# Django 2.2.9.dev20191202104323 文档

# django.template.engine 源代码

    import functools

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

    from .base import Context, Template
    from .context import _builtin_context_processors
    from .exceptions import TemplateDoesNotExist
    from .library import import_library

    class Engine:
        default_builtins = [
            'django.template.defaulttags',
            'django.template.defaultfilters',
            'django.template.loader_tags',
        ]

        def __init__(self, dirs=None, app_dirs=False, context_processors=None,
                     debug=False, loaders=None, string_if_invalid='',
                     file_charset='utf-8', libraries=None, builtins=None, autoescape=True):
            if dirs is None:
                dirs = []
            if context_processors is None:
                context_processors = []
            if loaders is None:
                loaders = ['django.template.loaders.filesystem.Loader']
                if app_dirs:
                    loaders += ['django.template.loaders.app_directories.Loader']
                if not debug:
                    loaders = [('django.template.loaders.cached.Loader', loaders)]
            else:
                if app_dirs:
                    raise ImproperlyConfigured(
                        "app_dirs must not be set when loaders is defined.")
            if libraries is None:
                libraries = {}
            if builtins is None:
                builtins = []

            self.dirs = dirs
            self.app_dirs = app_dirs
            self.autoescape = autoescape
            self.context_processors = context_processors
            self.debug = debug
            self.loaders = loaders
            self.string_if_invalid = string_if_invalid
            self.file_charset = file_charset
            self.libraries = libraries
            self.template_libraries = self.get_template_libraries(libraries)
            self.builtins = self.default_builtins + builtins
            self.template_builtins = self.get_template_builtins(self.builtins)

        @staticmethod
        @functools.lru_cache()
        def get_default():
            """
            Return the first DjangoTemplates backend that's configured, or raise
            ImproperlyConfigured if none are configured.

            This is required for preserving historical APIs that rely on a
            globally available, implicitly configured engine such as:

            >>> from django.template import Context, Template
            >>> template = Template("Hello {{ name }}!")
            >>> context = Context({'name': "world"})
            >>> template.render(context)
            'Hello world!'
            """
            # Since Engine is imported in django.template and since
            # DjangoTemplates is a wrapper around this Engine class,
            # local imports are required to avoid import loops.
            from django.template import engines
            from django.template.backends.django import DjangoTemplates
            for engine in engines.all():
                if isinstance(engine, DjangoTemplates):
                    return engine.engine
            raise ImproperlyConfigured('No DjangoTemplates backend is configured.')

        @cached_property
        def template_context_processors(self):
            context_processors = _builtin_context_processors
            context_processors += tuple(self.context_processors)
            return tuple(import_string(path) for path in context_processors)

        def get_template_builtins(self, builtins):
            return [import_library(x) for x in builtins]

        def get_template_libraries(self, libraries):
            loaded = {}
            for name, path in libraries.items():
                loaded[name] = import_library(path)
            return loaded

        @cached_property
        def template_loaders(self):
            return self.get_template_loaders(self.loaders)

        def get_template_loaders(self, template_loaders):
            loaders = []
            for template_loader in template_loaders:
                loader = self.find_template_loader(template_loader)
                if loader is not None:
                    loaders.append(loader)
            return loaders

        def find_template_loader(self, loader):
            if isinstance(loader, (tuple, list)):
                loader, *args = loader
            else:
                args = []

            if isinstance(loader, str):
                loader_class = import_string(loader)
                return loader_class(self, *args)
            else:
                raise ImproperlyConfigured(
                    "Invalid value in template loaders configuration: %r" % loader)

        def find_template(self, name, dirs=None, skip=None):
            tried = []
            for loader in self.template_loaders:
                try:
                    template = loader.get_template(name, skip=skip)
                    return template, template.origin
                except TemplateDoesNotExist as e:
                    tried.extend(e.tried)
            raise TemplateDoesNotExist(name, tried=tried)

        def from_string(self, template_code):
            """
            Return a compiled Template object for the given template code,
            handling template inheritance recursively.
            """
            return Template(template_code, engine=self)

        def get_template(self, template_name):
            """
            Return a compiled Template object for the given template name,
            handling template inheritance recursively.
            """
            template, origin = self.find_template(template_name)
            if not hasattr(template, 'render'):
                # template needs to be compiled
                template = Template(template, origin, template_name, engine=self)
            return template

        def render_to_string(self, template_name, context=None):
            """
            Render the template specified by template_name with the given context.
            For use in Django's test suite.
            """
            if isinstance(template_name, (list, tuple)):
                t = self.select_template(template_name)
            else:
                t = self.get_template(template_name)
            # Django < 1.8 accepted a Context in `context` even though that's
            # unintended. Preserve this ability but don't rewrap `context`.
            if isinstance(context, Context):
                return t.render(context)
            else:
                return t.render(Context(context))

        def select_template(self, template_name_list):
            """
            Given a list of template names, return the first that can be loaded.
            """
            if not template_name_list:
                raise TemplateDoesNotExist("No template names provided")
            not_found = []
            for template_name in template_name_list:
                try:
                    return self.get_template(template_name)
                except TemplateDoesNotExist as exc:
                    if exc.args[0] not in not_found:
                        not_found.append(exc.args[0])
                    continue
            # If we get here, none of the templates could be loaded
            raise TemplateDoesNotExist(', '.join(not_found))