# Django 2.2.9.dev20191202104323 文档

# django.template.backends.django 源代码

    from importlib import import_module
    from pkgutil import walk_packages

    from django.apps import apps
    from django.conf import settings
    from django.template import TemplateDoesNotExist
    from django.template.context import make_context
    from django.template.engine import Engine
    from django.template.library import InvalidTemplateLibrary

    from .base import BaseEngine

    class DjangoTemplates(BaseEngine):

        app_dirname = 'templates'

        def __init__(self, params):
            params = params.copy()
            options = params.pop('OPTIONS').copy()
            options.setdefault('autoescape', True)
            options.setdefault('debug', settings.DEBUG)
            options.setdefault('file_charset', settings.FILE_CHARSET)
            libraries = options.get('libraries', {})
            options['libraries'] = self.get_templatetag_libraries(libraries)
            super().__init__(params)
            self.engine = Engine(self.dirs, self.app_dirs, **options)

        def from_string(self, template_code):
            return Template(self.engine.from_string(template_code), self)

        def get_template(self, template_name):
            try:
                return Template(self.engine.get_template(template_name), self)
            except TemplateDoesNotExist as exc:
                reraise(exc, self)

        def get_templatetag_libraries(self, custom_libraries):
            """
            Return a collation of template tag libraries from installed
            applications and the supplied custom_libraries argument.
            """
            libraries = get_installed_libraries()
            libraries.update(custom_libraries)
            return libraries

    class Template:

        def __init__(self, template, backend):
            self.template = template
            self.backend = backend

        @property
        def origin(self):
            return self.template.origin

        def render(self, context=None, request=None):
            context = make_context(context, request, autoescape=self.backend.engine.autoescape)
            try:
                return self.template.render(context)
            except TemplateDoesNotExist as exc:
                reraise(exc, self.backend)

    def copy_exception(exc, backend=None):
        """
        Create a new TemplateDoesNotExist. Preserve its declared attributes and
        template debug data but discard __traceback__, __context__, and __cause__
        to make this object suitable for keeping around (in a cache, for example).
        """
        backend = backend or exc.backend
        new = exc.__class__(*exc.args, tried=exc.tried, backend=backend, chain=exc.chain)
        if hasattr(exc, 'template_debug'):
            new.template_debug = exc.template_debug
        return new

    def reraise(exc, backend):
        """
        Reraise TemplateDoesNotExist while maintaining template debug information.
        """
        new = copy_exception(exc, backend)
        raise new from exc

    def get_installed_libraries():
        """
        Return the built-in template tag libraries and those from installed
        applications. Libraries are stored in a dictionary where keys are the
        individual module names, not the full module paths. Example:
        django.templatetags.i18n is stored as i18n.
        """
        libraries = {}
        candidates = ['django.templatetags']
        candidates.extend(
            '%s.templatetags' % app_config.name
            for app_config in apps.get_app_configs())

        for candidate in candidates:
            try:
                pkg = import_module(candidate)
            except ImportError:
                # No templatetags package defined. This is safe to ignore.
                continue

            if hasattr(pkg, '__path__'):
                for name in get_package_libraries(pkg):
                    libraries[name[len(candidate) + 1:]] = name

        return libraries

    def get_package_libraries(pkg):
        """
        Recursively yield template tag libraries defined in submodules of a
        package.
        """
        for entry in walk_packages(pkg.__path__, pkg.__name__ + '.'):
            try:
                module = import_module(entry[1])
            except ImportError as e:
                raise InvalidTemplateLibrary(
                    "Invalid template library specified. ImportError raised when "
                    "trying to load '%s': %s" % (entry[1], e)
                )

            if hasattr(module, 'register'):
                yield entry[1]