('data_loader')
            endpoints = loader.load_data('endpoints')
            return EndpointResolver(endpoints)
        self._components.lazy_register_component(
            'endpoint_resolver', create_default_resolver)

    def _register_response_parser_factory(self):
        self._components.register_component('response_parser_factory',
                                            ResponseParserFactory())

    def _register_exceptions_factory(self):
        self._components.register_component(
            'exceptions_factory', ClientExceptionsFactory())

    def _register_builtin_handlers(self, events):
        for spec in handlers.BUILTIN_HANDLERS:
            if len(spec) == 2:
                event_name, handler = spec
                self.register(event_name, handler)
            else:
                event_name, handler, register_type = spec
                if register_type is handlers.REGISTER_FIRST:
                    self._events.register_first(event_name, handler)
                elif register_type is handlers.REGISTER_LAST:
                    self._events.register_last(event_name, handler)

    @property
    def available_profiles(self):
        return list(self._build_profile_map().keys())

    def _build_profile_map(self):
        # This will build the profile map if it has not been created,
        # otherwise it will return the cached value.  The profile map
        # is a list of profile names, to the config values for the profile.
        if self._profile_map is None:
            self._profile_map = self.full_config['profiles']
        return self._profile_map

    @property
    def profile(self):
        if self._profile is None:
            profile = self.get_config_variable('profile')
            self._profile = profile
        return self._profile

    def get_config_variable(self, logical_name,
                            methods=('instance', 'env', 'config')):
        """
        Retrieve the value associated with the specified logical_name
        from the environment or the config file.  Values found in the
        environment variable take precedence of values found in the
        config file.  If no value can be found, a None will be returned.

        :type logical_name: str
        :param logical_name: The logical name of the session variable
            you want to retrieve.  This name will be mapped to the
            appropriate environment variable name for this session as
            well as the appropriate config file entry.

        :type method: tuple
        :param method: Defines which methods will be used to find
            the variable value.  By default, all available methods
            are tried but you can limit which methods are used
            by supplying a different value to this parameter.
            Valid choices are: instance|env|config

        :returns: value of variable or None if not defined.

        """
        # Handle all the short circuit special cases first.
        if logical_name not in self.session_var_map:
            return
        # Do the actual lookups.  We need to handle
        # 'instance', 'env', and 'config' locations, in that order.
        value = None
        var_config = self.session_var_map[logical_name]
        if self._found_in_instance_vars(methods, logical_name):
            value = self._session_instance_vars[logical_name]
            logger.debug(
                "Loading variable %s from instance vars with value %r.",
                logical_name,
                value,
            )
            return value
        elif self._found_in_env(methods, var_config):
            value = self._retrieve_from_env(var_config[1], os.environ)
            logger.debug(
                "Loading variable %s from environment with value %r.",
                logical_name,
                value,
            )
        elif self._found_in_config_file(methods, var_config):
            value = self.get_scoped_config()[var_config[0]]
            logger.debug(
                "Loading variable %s from config file with value %r.",
                logical_name,
                value,
            )
        if value is None:
            logger.debug("Loading variable %s from defaults.", logical_name)
            value = var_config[2]
        if var_config[3] is not None:
            value = var_config[3](value)
        return value

    def _found_in_instance_vars(self, methods, logical_name):
        if 'instance' in methods:
            return logical_name in self._session_instance_vars
        return False

    def _found_in_env(self, methods, var_config):
        return (
            'env' in methods and
            var_config[1] is not None and
            self._retrieve_from_env(var_config[1], os.environ) is not None)

    def _found_in_config_file(self, methods, var_config):
        if 'config' in methods and var_config[0] is not None:
            return var_config[0] in self.get_scoped_config()
        return False

    def _retrieve_from_env(self, names, environ):
        # We need to handle the case where names is either
        # a single value or a list of variables.
        if not isinstance(names, list):
            names = [names]
        for name in names:
            if name in environ:
                return environ[name]
        return None

    def set_config_variable(self, logical_name, value):
        """Set a configuration variable to a specific value.

        By using this method, you can override the normal lookup
        process used in ``get_config_variable`` by explicitly setting
        a value.  Subsequent calls to ``get_config_variable`` will
        use the ``value``.  This gives you per-session specific
        configuration values.

        ::
            >>> # Assume logical name 'foo' maps to env var 'FOO'
            >>> os.environ['FOO'] = 'myvalue'
            >>> s.get_config_variable('foo')
            'myvalue'
            >>> s.set_config_variable('foo', 'othervalue')
            >>> s.get_config_variable('foo')
            'othervalue'

        :type logical_name: str
        :param logical_name: The logical name of the session variable
            you want to set.  These are the keys in ``SESSION_VARIABLES``.
        :param value: The value to associate with the config variable.

        """
        logger.debug(
            "Setting config variable for %s to %r",
            logical_name,
            value,
        )
        self._session_instance_vars[logical_name] = value

    def get_scoped_config(self):
        """
        Returns the config values from the config file scoped to the current
        profile.

        The configuration data is loaded **only** from the config file.
        It does not resolve variables based on different locations
        (e.g. first from the session instance, then from environment
        variables, then from the config file).  If you want this lookup
        behavior, use the ``get_config_variable`` method instead.

        Note that this configuration is specific to a single profile (the
        ``profile`` session variable).

        If the ``profile`` session variable is set and the profile does
        not exist in the config file, a ``ProfileNotFound`` exception
        will be raised.

        :raises: ConfigNotFound, ConfigParseError, ProfileNotFound
        :rtype: dict

        """
        profile_name = self.get_config_variable('profile')
        profile_map = self._build_profile_map()
        # If a profile is not explicitly set return the default
        # profile config or an empty config dict if we don't have
        # a default profile.
        if profile_name is None:
            return profile_map.get('default', {})
        elif profile_name not in profile_map:
            # Otherwise if they specified a profile, it has to
            # exist (even if it's the default profile) otherwise
            # we complain.
            raise ProfileNotFound(profile=profile_name)
        else:
            return profile_map[profile_name]

    @property
    def full_config(self):
        """Return the parsed config file.

        The ``get_config`` method returns the config associated with the
        specified profile.  This property returns the contents of the
        **entire** config file.

        :rtype: dict
        """
        if self._config is None:
            try:
                config_file = self.get_config_variable('config_file')
                self._config = botocore.configloader.load_config(config_file)
            except ConfigNotFound:
                self._config = {'profiles': {}}
            try:
                # Now we need to inject the profiles from the
                # credentials file.  We don't actually need the values
                # in the creds file, only the profile names so that we
                # can validate the user is not referring to a nonexistent
                # profile.
                cred_file = self.get_config_variable('credentials_file')
                cred_profiles = botocore.configloader.raw_config_parse(
                    cred_file)
                for profile in cred_profiles:
                    cred_vars = cred_profiles[profile]
                    if profile not in self._config['profiles']:
                        self._config['profiles'][profile] = cred_vars
                    else:
                        self._config['profiles'][profile].update(cred_vars)
            except ConfigNotFound:
                pass
        return self._config

    def get_default_client_config(self):
        """Retrieves the default config for creating clients

        :rtype: botocore.client.Config
        :returns: The default client config object when creating clients. If
            the value is ``None`` then there is no default config object
            attached to the session.
        """
        return self._client_config

    def set_default_client_config(self, client_config):
        """Sets the default config for creating clients

        :type client_config: botocore.client.Config
        :param client_config: The default client config object when creating
            clients. If the value is ``None`` then there is no default config
            object attached to the session.
        """
        self._client_config = client_config

    def set_credentials(self, access_key, secret_key, token=None):
        """
        Manually create credentials for this session.  If you would
        prefer to use botocore without a config file, environment variables,
        or IAM roles, you can pass explicit credentials into this
        method to establish credentials for this session.

        :type access_key: str
        :param access_key: The access key part of the credentials.

        :type secret_key: str
        :param secret_key: The secret key part of the credentials.

        :type token: str
        :param token: An option session token used by STS session
            credentials.
        """
        self._credentials = botocore.credentials.Credentials(access_key,
                                                             secret_key,
                                                             token)

    def get_credentials(self):
        """
        Return the :class:`botocore.credential.Credential` object
        associated with this session.  If the credentials have not
        yet been loaded, this will attempt to load them.  If they
        have already been loaded, this will return the cached
        credentials.

        """
        if self._credentials is None:
            self._credentials = self._components.get_component(
                'credential_provider').load_credentials()
        return self._credentials

    def user_agent(self):
        """
        Return a string suitable for use as a User-Agent header.
        The string will be of the form:

        <agent_name>/<agent_version> Python/<py_ver> <plat_name>/<plat_ver> <exec_env>

        Where:

         - agent_name is the value of the `user_agent_name` attribute
           of the session object (`Boto` by default).
         - agent_version is the value of the `user_agent_version`
           attribute of the session object (the botocore version by default).
           by default.
         - py_ver is the version of the Python interpreter beng used.
         - plat_name is the name of the platform (e.g. Darwin)
         - plat_ver is the version of the platform
         - exec_env is exec-env/$AWS_EXECUTION_ENV

        If ``user_agent_extra`` is not empty, then this value will be
        appended to the end of the user agent string.

        """
        base = '%s/%s Python/%s %s/%s' % (self.user_agent_name,
                                          self.user_agent_version,
                                          platform.python_version(),
                                          platform.system(),
                                          platform.release())
        if os.environ.get('AWS_EXECUTION_ENV') is not None:
            base += ' exec-env/%s' % os.environ.get('AWS_EXECUTION_ENV')
        if self.user_agent_extra:
            base += ' %s' % self.user_agent_extra

        return base

    def get_data(self, data_path):
        """
        Retrieve the data associated with `data_path`.

        :type data_path: str
        :param data_path: The path to the data you wish to retrieve.
        """
        return self.get_component('data_loader').load_data(data_path)

    def get_service_model(self, service_name, api_version=None):
        """Get the service model object.

        :type service_name: string
        :param service_name: The service name

        :type api_version: string
        :param api_version: The API version of the service.  If none is
            provided, then the latest API version will be used.

        :rtype: L{botocore.model.ServiceModel}
        :return: The botocore service model for the service.

        """
        service_description = self.get_service_data(service_name, api_version)
        return ServiceModel(service_description, service_name=service_name)

    def get_waiter_model(self, service_name, api_version=None):
        loader = self.get_component('data_loader')
        waiter_config = loader.load_service_model(
            service_name, 'waiters-2', api_version)
        return waiter.WaiterModel(waiter_config)

    def get_paginator_model(self, service_name, api_version=None):
        loader = self.get_component('data_loader')
        paginator_config = loader.load_service_model(
            service_name, 'paginators-1', api_version)
        return paginate.PaginatorModel(paginator_config)

    def get_service_data(self, service_name, api_version=None):
        """
        Retrieve the fully merged data associated with a service.
        """
        data_path = service_name
        service_data = self.get_component('data_loader').load_service_model(
            data_path,
            type_name='service-2',
            api_version=api_version
        )
        self._events.emit('service-data-loaded.%s' % service_name,
                          service_data=service_data,
                          service_name=service_name, session=self)
        return service_data

    def get_available_services(self):
        """
        Return a list of names of available services.
        """
        return self.get_component('data_loader')\
            .list_available_services(type_name='service-2')

    def set_debug_logger(self, logger_name='botocore'):
        """
        Convenience function to quickly configure full debug output
        to go to the console.
        """
        self.set_stream_logger(logger_name, logging.DEBUG)

    def set_stream_logger(self, logger_name, log_level, stream=None,
                          format_string=None):
        """
        Convenience method to configure a stream logger.

        :type logger_name: str
        :param logger_name: The name of the logger to configure

        :type log_level: str
        :param log_level: The log level to set for the logger.  This
            is any param supported by the ``.setLevel()`` method of
            a ``Log`` object.

        :type stream: file
        :param stream: A file like object to log to.  If none is provided
            then sys.stderr will be used.

        :type format_string: str
        :param format_string: The format string to use for the log
            formatter.  If none is provided this will default to
            ``self.LOG_FORMAT``.

        """
        log = logging.getLogger(logger_name)
        log.setLevel(logging.DEBUG)

        ch = logging.StreamHandler(stream)
        ch.setLevel(log_level)

        # create formatter
        if format_string is None:
            format_string = self.LOG_FORMAT
        formatter = logging.Formatter(format_string)

        # add formatter to ch
        ch.setFormatter(formatter)

        # add ch to logger
        log.addHandler(ch)

    def set_file_logger(self, log_level, path, logger_name='botocore'):
        """
        Convenience function to quickly configure any level of logging
        to a file.

        :type log_level: int
        :param log_level: A log level as specified in the `logging` module

        :type path: string
        :param path: Path to the log file.  The file will be created
            if it doesn't already exist.
        """
        log = logging.getLogger(logger_name)
        log.setLevel(logging.DEBUG)

        # create console handler and set level to debug
        ch = logging.FileHandler(path)
        ch.setLevel(log_level)

        # create formatter
        formatter = logging.Formatter(self.LOG_FORMAT)

        # add formatter to ch
        ch.setFormatter(formatter)

        # add ch to logger
        log.addHandler(ch)

    def register(self, event_name, handler, unique_id=None,
                 unique_id_uses_count=False):
        """Register a handler with an event.

        :type event_name: str
        :param event_name: The name of the event.

        :type handler: callable
        :param handler: The callback to invoke when the event
            is emitted.  This object must be callable, and must
            accept ``**kwargs``.  If either of these preconditions are
            not met, a ``ValueError`` will be raised.

        :type unique_id: str
        :param unique_id: An optional identifier to associate with the
            registration.  A unique_id can only be used once for
            the entire session registration (unless it is unregistered).
            This can be used to prevent an event handler from being
            registered twice.

        :param unique_id_uses_count: boolean
        :param unique_id_uses_count: Specifies if the event should maintain
            a count when a ``unique_id`` is registered and unregisted. The
            event can only be completely unregistered once every register call
            using the unique id has been matched by an ``unregister`` call.
            If ``unique_id`` is specified, subsequent ``register``
            calls must use the same value for  ``unique_id_uses_count``
            as the ``register`` call that first registered the event.

        :raises ValueError: If the call to ``register`` uses ``unique_id``
            but the value for ``unique_id_uses_count`` differs from the
            ``unique_id_uses_count`` value declared by the very first
            ``register`` call for that ``unique_id``.
        """
        self._events.register(event_name, handler, unique_id,
                              unique_id_uses_count=unique_id_uses_count)

    def unregister(self, event_name, handler=None, unique_id=None,
                   unique_id_uses_count=False):
        """Unregister a handler with an event.

        :type event_name: str
        :param event_name: The name of the event.

        :type handler: callable
        :param handler: The callback to unregister.

        :type unique_id: str
        :param unique_id: A unique identifier identifying the callback
            to unregister.  You can provide either the handler or the
            unique_id, you do not have to provide both.

        :param unique_id_uses_count: boolean
        :param unique_id_uses_count: Specifies if the event should maintain
            a count when a ``unique_id`` is registered and unregisted. The
            event can only be completely unregistered once every ``register``
            call using the ``unique_id`` has been matched by an ``unregister``
            call. If the ``unique_id`` is specified, subsequent
            ``unregister`` calls must use the same value for
            ``unique_id_uses_count`` as the ``register`` call that first
            registered the event.

        :raises ValueError: If the call to ``unregister`` uses ``unique_id``
            but the value for ``unique_id_uses_count`` differs from the
            ``unique_id_uses_count`` value declared by the very first
            ``register`` call for that ``unique_id``.
        """
        self._events.unregister(event_name, handler=handler,
                                unique_id=unique_id,
                                unique_id_uses_count=unique_id_uses_count)

    def emit(self, event_name, **kwargs):
        return self._events.emit(event_name, **kwargs)

    def emit_first_non_none_response(self, event_name, **kwargs):
        responses = self._events.emit(event_name, **kwargs)
        return first_non_none_response(responses)

    def get_component(self, name):
        return self._components.get_component(name)

    def register_component(self, name, component):
        self._components.register_component(name, component)

    def lazy_register_component(self, name, component):
        self._components.lazy_register_component(name, component)

    def create_client(self, service_name, region_name=None, api_version=None,
                      use_ssl=True, verify=None, endpoint_url=None,
                      aws_access_key_id=None, aws_secret_access_key=None,
                      aws_session_token=None, config=None):
        """Create a botocore client.

        :type service_name: string
        :param service_name: The name of the service for which a client will
            be created.  You can use the ``Sesssion.get_available_services()``
            method to get a list of all available service names.

        :type region_name: string
        :param region_name: The name of the region associated with the client.
            A client is associated with a single region.

        :type api_version: string
        :param api_version: The API version to use.  By default, botocore will
            use the latest API version when creating a client.  You only need
            to specify this parameter if you want to use a previous API version
            of the client.

        :type use_ssl: boolean
        :param use_ssl: Whether or not to use SSL.  By default, SSL is used.
            Note that not all services support non-ssl connections.

        :type verify: boolean/string
        :param verify: Whether or not to verify SSL certificates.
            By default SSL certificates are verified.  You can provide the
            following values:

            * False - do not validate SSL certificates.  SSL will still be
              used (unless use_ssl is False), but SSL certificates
              will not be verified.
            * path/to/cert/bundle.pem - A filename of the CA cert bundle to
              uses.  You can specify this argument if you want to use a
              different CA cert bundle than the one used by botocore.

        :type endpoint_url: string
        :param endpoint_url: The complete URL to use for the constructed
            client.  Normally, botocore will automatically construct the
            appropriate URL to use when communicating with a service.  You can
            specify a complete URL (including the "http/https" scheme) to
            override this behavior.  If this value is provided, then
            ``use_ssl`` is ignored.

        :type aws_access_key_id: string
        :param aws_access_key_id: The access key to use when creating
            the client.  This is entirely optional, and if not provided,
            the credentials configured for the session will automatically
            be used.  You only need to provide this argument if you want
            to override the credentials used for this specific client.

        :type aws_secret_access_key: string
        :param aws_secret_access_key: The secret key to use when creating
            the client.  Same semantics as aws_access_key_id above.

        :type aws_session_token: string
        :param aws_session_token: The session token to use when creating
            the client.  Same semantics as aws_access_key_id above.

        :type config: botocore.client.Config
        :param config: Advanced client configuration options. If a value
            is specified in the client config, its value will take precedence
            over environment variables and configuration values, but not over
            a value passed explicitly to the method. If a default config
            object is set on the session, the config object used when creating
            the client will be the result of calling ``merge()`` on the
            default config with the config provided to this call.

        :rtype: botocore.client.BaseClient
        :return: A botocore client instance

        """
        default_client_config = self.get_default_client_config()
        # If a config is provided and a default config is set, then
        # use the config resulting from merging the two.
        if config is not None and default_client_config is not None:
            config = default_client_config.merge(config)
        # If a config was not provided then use the default
        # client config from the session
        elif default_client_config is not None:
            config = default_client_config

        # Figure out the user-provided region based on the various
        # configuration options.
        if region_name is None:
            if config and config.region_name is not None:
                region_name = config.region_name
            else:
                region_name = self.get_config_variable('region')

        # Figure out the verify value base on the various
        # configuration options.
        if verify is None:
            verify = self.get_config_variable('ca_bundle')

        if api_version is None:
            api_version = self.get_config_variable('api_versions').get(
                service_name, None)

        loader = self.get_component('data_loader')
        event_emitter = self.get_component('event_emitter')
        response_parser_factory = self.get_component(
            'response_parser_factory')
        if aws_access_key_id is not None and aws_secret_access_key is not None:
            credentials = botocore.credentials.Credentials(
                access_key=aws_access_key_id,
                secret_key=aws_secret_access_key,
                token=aws_session_token)
        elif self._missing_cred_vars(aws_access_key_id,
                                     aws_secret_access_key):
            raise PartialCredentialsError(
                provider='explicit',
                cred_var=self._missing_cred_vars(aws_access_key_id,
                                                 aws_secret_access_key))
        else:
            credentials = self.get_credentials()
        endpoint_resolver = self.get_component('endpoint_resolver')
        exceptions_factory = self.get_component('exceptions_factory')
        client_creator = botocore.client.ClientCreator(
            loader, endpoint_resolver, self.user_agent(), event_emitter,
            retryhandler, translate, response_parser_factory,
            exceptions_factory)
        client = client_creator.create_client(
            service_name=service_name, region_name=region_name,
            is_secure=use_ssl, endpoint_url=endpoint_url, verify=verify,
            credentials=credentials, scoped_config=self.get_scoped_config(),
            client_config=config, api_version=api_version)
        return client

    def _missing_cred_vars(self, access_key, secret_key):
        if access_key is not None and secret_key is None:
            return 'aws_secret_access_key'
        if secret_key is not None and access_key is None:
            return 'aws_access_key_id'
        return None

    def get_available_partitions(self):
        """Lists the available partitions found on disk

        :rtype: list
        :return: Returns a list of partition names (e.g., ["aws", "aws-cn"])
        """
        resolver = self.get_component('endpoint_resolver')
        return resolver.get_available_partitions()

    def get_available_regions(self, service_name, partition_name='aws',
                              allow_non_regional=False):
        """Lists the region and endpoint names of a particular partition.

        :type service_name: string
        :param service_name: Name of a service to list endpoint for (e.g., s3).
            This parameter accepts a service name (e.g., "elb") or endpoint
            prefix (e.g., "elasticloadbalancing").

        :type partition_name: string
        :param partition_name: Name of the partition to limit endpoints to.
            (e.g., aws for the public AWS endpoints, aws-cn for AWS China
            endpoints, aws-us-gov for AWS GovCloud (US) Endpoints, etc.

        :type allow_non_regional: bool
        :param allow_non_regional: Set to True to include endpoints that are
             not regional endpoints (e.g., s3-external-1,
             fips-us-gov-west-1, etc).
        :return: Returns a list of endpoint names (e.g., ["us-east-1"]).
        """
        resolver = self.get_component('endpoint_resolver')
        results = []
        try:
            service_data = self.get_service_data(service_name)
            endpoint_prefix = service_data['metadata'].get(
                'endpointPrefix', service_name)
            results = resolver.get_available_endpoints(
                endpoint_prefix, partition_name, allow_non_regional)
        except UnknownServiceError:
            pass
        return results


class ComponentLocator(object):
    """Service locator for session components."""
    def __init__(self):
        self._components = {}
        self._deferred = {}

    def get_component(self, name):
        if name in self._deferred:
            factory = self._deferred[name]
            self._components[name] = factory()
            # Only delete the component from the deferred dict after
            # successfully creating the object from the factory as well as
            # injecting the instantiated value into the _components dict.
            del self._deferred[name]
        try:
            return self._components[name]
        except KeyError:
            raise ValueError("Unknown component: %s" % name)

    def register_component(self, name, component):
        self._components[name] = component
        try:
            del self._deferred[name]
        except KeyError:
            pass

    def lazy_register_component(self, name, no_arg_factory):
        self._deferred[name] = no_arg_factory
        try:
            del self._components[name]
        except KeyError:
            pass


def get_session(env_vars=None):
    """
    Return a new session object.
    """
    return Session(env_vars)
