Source code for django.template.utils

import functools
from collections import Counter, OrderedDict
from pathlib import Path

from django.apps import apps
from django.conf import settings
from django.core.exceptions import ImproperlyConfigured
from django.utils.functional import cached_property
from django.utils.module_loading import import_string


class InvalidTemplateEngineError(ImproperlyConfigured):
    pass


class EngineHandler:
    def __init__(self, templates=None):
        """
        templates is an optional list of template engine definitions
        (structured like settings.TEMPLATES).
        """
        self._templates = templates
        self._engines = {}

    @cached_property
    def templates(self):
        if self._templates is None:
            self._templates = settings.TEMPLATES

        templates = OrderedDict()
        backend_names = []
        for tpl in self._templates:
            try:
                # This will raise an exception if 'BACKEND' doesn't exist or
                # isn't a string containing at least one dot.
                default_name = tpl['BACKEND'].rsplit('.', 2)[-2]
            except Exception:
                invalid_backend = tpl.get('BACKEND', '<not defined>')
                raise ImproperlyConfigured(
                    "Invalid BACKEND for a template engine: {}. Check "
                    "your TEMPLATES setting.".format(invalid_backend))

            tpl = {
                'NAME': default_name,
                'DIRS': [],
                'APP_DIRS': False,
                'OPTIONS': {},
                **tpl,
            }

            templates[tpl['NAME']] = tpl
            backend_names.append(tpl['NAME'])

        counts = Counter(backend_names)
        duplicates = [alias for alias, count in counts.most_common() if count > 1]
        if duplicates:
            raise ImproperlyConfigured(
                "Template engine aliases aren't unique, duplicates: {}. "
                "Set a unique NAME for each engine in settings.TEMPLATES."
                .format(", ".join(duplicates)))

        return templates

    def __getitem__(self, alias):
        try:
            return self._engines[alias]
        except KeyError:
            try:
                params = self.templates[alias]
            except KeyError:
                raise InvalidTemplateEngineError(
                    "Could not find config for '{}' "
                    "in settings.TEMPLATES".format(alias))

            # If importing or initializing the backend raises an exception,
            # self._engines[alias] isn't set and this code may get executed
            # again, so we must preserve the original params. See #24265.
            params = params.copy()
            backend = params.pop('BACKEND')
            engine_cls = import_string(backend)
            engine = engine_cls(params)

            self._engines[alias] = engine
            return engine

    def __iter__(self):
        return iter(self.templates)

    def all(self):
        return [self[alias] for alias in self]


@functools.lru_cache()
def get_app_template_dirs(dirname):
    """
    Return an iterable of paths of directories to load app templates from.

    dirname is the name of the subdirectory containing templates inside
    installed applications.
    """
    template_dirs = [
        str(Path(app_config.path) / dirname)
        for app_config in apps.get_app_configs()
        if app_config.path and (Path(app_config.path) / dirname).is_dir()
    ]
    # Immutable return value because it will be cached and shared by callers.
    return tuple(template_dirs)
up