Imported BaseEngine from the DEP.

i18n is left aside for now.

django/template/backends/base.py

+from django.core.exceptions import (
+    ImproperlyConfigured, SuspiciousFileOperation)
+from django.template.utils import get_app_template_dirs
+from django.utils._os import safe_join
+from django.utils.functional import cached_property
+
+
+class BaseEngine(object):
+
+    # Core methods: engines have to provide their own implementation
+    #               (except for from_string which is optional).
+
+    def __init__(self, params):
+        """
+        Initializes the template engine.
+
+        Receives the configuration settings as a dict.
+        """
+        params = params.copy()
+        self.name = params.pop('NAME')
+        self.dirs = list(params.pop('DIRS'))
+        self.app_dirs = bool(params.pop('APP_DIRS'))
+        if params:
+            raise ImproperlyConfigured(
+                "Unknown parameters: {}".format(", ".join(params)))
+
+    @property
+    def app_dirname(self):
+        raise ImproperlyConfigured(
+            "{} doesn't support loading templates from installed "
+            "applications.".format(self.__class__.__name__))
+
+    def from_string(self, template_code):
+        """
+        Creates and returns a template for the given source code.
+
+        This method is optional.
+        """
+        raise NotImplementedError(
+            "subclasses of BaseEngine should provide "
+            "a from_string() method")
+
+    def get_template(self, template_name):
+        """
+        Loads and returns a template for the given name.
+
+        Raises TemplateDoesNotExist if no such template exists.
+        """
+        raise NotImplementedError(
+            "subclasses of BaseEngine must provide "
+            "a get_template() method")
+
+    # Utility methods: they are provided to minimize code duplication and
+    #                  security issues in third-party backends.
+
+    @cached_property
+    def template_dirs(self):
+        """
+        Returns a list of directories to search for templates.
+        """
+        # Immutable return value because it's cached and shared by callers.
+        template_dirs = tuple(self.dirs)
+        if self.app_dirs:
+            template_dirs += get_app_template_dirs(self.app_dirname)
+        return template_dirs
+
+    def iter_template_filenames(self, template_name):
+        """
+        Iterates over candidate files for template_name.
+
+        Ignores files that don't lie inside configured template dirs to avoid
+        directory traversal attacks.
+        """
+        for template_dir in self.template_dirs:
+            try:
+                yield safe_join(template_dir, template_name)
+            except SuspiciousFileOperation:
+                # The joined path was located outside of this template_dir
+                # (it might be inside another one, so this isn't fatal).
+                pass