# J2GPP - Jinja2-based General Purpose Preprocessor

`j2gpp` is a command-line tool for rendering templates using the Jinja2 syntax. It's intended purpose is to serve as a preprocessor for any programming or markup language with a unified syntax and flow across languages.

- [J2GPP - Jinja2-based General Purpose Preprocessor](#j2gpp---jinja2-based-general-purpose-preprocessor)
  - [Installation](#installation)
  - [Basic usage](#basic-usage)
    - [J2GPP CLI quick start](#j2gpp-cli-quick-start)
    - [J2GPP API quick start](#j2gpp-api-quick-start)
  - [Additional template features](#additional-template-features)
    - [Context variables](#context-variables)
    - [Additional built-in filters](#additional-built-in-filters)
    - [Additional built-in tests](#additional-built-in-tests)
    - [Additional built-in globals](#additional-built-in-globals)
  - [CLI reference](#cli-reference)
    - [Command line arguments](#command-line-arguments)
    - [User configuration file](#user-configuration-file)
    - [Exception handling and errors](#exception-handling-and-errors)
  - [API reference](#api-reference)
    - [Rendering methods](#rendering-methods)
    - [Variable management](#variable-management)
    - [Configuration methods](#configuration-methods)
    - [Advanced configuration](#advanced-configuration)
    - [Exception handling mode](#exception-handling-mode)
    - [Configuration inspection](#configuration-inspection)
    - [Template validation](#template-validation)
    - [Template discovery \& analysis](#template-discovery--analysis)
    - [Extension management](#extension-management)
    - [Result objects](#result-objects)
  - [Advanced usage](#advanced-usage)
    - [Specify output directory](#specify-output-directory)
    - [Specifying output file](#specifying-output-file)
    - [Template search directories](#template-search-directories)
    - [Include search directory](#include-search-directory)
    - [Passing global variables in command line](#passing-global-variables-in-command-line)
    - [Loading global variables from files](#loading-global-variables-from-files)
    - [Loading global variables from environment](#loading-global-variables-from-environment)
    - [Loading custom Jinja2 filters](#loading-custom-jinja2-filters)
    - [Loading custom Jinja2 tests](#loading-custom-jinja2-tests)
    - [Loading custom Jinja2 globals](#loading-custom-jinja2-globals)
    - [Processing variables before rendering](#processing-variables-before-rendering)
    - [Process directories](#process-directories)
  - [Supported formats for variables](#supported-formats-for-variables)
    - [CLI define](#cli-define)
    - [API define](#api-define)
    - [YAML](#yaml)
    - [JSON](#json)
    - [HJSON](#hjson)
    - [XML](#xml)
    - [TOML](#toml)
    - [INI/CFG](#inicfg)
    - [ENV](#env)
    - [CSV/TSV](#csvtsv)
    - [PLIST](#plist)
  - [Scripting in J2GPP templates](#scripting-in-j2gpp-templates)
    - [Conditional and do extension](#conditional-and-do-extension)
    - [Conditional and filters](#conditional-and-filters)
    - [Throw errors and warnings from template](#throw-errors-and-warnings-from-template)
    - [Writing files](#writing-files)
      - [Write example](#write-example)
      - [Append example](#append-example)
      - [Path argument](#path-argument)
      - [Writing to both files](#writing-to-both-files)
      - [Skipping the parent file](#skipping-the-parent-file)
  - [Extensions](#extensions)
    - [Using extensions](#using-extensions)
    - [Creating extensions](#creating-extensions)


## Installation

With `Python >= 3.7`, Install from Pypi :

``` shell
pip install j2gpp
```

## Basic usage

### J2GPP CLI quick start

`j2gpp` requires at least one source be provided. The source paths can be files or directories, relative or absolute, and can use UNIX-style patterns such as wildcards. Template file names must end with the `.j2` extension which will be stripped at render.

For more information about the Jinja2 syntax, see the documentation at [jinja.palletsprojects.com](https://jinja.palletsprojects.com/).

For example, suppose we have a templatized source file `foo.c.j2` :

``` c
#include <stdio.h>

{% set message = "Hello, world!" %}

int main() {
  printf("{{message}}");
  return 0;
}
```

We can render the template using `j2gpp` :

``` shell
j2gpp ./foo.c.j2
```

The output is written to `foo.c` next to the source template :

``` c
#include <stdio.h>

int main() {
  printf("Hello, world!");
  return 0;
}
```

### J2GPP API quick start

J2GPP can also be called from another Python script using the API view. To perform the same action as the example above, the following Python script can be used.

``` python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.render_file("./foo.c.j2")
```

Options that can be passed to the CLI can also be passed to the instance of the class `J2GPP` using methods that are chainable for fluent configuration.

The rendering methods `J2GPP.render_file()` and `J2GPP.render_directory()` return a `FileRenderResult` and `RenderResult` object containing information about the render, such as success, error message, and output path. The API can also render strings using `J2GPP.render_string()`.

``` python
from j2gpp import J2GPP

# Complex configuration example
result = (J2GPP()
  .add_include_directory("./includes/")
  .load_variables_from_file("./config.yml")
  .define_variable("debug", True)
  .set_option("trim_whitespace", True)
  .load_filters_from_file("./custom_filters.py")
  .render_file("./template.j2", output_path="./output.txt")
)

if result.success:
  print(f"Template rendered successfully to: {result.output_path}")
else:
  print(f"Error: {result.error_message}")
```

## Additional template features

J2GPP adds a bunch of features to Jinja2 to be used inside templates.

### Context variables

Useful context variables are added before any other variable is loaded. Some are global for all templates rendered, and some are template-specific.

| Variable                | Scope    | Description                                   |
| ----------------------- | -------- | --------------------------------------------- |
| `__python_version__`    | Global   | Python version                                |
| `__jinja2_version__`    | Global   | Jinja2 version                                |
| `__j2gpp_version__`     | Global   | J2GPP version                                 |
| `__user__`              | Global   | Name of the current user                      |
| `__pid__`               | Global   | Process ID of the current process             |
| `__ppid__`              | Global   | Process ID of the parent process              |
| `__date__`              | Global   | Date in the format `DD-MM-YYYY`               |
| `__date_inv__`          | Global   | Date in the format `YYYY-MM-DD`               |
| `__time__`              | Global   | Time in the format `hh:mm:ss`                 |
| `__datetime__`          | Global   | Timestamp in the format `YYYY-MM-DD hh:mm:ss` |
| `__working_directory__` | Global   | Working directory                             |
| `__output_directory__`  | Global   | Output directory                              |
| `__source_path__`       | Template | Path of the source template file              |
| `__output_path__`       | Template | Path where the template is rendered           |

### Additional built-in filters

In addition to the [Jinja2 built-in filters](https://jinja.palletsprojects.com/en/latest/templates/#builtin-filters), J2GPP also defines many useful filter functions.

|               |              |              |                     |                                         |
| ------------- | ------------ | ------------ | ------------------- | --------------------------------------- |
| `warning()`   | `sha224()`   | `rjust()`    | `reindent()`        | `product()`                             |
| `error()`     | `sha256()`   | `center()`   | `autoindent()`      | `permutations()`                        |
| `list_add()`  | `sha384()`   | `strip()`    | `align()`           | `combinations()`                        |
| `list_sub()`  | `sha512()`   | `lstrip()`   | `restructure()`     | `combinations_with_replacement()`       |
| `list_mult()` | `sha3_224()` | `rstrip()`   | `el_of_max_attr()`  | `permutations_range()`                  |
| `list_div()`  | `sha3_256()` | `title()`    | `el_of_min_attr()`  | `combinations_range()`                  |
| `list_mod()`  | `sha3_384()` | `swapcase()` | `key_of_max_attr()` | `combinations_with_replacement_range()` |
| `list_rem()`  | `sha3_512()` | `camel()`    | `key_of_min_attr()` | `write()`                               |
| `list_exp()`  | `blake2b()`  | `pascal()`   | `accumulate()`      | `append()`                              |
| `md5()`       | `blake2s()`  | `snake()`    | `count()`           | `type()`                                |
| `sha1()`      | `ljust()`    | `kebab()`    | `pairwise()`        |                                         |

All functions from the Python libraries [`math`](https://docs.python.org/3/library/math.html) and [`statistics`](https://docs.python.org/3/library/statistics.html) are made available as filters. This includes useful functions such as `sqrt`, `pow`, `log`, `sin`, `cos`, `floor`, `ceil`, `mean`, `median`, `variance`, `stdev`, ...

The `warning` and `error` filters can be used to throw warnings and errors from the template that will be displayed in the J2GPP logs. The filter is applied to a block, replaces the block with nothing and throws the warning or error with the content of the block as comment. The filter works with conditional blocks if the version of Jinja2 installed supports the `@render_time_only` decorator.

An operation can be applied to all elements of a list with the filters `list_add`, `list_sub`, `list_mult`, `list_div`, `list_mod`, `list_rem` and `list_exp` respectively for the Python operators `+`, `-`, `*`, `/`, `%`, `//` and `**`.

Representation of integers in different bases are available with the filters `bin`, `ter`, `oct`, `doz`, `hex` and their aliases `binary`, `ternary`, `octal`, `duodecimal`, and `hexadecimal` respectively. If the integer is negative, the representation will be that of the absolute value. An optional length can be provided and the output will be padded with zeros. No prefix such as `0b` or `0x` are added. For duodecimal, arguments `ten='a'` and `eleven='b'` can be used to overwrite the characters for 10 and 11.

All hash algorithms provided by the `hashlib` library are provided as filters. The input value or string will be sanitized into JSON and encoded into UTF-8, then hashed and digested in hexadecimal. The number of symbols returned depends on the hash algorithm. The algorithms supported are : `md5`, `sha1`, `sha224`, `sha256`, `sha384`, `sha512`, `sha3_224`, `sha3_256`, `sha3_384`, `sha3_512`, `blake2b`, `blake2s`. The hash functions `adler32` and `crc32` from `zlib` are also provided.

Text alignment can be controlled with the Python functions `ljust`, `rjust` and `center`.

Stripping whitespaces or other characters from the start or end of strings can be done with the Python functions `strip`, `lstrip`, and `rstrip`.

Case and formatting can be controlled with the Python functions `title` and `swapcase`, or the added functions `camel`, `pascal`, `snake` and `kebab`. `camel` and `pascal` will remove the underscores and hyphens by default but leave the dots ; that behavior can be changed by providing the filter arguments `remove_underscore`, `remove_hyphen` and `remove_dot` as `True` or `False`. `snake` and `kebab` will group capitalized letters and preserve capitalization by default ; that behavior can be changed by providing the filter arguments `preserve_caps` and `group_caps` as `True` or `False`.

Paragraph formatting is facilitated by multiple filters that should be used on a `filter` block. `reindent` removes pre-existing indentation and sets new one. `autoindent`/`auto_indent` removes pre-existing indentation and sets new indent by incrementing and decrementing the depth based on start and end delimiters of blocks provided by the `starts` and `ends` lists of strings provided by argument (curly braces by default). `align` aligns every line of the paragraph by columns, left before `§`, right before `§§`.

Restructuring of large blocks is facilitated by the filter `restructure` that should be used on a `filter` block. The block will be parsed for structure tags, which is an addition to the Jinja2 syntax and uses the `{§...§}` syntax. The only structure tag currently implemented is `spacing`. It takes a single parameter representing a number of line. When the restructuring filter processes the tag, it will strip all line breaks before and after the tag and replace them with the number of line breaks specified. This is useful to add spacing to the template for readability while keeping the spacing of the generate file clean.

When using a list of dictionaries with a key in common, you can get the list element with the minimum or maximum value of that attribute using the filters `el_of_max_attr` or `el_of_min_attr`.

When using two-level dictionaries, the key corresponding to the minimum or maximum with regards to a specified attribute using the filters `key_of_max_attr` or `key_of_min_attr`.

The sum of elements in a list or other iterable object can be computed using the function `accumulate`.

You can count the number of occurrences of a value in a list using the `count` filter.

To perform combinatorics on a list, the following functions from the Python library [`itertools`](https://docs.python.org/3/library/itertools.html) are provided : `pairwise`, `product` (cartesian product), `permutations`, `combinations`, and `combinations_with_replacement`. They all work on lists and may take an additional parameter for the length of the permutations or combinations. In addition, the following functions are used to generate permutations and combinations of a range : `permutations_range`, `combinations_range`, and `combinations_with_replacement_range`. They all work in lists and take two additional parameters : the start and stop index of the range of lengths.

The `write` and `append` filters can be used to export the content of a filter to another file whose path is provided as argument to the filter. The path can be absolute or relative to the output rendered base template. By default, the content of the filter is not written to the base rendered template ; this behavior can be changed by providing the filter argument `preserve` as `True`. The source template can also be prevented from resulting in a generated file by providing the filter argument `write_source` as `False`, and only the content of `write` and `append` blocks will generate files.

To get the name of the type of a variable as a string, use the `type` filter.

### Additional built-in tests

In addition to the [Jinja2 built-in tests](https://jinja.palletsprojects.com/en/latest/templates/#builtin-tests), J2GPP also defines many useful test functions.

|             |                |               |                       |                    |
| ----------- | -------------- | ------------- | --------------------- | ------------------ |
| `decimal()` | `identifier()` | `printable()` | `defined_and_true()`  | `defined_and_le()` |
| `digit()`   | `space()`      | `empty()`     | `defined_and_false()` | `defined_and_gt()` |
| `numeric()` | `lower()`      | `singleton()` | `defined_and_eq()`    | `defined_and_ge()` |
| `alpha()`   | `upper()`      | `list()`      | `defined_and_ne()`    |                    |
| `alnum()`   | `title()`      | `dict()`      | `defined_and_lt()`    |                    |

The Python test functions `str.isdecimal()`, `str.isdigit()`, `str.isnumeric()`, `str.isalpha()`, `str.isalnum()`, `str.isidentifier()`, `str.isspace()`, `str.islower()`, `str.isupper()`, `str.istitle()`, and `str.isprintable()` are made available as tests `decimal`, `digit`, `numeric`, `alpha`, `alnum`, `identifier`, `space`, `lower`, `upper`, `title`, and `printable`. They automatically cast the input value to a string.

For lists, the tests `empty` and `singleton` returns whether the list is empty or contains a single item respectively.

Jinja2 provides tests for type testing integers, floats, and strings. J2GPP adds type testing for lists and dictionaries with the tests `list` and `dict` respectively.

Jinja2 provides the `defined` test. To facilitate testing if a variable is defined and an additional condition on its value, J2GPP adds the tests `defined_and_true`, `defined_and_false`, `defined_and_eq` (equal, `==`), `defined_and_ne` (not equal, `!=`), `defined_and_lt` (less than, `<`), `defined_and_le` (less than or equal to, `<=`), `defined_and_gt` (greater than, `>`), and `defined_and_ge` (greater than or equal to, `>=`).

### Additional built-in globals

The global [context variables](#context-variables) are defined inside J2GPP as global variables.

Some useful mathematical constants are provided: `nan`, `inf`, `pi`, `tau`, `e`, `phi`, `sqrt2`, `sqrt3`, `cbrt2`, `cbrt3`, and `ln2`.

The `Accumulator` class represents a variable that can be mutated with many operator that return the current value of the accumulator. The mutator methods are `reset()`, `clear()`, `set(value)`, `get()`, `increment()`, `decrement()`, `add(value)`, `subtract(value)`, `multiply(value)`, `divide(value)`, `modulo(value)`, `floor_divide(value)`, `square()`, `cube()`, `square_root()`, `cube_root()`, `exponentiate(value=2)`, `logarithm(value=2)`. There are additional `*_after()` methods that return the value before the operation (it performs the operation after the return, hence the name). The accumulator can be instantiated with an initial value. The `clear()` method sets the value of the accumulator to zero, while the `reset()` method sets it back to the initial value (or zero if none provided).

The `Cycler` class can be used to cycle through a list of items. The method `next()` returns the next item in the cycle, and `reset()` restarts the cycle from the beginning. The cycler is instantiated with a list of items.

Multiple standard and third party libraries are provided: `math`, `statistics`, `itertools`, `random`, `secrets`, `ospath` (os.path), `datetime`, `calendar`, `pprint`, `colorsys`, `colour` (https://pypi.org/project/colour/).

## CLI reference

### Command line arguments

The following arguments can be added to the command for additional features. The details of each options are explained in the sections below.

| Argument                   | Description                                                             |
| -------------------------- | ----------------------------------------------------------------------- |
| `-O/--outdir`              | Output directory for all rendered templates                             |
| `-o/--output`              | Output file for single template                                         |
| `-I/--incdir`              | Include search directory for include and import statements              |
| `-D/--define`              | Inline global variables for all templates                               |
| `-V/--varfile`             | Global variables files for all templates                                |
| `--envvar`                 | Loads environment variables as global variables                         |
| `--filters`                | Load extra Jinja2 filters from a Python file                            |
| `--tests`                  | Load extra Jinja2 tests from a Python file                              |
| `--globals`                | Load extra Jinja2 globals from a Python file                            |
| `--file-vars-adapter`      | Load a Python function to process variables after loading from a file   |
| `--global-vars-adapter`    | Load a Python function to process all variables before rendering        |
| `--overwrite-outdir`       | Overwrite output directory                                              |
| `--warn-overwrite`         | Warn when overwriting files                                             |
| `--no-overwrite`           | Prevent overwriting files                                               |
| `--no-strict-undefined`    | Disable error with undefined variable in template                       |
| `--no-check-identifier`    | Disable warning when attributes are not valid identifiers               |
| `--fix-identifiers`        | Replace invalid characters from identifiers with underscore             |
| `--chdir-src`              | Change working directory to source before rendering instead of output   |
| `--no-chdir`               | Disable changing working directory to output directory before rendering |
| `--trim-whitespace`        | Trim trailing whitespace in generated files                             |
| `--csv-delimiter`          | CSV delimiter (default: '`,`')                                          |
| `--csv-escape-char`        | CSV escape character (default: None)                                    |
| `--csv-dont-strip`         | Disable stripping whitespace of CSV values                              |
| `--xml-convert-attributes` | Convert XML attributes to normal element without the '@' prefix         |
| `--xml-remove-namespaces`  | Remove XML namespace prefixes from tags                                 |
| `--render-non-template`    | Process also source files that are not recognized as templates          |
| `--copy-non-template`      | Copy source files that are not templates to output directory            |
| `--force-glob`             | Glob UNIX-like patterns in path even when quoted                        |
| `--debug-vars`             | Display available variables at the top of rendered templates            |
| `--stdout-errors`          | Display errors on stdout instead of stderr                              |
| `--perf`                   | Measure the execution time for performance testing                      |
| `--version`                | Print J2GPP version and quits                                           |
| `--license`                | Print J2GPP license and quits                                           |
| `--no-auto-extensions`     | Disable automatic loading of installed extensions                       |
| `--extension`              | Explicitly load an extension by name                                    |
| `--disable-extension`      | Disable a specific extension from being loaded                          |
| `-c/--config`              | Path to user configuration file                                         |
| `--no-config`              | Disable loading user configuration file                                 |

### User configuration file

J2GPP supports a user configuration file to set default values for CLI options. The configuration file is searched in the following locations (first found wins):

1. `~/.config/j2gpp/j2gpp.json` (XDG standard)
2. `~/.j2gpp.json` (simple dotfile)

Options are resolved with the following priority (highest to lowest):

1. **CLI arguments** - Always take precedence
2. **User configuration file** - Second priority
3. **Hardcoded defaults** - Fallback values

You can specify an explicit configuration file path with the `-c/--config` argument, or disable configuration file loading entirely with `--no-config`.

**Example configuration file:**

```json
{
  "options": {
    "trim_whitespace": true,
    "csv_delimiter": ";",
    "warn_overwrite": true
  },
  "include_dirs": [
    "~/templates/includes",
    "/shared/jinja2/macros"
  ],
  "no_auto_extensions": false,
  "disable_extensions": ["problematic_extension"],
  "extensions": {
    "my_extension": {
      "api_url": "https://api.example.com",
      "timeout": 30
    }
  }
}
```

**Supported options:**

The `options` object supports the following keys, corresponding to CLI arguments:

| Option Key              | CLI Equivalent           |
| ----------------------- | ------------------------ |
| `no_strict_undefined`   | `--no-strict-undefined`  |
| `debug_vars`            | `--debug-vars`           |
| `no_check_identifier`   | `--no-check-identifier`  |
| `fix_identifiers`       | `--fix-identifiers`      |
| `chdir_src`             | `--chdir-src`            |
| `no_chdir`              | `--no-chdir`             |
| `trim_whitespace`       | `--trim-whitespace`      |
| `csv_delimiter`         | `--csv-delimiter`        |
| `csv_escape_char`       | `--csv-escape-char`      |
| `csv_dont_strip`        | `--csv-dont-strip`       |
| `xml_convert_attributes`| `--xml-convert-attributes`|
| `xml_remove_namespaces` | `--xml-remove-namespaces`|
| `render_non_template`   | `--render-non-template`  |
| `copy_non_template`     | `--copy-non-template`    |
| `warn_overwrite`        | `--warn-overwrite`       |
| `no_overwrite`          | `--no-overwrite`         |
| `overwrite_outdir`      | `--overwrite-outdir`     |

**Extension configuration:**

Extensions can receive configuration from the `extensions` section. Each key under `extensions` corresponds to an extension name, and its value is a dictionary of configuration options that will be passed to the extension's `on_configure` callback.

### Exception handling and errors

When exceptions are encountered during the execution of J2GPP - either in the template, the variables file loaders, or the variable adapter functions - it will display the errors gracefully including, if possible, an error message explaining the issue, and the traceback information to locate the issue in the template.

The errors and warnings can occur in different steps of the execution of J2GPP. They are collected and displayed again at the end of the execution alongside an error and warning counter. Warnings are printed on the standard output `stdout` of the shell, while errors are printed on the error stream `stderr`. If errors occurred, the J2GPP process will also return an error status.

## API reference

The `J2GPP` class provides a comprehensive API for programmatic template rendering. All configuration methods are chainable, allowing for fluent configuration. The details of each methods are explained in the sections below.

### Rendering methods

``` python
J2GPP.render_file(source_path, output_path=None, output_dir=None, variables=None)
```

Renders a single template file. Returns `FileRenderResult` with information about the operation.

``` python
J2GPP.render_directory(source_dir, output_dir=None, variables=None)
```

Renders all templates in a directory tree. Returns `RenderResult` containing information about all processed files.

``` python
J2GPP.render_string(template_string, variables=None)
```

Renders a template string directly. Returns the rendered string.

``` python
J2GPP.render_file_to_string(source_path, variables=None)
```

Renders a template file and returns the result as a string without writing to disk. Useful for testing, preview, or programmatic string generation from file-based templates.

### Variable management

``` python
J2GPP.define_variable("key", "value")        # Define single variable with auto-casting of strings and support for dot noration
J2GPP.define_variables({"key": "value"})     # Add dictionary of variables
J2GPP.load_variables_from_file("./vars.yml") # Load from variables file
J2GPP.load_variables_from_env()              # Load environment variables
J2GPP.load_variables_from_env("ENV")         # Load environment variables under "ENV" prefix
J2GPP.clear_variables()                      # Clear all variables
J2GPP.remove_variable("key")                 # Remove variable (supports dot notation)
```

### Configuration methods

``` python
J2GPP.set_output_directory("./output/")                 # Set default output directory for all rendering
J2GPP.add_include_directory("./includes/")              # Add include directory for Jinja2 imports
J2GPP.set_include_directories(["./inc1/", "./inc2/"])   # Set multiple include directories
J2GPP.add_filter("my_filter", my_filter_function)       # Add single filter function directly
J2GPP.add_test("my_test", my_test_function)             # Add single test function directly
J2GPP.add_global("my_global", my_global_function)       # Add single global function directly
J2GPP.add_filters({"filter1": func1, "filter2": func2}) # Add multiple filter functions
J2GPP.add_tests({"test1": func1, "test2": func2})       # Add multiple test functions
J2GPP.add_globals({"global1": var1, "global2": class1}) # Add multiple global functions
J2GPP.load_filters_from_file("./filters.py")            # Load custom Jinja2 filters from Python file
J2GPP.load_tests_from_file("./tests.py")                # Load custom Jinja2 tests from Python file
J2GPP.load_globals_from_file("./globals.py")            # Load custom Jinja2 globals from Python file
J2GPP.set_option("trim_whitespace", True)               # Set rendering options
J2GPP.add_search_directory("./templates/")              # Add template search directory
J2GPP.set_search_directories(["./t1/", "./t2/"])        # Set multiple search directories
J2GPP.remove_search_directory("./templates/")           # Remove a search directory
J2GPP.clear_search_directories()                        # Remove all search directories
```

### Advanced configuration

``` python
J2GPP.set_file_vars_adapter(my_function)   # Process variables after loading from files
J2GPP.set_global_vars_adapter(my_function) # Process all variables before rendering
J2GPP.set_raise_exceptions(True)           # Raise exceptions on errors instead of returning error results
J2GPP.jinja2_env                           # Access Jinja2 Environment instance for advanced operations
```

The `jinja_env` property provides direct access to the underlying `jinja2.Environment` instance. This allows power users to perform advanced operations on the Jinja2 environment, such as using Jinja2 overlays or adding native extensions. The environment is created lazily and automatically rebuilt when configuration changes.

### Exception handling mode

By default, the J2GPP API returns error information in result objects without raising exceptions. This allows for graceful handling of errors in batch processing scenarios.

To enable exception mode, where errors raise a `J2GPPRenderError` exception:

``` python
from j2gpp import J2GPP, J2GPPRenderError

# Enable via constructor
j2gpp = J2GPP(raise_exceptions=True)

# Or enable via setter (chainable)
j2gpp = J2GPP().set_raise_exceptions(True)

# Handle exceptions
try:
  j2gpp.render_file("template.j2")
except J2GPPRenderError as e:
  print(f"Render failed: {e.message}")
  # e.result contains the FileRenderResult or RenderResult object
```

### Configuration inspection

``` python
# Get current configuration state
J2GPP.get_variables()           # Return copy of current variables dictionary
J2GPP.get_options()             # Return copy of current options dictionary
J2GPP.get_filters()             # Return copy of available filters dictionary
J2GPP.get_tests()               # Return copy of available tests dictionary
J2GPP.get_globals()             # Return copy of available globals dictionary
J2GPP.get_include_directories() # Return copy of include directories list
J2GPP.get_output_directory()    # Return current output directory (None if not set)

# Check configuration state
J2GPP.has_variable("key")                  # Check if variable exists (supports dot notation)
J2GPP.has_filter("my_filter")              # Check if filter is loaded
J2GPP.has_test("my_test")                  # Check if test is loaded
J2GPP.has_global("my_global")              # Check if global is loaded
J2GPP.has_include_directory("./includes/") # Check if include directory is configured
J2GPP.get_search_directories()             # Return copy of search directories list
J2GPP.has_search_directory("./templates/") # Check if search directory is configured
```

### Template validation

``` python
# Validate template syntax without rendering
J2GPP.validate_template_string(template_str)    # Validate template string syntax
J2GPP.validate_template_file("template.j2")     # Validate template file syntax
J2GPP.validate_directory("./templates/")        # Validate all templates in directory (recursive by default)
J2GPP.validate_directory("./templates/", False) # Validate templates in directory (non-recursive)
```

### Template discovery & analysis

``` python
# Find template files
J2GPP.find_templates("./templates/")         # Find all .j2 files (recursive by default)
J2GPP.find_templates("./templates/", False)  # Find .j2 files (non-recursive)

# Analyze template dependencies and structure
J2GPP.find_template_dependencies("main.j2") # Find included/imported templates
J2GPP.analyze_template_variables("main.j2") # Extract variables used in template
```

### Extension management

``` python
# Extension loading
J2GPP.load_extensions()              # Discover and load all installed extensions
J2GPP.load_extension("ext_name")     # Load a specific extension by name
J2GPP.disable_extension("ext_name")  # Disable an extension from auto-loading

# Extension inspection
J2GPP.get_loaded_extensions()        # Get metadata for all loaded extensions
J2GPP.has_extension("ext_name")      # Check if an extension is loaded
```

### Result objects

``` python
FileRenderResult               # Returned by J2GPP.render_file()
FileRenderResult.success       # Boolean indicating success
FileRenderResult.output_path   # Path where file was written
FileRenderResult.error_message # Error description if failed
FileRenderResult.source_path   # Original template path

RenderResult               # Returned by J2GPP.render_directory()
RenderResult.success       # Boolean indicating overall success
RenderResult.file_results  # List of `FileRenderResult` objects for each processed file
RenderResult.error_message # Error description if failed

ValidationResult               # Returned by template validation methods
ValidationResult.is_valid      # Boolean indicating if template syntax is valid
ValidationResult.template_path # Path to template file (None for string validation)
ValidationResult.error_message # Error description if validation failed
ValidationResult.error_line    # Line number where error occurred (if available)
ValidationResult.error_column  # Column number where error occurred (if available)

DirectoryValidationResult                   # Returned by J2GPP.validate_directory()
DirectoryValidationResult.is_valid          # Boolean indicating if all templates are valid
DirectoryValidationResult.directory_path    # Path to validated directory
DirectoryValidationResult.total_templates   # Total number of templates found
DirectoryValidationResult.valid_templates   # Number of valid templates
DirectoryValidationResult.invalid_templates # Number of invalid templates
DirectoryValidationResult.template_results  # List of `ValidationResult` objects for each template
```

## Advanced usage

### Specify output directory

By default the rendered files are saved next to the source templates. You can provide an output directory with the `-O/--outdir` argument. The output directory path can be relative or absolute. If the directory doesn't exist, it will be created.

For instance the following command or script will write the rendered file to `./bar/foo.c`.

``` shell
j2gpp ./foo.c.j2 --outdir ./bar/
```

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.render_file("./foo.c.j2", output_dir="./bar/")
```

In API mode, you can also set the default output directory for all rendering operations using the `set_output_directory()` method. This new default directory is overwritten by the `output_path` and `output_dir` parameters of the render methods.

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.set_output_directory("./bar/")
j2gpp.render_file("./foo.c.j2")
```

### Specifying output file

By default the rendered files are saved next to the source templates. If a single source template is provided, you can specify the output file directory and name with the `-o/--output` argument. The output file path can be relative or absolute. If the directory doesn't exist, it will be created.

For instance the following command or script will write the rendered file to `./bar.c`.

``` shell
j2gpp ./foo.c.j2 --output ./bar.c
```

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.render_file("./foo.c.j2", output_path="./bar.c")
```

### Include search directory

The `include` and `import` Jinja2 statements require specifying the directory in which the renderer will search. That is provided using the `-I/--incdir` argument.

For instance, with the following command or script, the files in the directory `./includes/` will be available to `include` and `import` statements when rendering the template `foo.c.j2`.

``` shell
j2gpp ./foo.c.j2 --incdir ./includes/
```

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.add_include_directory("./includes/").render_file("./foo.c.j2")
```

### Template search directories

In API mode, you can register template search directories to avoid specifying the full path to every template file. When a source path is provided to a render method, J2GPP resolves it in the following order:

1. If the path exists relative to the current working directory, use it directly
2. Otherwise, search through registered search directories in the order they were added
3. If the file is not found in any search directory, the path is passed through as-is (producing the normal "file not found" error)

```python
from j2gpp import J2GPP

j2gpp = J2GPP()
j2gpp.add_search_directory("./project/templates/")
j2gpp.add_search_directory("./shared/templates/")

# Render by name — resolved through search directories
j2gpp.render_file("header.h.j2")

# Render to string by name
result = j2gpp.render_file_to_string("config.yml.j2")

# Subdirectory paths are also supported
j2gpp.render_file("components/widget.html.j2")
```

Search directories also apply to `validate_template_file()`, `find_template_dependencies()`, and `analyze_template_variables()`.

### Passing global variables in command line

You can pass global variables to all templates rendered using the `-D/--define` argument with a list of variables in the format `name=value`. Values are parsed to cast to the correct Python type as explained [later](#command-line-define). Dictionary attributes to any depth can be assigned using dots "`.`" to separate the keys. Global variables defined in the command line overwrite the global variables set by loading files.

For instance, with the following command or script, the variable `bar` will have the value `42` when rendering the template `foo.c.j2`.

``` shell
j2gpp ./foo.c.j2 --define bar=42
```

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.define_variable("bar", 42).render_file("./foo.c.j2")
```

### Loading global variables from files

You can load global variables from files using the `-V/--varfile` argument with a list of files. The file paths can be relative or absolute, and can use UNIX-style patterns such as wildcards. Variables file types supported right now are YAML, JSON, HJSON, XML, TOML, INI/CFG, ENV, CSV, TSV and PLIST. Global variables loaded from files are overwritten by variables defined in the command line.

For instance, with the following command or script, the variable `bar` will have the value `42` when rendering the template `foo.c.j2`.

``` shell
j2gpp ./foo.c.j2 --varfile ./qux.yml
```

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.load_variables_from_file("./qux.yml").render_file("./foo.c.j2")
```

With the variables file `qux.yml` :

``` yml
bar: 42
```

### Loading global variables from environment

You can import the environment variables of the shell as global variables using the `--envvar` argument. The name of the variables will be that of the environment variable and the value will be cast automatically to the proper Python/Jinja2 type.

For instance, with the following command or script, the variable `BAR` will have the value `42` when rendering the template `foo.c.j2`.

``` shell
export BAR=42
j2gpp ./foo.c.j2 --envvar
```

```python
import os
from j2gpp import J2GPP
os.environ["BAR"] = "42"
j2gpp = J2GPP()
j2gpp.load_variables_from_env().render_file("./foo.c.j2")
```

If a string is provided after the `--envvar` argument, the environment variables will be stored in an object of the name provided instead of at the root.

For instance, with the following command or script, the variable `ENV.BAR` will have the value `42` when rendering the template `foo.c.j2`.

``` shell
export BAR=42
j2gpp ./foo.c.j2 --envvar ENV
```

```python
import os
from j2gpp import J2GPP
os.environ["BAR"] = "42"
j2gpp = J2GPP()
j2gpp.load_variables_from_env("ENV").render_file("./foo.c.j2")
```

### Loading custom Jinja2 filters

You can import custom Jinja2 filters by providing Python files with the `--filters` argument. All functions defined in the python files will be available as Jinja2 filters in the templates.

For instance, with the following command or script and python file, the filter `right_justify` will be available when rendering the template `foo.c.j2`.

``` shell
j2gpp ./foo.c.j2 --filters ./bar.py
```

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.load_filters_from_file("./bar.py").render_file("./foo.c.j2")
```

Or add filter functions directly:

```python
from j2gpp import J2GPP

def right_justify(s, length=0):
  return s.rjust(length)

j2gpp = J2GPP()
j2gpp.add_filter("right_justify", right_justify).render_file("./foo.c.j2")
```

With the filter script `bar.py` :

``` python
def right_justify(s, length=0):
  return s.rjust(length)
```

### Loading custom Jinja2 tests

You can import custom Jinja2 tests by providing Python files with the `--tests` argument. All functions defined in the python files will be available as Jinja2 tests in the templates.

For instance, with the following command or script and python file, the test `prime` will be available when rendering the template `foo.c.j2`.

``` shell
j2gpp ./foo.c.j2 --tests ./bar.py
```

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.load_tests_from_file("./bar.py").render_file("./foo.c.j2")
```

Or add test functions directly:

```python
import math
from j2gpp import J2GPP

def prime(x):
  if x<=1: return False
  for i in range(2,int(math.sqrt(x))+1):
    if (x%i) == 0:
      return False
  return True

j2gpp = J2GPP()
j2gpp.add_test("prime", prime).render_file("./foo.c.j2")
```

With the test script `bar.py` :

``` python
import math
def prime(x):
  if x<=1: return False
  for i in range(2,int(math.sqrt(x))+1):
    if (x%i) == 0:
      return False
  return True
```

### Loading custom Jinja2 globals

You can import custom Jinja2 global objects (variables, functions, classes) by providing Python files with the `--globals` argument. All objects defined in the python files will be available as Jinja2 globals in the templates.

For instance, with the following command or script and python file, the global `prime` will be available when rendering the template `foo.c.j2`.

``` shell
j2gpp ./foo.c.j2 --globals ./bar.py
```

```python
from j2gpp import J2GPP
j2gpp = J2GPP()
j2gpp.load_globals_from_file("./bar.py").render_file("./foo.c.j2")
```

Or add global functions directly:

```python
from j2gpp import J2GPP

class AddAccumulator:
  def __init__(self):
    self.accumulation = 0
  def add_accumulate(self, value):
    self.accumulation += value
    return self.accumulation

j2gpp = J2GPP()
j2gpp.add_global("AddAccumulator", AddAccumulator).render_file("./foo.c.j2")
```

With the global script `bar.py` :

``` python
class AddAccumulator:
  def __init__(self):
    self.accumulation = 0
  def add_accumulate(self, value):
    self.accumulation += value
    return self.accumulation
```

### Processing variables before rendering

You can perform transformations on the dictionary storing the variables before rendering templates by providing a Python function with the argument `--global-vars-adapter`. It takes two arguments, the first is the path to the Python script, and the second is the name of the function to use. The function must take in a single argument, the variables dictionary, and modify it as a reference (not return the modified dictionary). You can also provide an adapter function to be run on the variables loaded from each file before they are written to the global variables object by providing a function with the argument `--file-vars-adapter`.

For instance, with the following command or script and python file, the variable dictionary loaded from the file `qux.yml` will be processed by the function `shout_values()` before rendering the template `foo.c.j2`.

``` shell
j2gpp ./foo.c.j2 --varfile ./qux.yml --file-vars-adapter ./bar.py shout_values
```

```python
from j2gpp import J2GPP

def shout_values(var_dict):
  for key, val in var_dict.items():
    if isinstance(val, str):
      var_dict[key] = val.upper()

j2gpp = J2GPP()
j2gpp.set_file_vars_adapter(shout_values).load_variables_from_file("./qux.yml").render_file("./foo.c.j2")
```

With the processing script `bar.py` :

``` python
def shout_values(var_dict):
  for key,val in var_dict.items():
    if isinstance(val, str):
      var_dict[key] = val.upper()
```

### Process directories

When the source path provided corresponds to a directory, J2GPP will look for any template files in the source directory tree. If no output directory argument is provided, the rendered files will be written next to the source templates. If an output directory is provided, the source directory tree structure will be copied to the output directory with only the rendered files.

For instance, suppose we have the following directory structure :

``` txt
.
└── test_dir
    ├── sub_dir_1
    │   ├── deep_dir
    │   │   └── template_1.txt.j2
    │   └── non_template_1.txt
    ├── sub_dir_2
    │   └── non_template_2.txt
    └── template_2.txt.j2
```

When we execute the command `j2gpp ./test_dir/` or the script `J2GPP().render_directory("./test_dir/")`, we will get :

``` txt
.
└── test_dir
    ├── sub_dir_1
    │   ├── deep_dir
    │   │   ├── template_1.txt
    │   │   └── template_1.txt.j2
    │   └── non_template_1.txt
    ├── sub_dir_2
    │   └── non_template_2.txt
    ├── template_2.txt
    └── template_2.txt.j2
```

But if we provide an output directory with the command `j2gpp ./test_dir/ --outdir ./out_dir/` or the script `J2GPP().render_directory("./test_dir/", "./out_dir/")`, we will get :

``` txt
.
├── test_dir
│   ├── sub_dir_1
│   │   ├── deep_dir
│   │   │   └── template_1.txt.j2
│   │   └── non_template_1.txt
│   ├── sub_dir_2
│   │   └── non_template_2.txt
│   └── template_2.txt.j2
└── out_dir
    ├── sub_dir_1
    │   └── deep_dir
    │       └── template_1.txt
    └── template_2.txt
```

We can also tell J2GPP to copy the non-template files with the command `j2gpp ./test_dir/ --outdir ./out_dir/ --copy-non-template` or the script `J2GPP().set_option("copy_non_template", True).render_directory("./test_dir/", "./out_dir/")`, then we will get :

``` txt
.
├── test_dir
│   ├── sub_dir_1
│   │   ├── deep_dir
│   │   │   └── template_1.txt.j2
│   │   └── non_template_1.txt
│   ├── sub_dir_2
│   │   └── non_template_2.txt
│   └── template_2.txt.j2
└── out_dir
    ├── sub_dir_1
    │   ├── deep_dir
    │   │   └── template_1.txt
    │   └── non_template_1.txt
    ├── sub_dir_2
    │   └── non_template_2.txt
    └── template_2.txt
```

Or even to process non-templates files as templates anyway with the command `j2gpp ./test_dir/ --outdir ./out_dir/ --render-non-template` or the script `J2GPP().set_option("render_non_template", True).render_directory("./test_dir/", "./out_dir/")`, then we will get :

``` txt
.
├── test_dir
│   ├── sub_dir_1
│   │   ├── deep_dir
│   │   │   └── template_1.txt.j2
│   │   └── non_template_1.txt
│   ├── sub_dir_2
│   │   └── non_template_2.txt
│   └── template_2.txt.j2
└── out_dir
    ├── sub_dir_1
    │   ├── deep_dir
    │   │   └── template_1.txt
    │   └── non_template_1_j2gpp.txt
    ├── sub_dir_2
    │   └── non_template_2_j2gpp.txt
    └── template_2.txt
```

## Supported formats for variables

Jinja2 supports variables types from python. The main types are None, Boolean, Integer, Float, String, Tuple, List and Dictionary. J2GPP provides many ways to set variables and not all types are supported by each format.

### CLI define

Defines passed by the command line are interpreted by the Python [ast.literal_eval()](https://docs.python.org/3/library/ast.html#ast.literal_eval) function which supports Python syntax and some additional types such as `set()`.

``` shell
j2gpp ./foo.c.j2 --define test_none=None             \
                 --define test_bool=True             \
                 --define test_int=42                \
                 --define test_float=3.141592        \
                 --define test_string1=lorem         \
                 --define test_string2="lorem ipsum" \
                 --define test_tuple="(1,2,3)"       \
                 --define test_list="[1,2,3]"        \
                 --define test_dict="{'key1': value1, 'key2': value2}" \
                 --define test_dict.key3=value3
```

### API define

Variables can be defined programmatically using the J2GPP API. The `define_variable()` method supports dot notation for nested dictionary keys, while `add_variables()` accepts a complete dictionary.

``` python
from j2gpp import J2GPP
j2gpp = J2GPP()

# Define individual variables
j2gpp.define_variable("test_none", None)
j2gpp.define_variable("test_bool", True)
j2gpp.define_variable("test_int", 42)
j2gpp.define_variable("test_float", 3.141592)
j2gpp.define_variable("test_string1", "lorem")
j2gpp.define_variable("test_string2", "lorem ipsum")
j2gpp.define_variable("test_tuple", (1, 2, 3))
j2gpp.define_variable("test_list", [1, 2, 3])
j2gpp.define_variable("test_dict", {"key1": "value1", "key2": "value2"})
j2gpp.define_variable("test_dict.key3", "value3")

# Or define all variables at once
j2gpp.add_variables({
  "test_none": None,
  "test_bool": True,
  "test_int": 42,
  "test_float": 3.141592,
  "test_string1": "lorem",
  "test_string2": "lorem ipsum",
  "test_tuple": (1, 2, 3),
  "test_list": [1, 2, 3],
  "test_dict": {
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
  }
})

j2gpp.render_file("./foo.c.j2")
```

### YAML

``` yml
test_none1:
test_none2: null

test_bool1: true
test_bool2: false

test_int: 42
test_float: 3.141592

test_string1: lorem ipsum
test_string2:
  single
  line
  text
test_string3: |
  multi
  line
  text

test_list1: [1,2,3]
test_list2:
  - 1
  - 2
  - 3

test_dict:
  key1: value1
  key2: value2
  key3: value3
```

### JSON

``` json
{
  "test_none": null,

  "test_bool1": true,
  "test_bool2": false,

  "test_int": 42,
  "test_float": 3.141592,

  "test_string": "lorem ipsum",

  "test_list": [1,2,3],

  "test_dict": {
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
  }
}
```

### HJSON

``` hjson
{
  # None
  test_none: null,

  # Boolean
  test_bool1: true,
  test_bool2: false,

  # Numbers
  test_int: 42,
  test_float: 3.141592,

  # String
  test_string1: "lorem ipsum",
  test_string2: lorem ipsum,
  test_string3: '''
                multi
                line
                string
                '''

  # List
  test_list: [1,2,3],

  # Dictionary
  test_dict: {
    key1: value1,
    key2: value2,
    key3: value3
  }
}
```

### XML

Note that XML expects a single root element. To avoid having to specify the root element when using the variables in a template, J2GPP automatically removes the root element level if it is named "`_`".

XML attributes are converted to children with the attribute being prefixed by '`@`'. This prefix can be removed by setting the option `--xml-convert-attributes`.

By default, XML tag prefixes and namespaces are kept, but they can be stripped by setting the option `--xml-remove-namespaces`.

``` xml
<_>
  <test_none></test_none>

  <test_bool1>true</test_bool1>
  <test_bool2>false</test_bool2>

  <test_int>42</test_int>
  <test_float>3.141592</test_float>

  <test_string>lorem ipsum</test_string>

  <test_list>1</test_list>
  <test_list>2</test_list>
  <test_list>3</test_list>

  <test_dict>
    <key1>value1</key1>
    <key2>value2</key2>
    <key3>value3</key3>
  </test_dict>
</_>
```

### TOML

``` toml
test_bool1 = true
test_bool2 = false

test_int = 42
test_float = 3.141592

test_string1 = "lorem ipsum"
test_string2 = """
multi
line
text"""

test_list = [1,2,3]

[test_dict]
key1 = "value1"
key2 = "value2"
key3 = "value3"
```

### INI/CFG

Note that INI file expects data to be divided in sections with a header in square brackets. To avoid having to specify the root element when using the variables in a template, J2GPP automatically flattens the section whose header is "`_`".

``` ini
[_]
test_bool1 = True
test_bool2 = False

test_int = 42
test_float = 3.141592

test_string = "lorem ipsum"

test_list = [1,2,3]

[test_dict]
key1 = value1
key2 = value2
key3 = value3
```

### ENV

``` env
test_bool1 = True
test_bool2 = False

test_int = 42
test_float = 3.141592

test_string = lorem ipsum

test_list = [1,2,3]

test_dict = {'key1':'value1','key2':'value2','key3':'value3'}
```

### CSV/TSV

CSV and TSV are interpreted as a list of objects with the same attributes. They are converted to a list of dictionaries whose name is the first cell of each line and the keys are the headers of each column.

CSV and TSV use the same loader, just with different delimiters. A different delimiter can be provided with the argument `--csv-delimiter`. To use the delimiter in a value, it can be escaped by defining an escape character with the argument `--csv-escapechar`, for instance the backslash "`\`".

By default, the whitespace around the keys and values in the CSV is stripped. This behavior can be disabled with the argument `--csv-dontstrip`.

``` csv
keys,key1,key2,key3
test_dict1,1,2,3
test_dict2,11,12,13
test_dict3,21,22,23
```

``` tsv
keys  key1  key2  key3
test_dict1  1  2  3
test_dict2  11  12  13
test_dict3  21  22  23
```

### PLIST

Apple's Property List (PLIST) files are supported.

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>test_bool1</key>
	<true/>
	<key>test_bool2</key>
	<false/>
	<key>test_int</key>
	<integer>42</integer>
	<key>test_float</key>
	<real>3.141592</real>
	<key>test_string</key>
	<string>lorem ipsum</string>
	<key>test_list</key>
	<array>
		<integer>1</integer>
		<integer>2</integer>
		<integer>3</integer>
	</array>
	<key>test_dict</key>
	<dict>
		<key>key1</key>
		<string>value1</string>
		<key>key2</key>
		<string>value2</string>
		<key>key3</key>
		<string>value3</string>
	</dict>
</dict>
</plist>
```

## Scripting in J2GPP templates

An advanced use of the template feature and filters of Jinja2 and J2GPP allow this tool to do some amount of scripting. The main features allowing this usage are explained in this section.

### Conditional and do extension

The basic Jinja2 feature of conditional blocks `if`/`elif`/`else` can be used alongside the `do` extension to more easily manipulate variables and complex objects such as Python dictionaries.

Note that, if possible, it is preferable to process the variables after loading and before rendering by providing a Python function with the `--vars-post-processor` command line argument.

### Conditional and filters

As filters are Python functions, they can be very powerful, especially when coupled with the use of conditional blocks. However, Jinja2 optimizes processing by running some filters during the compilation phase, while conditional blocks are resolved at render time.

To fix this, you can use the `@render_time_only` decorator to force a filter or test to execute at render time only. This decorator is [currently](https://github.com/pallets/jinja/pull/1759) only available by installing a custom fork of Jinja2 :

``` shell
git clone https://github.com/Louis-DR/jinja.git
cd jinja
pip3 install ./
```

### Throw errors and warnings from template

If the version of Jinja2 installed supports the `@render_time_only` decorator, then the `warning` and `error` filters allow to throw warnings and errors from the template and display them in the J2GPP logs. This is useful with conditionals to perform data sanity checks for instance.

### Writing files

The J2GPP filters `write` and `append` allow exporting the content of a block to another file. This can be used for a file combining elements contributed by multiple templates, for alternative versions of a file from a single template, for generating small annex files to a large template, for generating a files for each element in a list, etc. When coupled with the `include` or `macro` statements with nested templates, it allows for even more complex outputs.

Note that if the installed Jinja2 version doesn't support the `@render_time_only` decorator, using the `write` and `append` filters in conditional blocks may results in unwanted behavior.

#### Write example

For example, with the parent template `parent.txt.j2`:

``` jinja2
This will be rendered to the parent output file
{% filter write("child.txt") %}
This will be rendered to the child output file
{% endfilter %}
```

``` shell
>>> tree
.
└── parent.txt.j2

>>> j2gpp ./parent.txt.j2
[...]

>>> tree
.
├── child.txt
├── parent.txt
└── parent.txt.j2

>>> cat parent.txt
This will be rendered to the parent output file

>>> cat child.txt
This will be rendered to the child output file
```

#### Append example

If the file doesn't exists, the `append` filter will create it and be equivalent to `write`. However, if the file already exists, `write` will override it while `append` will append to the end of it. For example, with the parent template `parent.txt.j2`:

``` jinja2
This will be rendered to the parent output file
{% filter append("child.txt") %}
This will be rendered to the child output file
{% endfilter %}
```

``` shell
>>> tree
.
└── parent.txt.j2

>>> j2gpp ./parent.txt.j2
[...]

>>> tree
.
├── child.txt
├── parent.txt
└── parent.txt.j2

>>> cat parent.txt
This will be rendered to the parent output file

>>> cat child.txt
This will be rendered to the child output file

>>> j2gpp ./parent.txt.j2
[...]

>>> cat child.txt
This will be rendered to the child output file
This will be rendered to the child output file

>>> j2gpp ./parent.txt.j2
[...]

>>> cat child.txt
This will be rendered to the child output file
This will be rendered to the child output file
This will be rendered to the child output file
```

#### Path argument

The `write` and `append` filters require at least one argument, the name or path of the second file to generate. The path can be absolute, or relative to the generated file of the parent template. Non-existing directories will be created. For example, with the parent template `parent.txt.j2`:

``` jinja2
{% filter write("foo/child.txt") %}
This will be rendered to the child output file
{% endfilter %}
```

``` shell
>>> tree
.
└── src
    └── parent.txt.j2

>>> j2gpp ./src/parent.txt.j2 --outdir ./gen/
[...]

>>> tree
.
├── src
│   └── parent.txt.j2
└── gen
    ├── foo
    │   └── child.txt
    └── parent.txt
```

#### Writing to both files

By default, the content of the block is only written to the child file, and is not written to the parent rendered template. This behaviour can be changed by providing the filter argument `preserve` as `True`. For example, with the parent template `parent.txt.j2`:

``` jinja2
{% filter write("child.txt", preserve=True) %}
This will be rendered to both parent and child output files
{% endfilter %}
```

``` shell
>>> j2gpp ./parent.txt.j2 --outdir ./gen/
[...]

>>> cat ./parent.txt
This will be rendered to both parent and child output files

>>> cat ./child.txt
This will be rendered to both parent and child output files
```

#### Skipping the parent file

The source template can also be prevented from resulting in a generated file by providing the filter argument `write_source` as `False`, and only the content of `write` and `append` blocks will generate files. For example, with the parent template `parent.txt.j2`:

``` jinja2
This will not be written to any file
{% filter write("child.txt") %}
This will be rendered to the child output file
{% endfilter %}
```

``` shell
>>> tree
.
└── parent.txt.j2

>>> j2gpp ./parent.txt.j2
[...]

>>> tree
.
├── child.txt
└── parent.txt.j2

>>> cat child.txt
This will be rendered to the child output file
```

## Extensions

J2GPP supports extensions that can add custom filters, tests, and globals. Extensions are Python packages that register themselves via entry points, allowing them to be automatically discovered when installed.

### Using extensions

By default, J2GPP automatically discovers and loads all installed extensions. Extensions are loaded during engine initialization and their filters, tests, and globals become available to templates.

**CLI usage:**

``` shell
# Auto-load all installed extensions (default behavior)
j2gpp ./template.j2

# Disable automatic extension loading
j2gpp ./template.j2 --no-auto-extensions

# Explicitly load a specific extension
j2gpp ./template.j2 --extension my_extension

# Disable a specific extension from being loaded
j2gpp ./template.j2 --disable-extension unwanted_extension
```

**API usage:**

``` python
from j2gpp import J2GPP

# Auto-load all installed extensions
engine = J2GPP()
engine.load_extensions()

# Load a specific extension by name
engine.load_extension("my_extension")

# Disable an extension before auto-loading
engine = J2GPP()
engine.disable_extension("unwanted_extension")
engine.load_extensions()

# Inspect loaded extensions
loaded = engine.get_loaded_extensions()
for name, info in loaded.items():
    print(f"{name} v{info['version']}: {info['filter_count']} filters")

# Check if an extension is loaded
if engine.has_extension("my_extension"):
    print("Extension is available")
```

### Creating extensions

Extensions are Python packages that expose a dictionary via entry points. The dictionary contains the extension's metadata and the filters, tests, and globals it provides.

**1. Create the extension package:**

``` python
# my_j2gpp_extension/__init__.py

j2gpp_extension = {
    "name": "my_extension",
    "version": "1.0.0",
    "dependencies": [],  # Optional: list of extension names this depends on
    "filters": {
        "shout": lambda x: str(x).upper() + "!",
        "whisper": lambda x: str(x).lower(),
    },
    "tests": {
        "loud": lambda x: str(x).isupper(),
    },
    "globals": {
        "EXTENSION_VERSION": "1.0.0",
    },
}
```

**2. Register the entry point in `pyproject.toml`:**

``` toml
[project.entry-points."j2gpp.extensions"]
my_extension = "my_j2gpp_extension:j2gpp_extension"
```

**3. Install the package:**

``` shell
pip install ./my_j2gpp_extension
```

Once installed, the extension will be automatically discovered by J2GPP and its filters, tests, and globals will be available in templates.

**Extension interface:**

| Field          | Required | Description                                           |
| -------------- | -------- | ----------------------------------------------------- |
| `name`         | Yes      | Unique identifier for the extension                   |
| `version`      | No       | Version string (defaults to "unknown")                |
| `dependencies` | No       | List of extension names this extension depends on     |
| `filters`      | No       | Dictionary of filter name to callable                 |
| `tests`        | No       | Dictionary of test name to callable                   |
| `globals`      | No       | Dictionary of global name to value (any type)         |

**Conflict handling:**

If multiple extensions or custom filters/tests/globals define the same name, the later one will override the earlier one and a warning will be displayed. Extensions are loaded in dependency order, then alphabetically.
