Fixed #15667 -- Added template-based widget rendering.

Thanks Carl Meyer and Tim Graham for contributing to the patch.

django/forms/renderers.py

@@ -0,0 +1,71 @@
+import os
+
+from django.conf import settings
+from django.template.backends.django import DjangoTemplates
+from django.template.loader import get_template
+from django.utils import lru_cache
+from django.utils._os import upath
+from django.utils.functional import cached_property
+from django.utils.module_loading import import_string
+
+try:
+    from django.template.backends.jinja2 import Jinja2
+except ImportError:
+    def Jinja2(params):
+        raise ImportError("jinja2 isn't installed")
+
+ROOT = upath(os.path.dirname(__file__))
+
+
+@lru_cache.lru_cache()
+def get_default_renderer():
+    renderer_class = import_string(settings.FORM_RENDERER)
+    return renderer_class()
+
+
+class BaseRenderer(object):
+    def get_template(self, template_name):
+        raise NotImplementedError('subclasses must implement get_template()')
+
+    def render(self, template_name, context, request=None):
+        template = self.get_template(template_name)
+        return template.render(context, request=request).strip()
+
+
+class EngineMixin(object):
+    def get_template(self, template_name):
+        return self.engine.get_template(template_name)
+
+    @cached_property
+    def engine(self):
+        return self.backend({
+            'APP_DIRS': True,
+            'DIRS': [os.path.join(ROOT, self.backend.app_dirname)],
+            'NAME': 'djangoforms',
+            'OPTIONS': {},
+        })
+
+
+class DjangoTemplates(EngineMixin, BaseRenderer):
+    """
+    Load Django templates from the built-in widget templates in
+    django/forms/templates and from apps' 'templates' directory.
+    """
+    backend = DjangoTemplates
+
+
+class Jinja2(EngineMixin, BaseRenderer):
+    """
+    Load Jinja2 templates from the built-in widget templates in
+    django/forms/jinja2 and from apps' 'jinja2' directory.
+    """
+    backend = Jinja2
+
+
+class TemplatesSetting(BaseRenderer):
+    """
+    Load templates using template.loader.get_template() which is configured
+    based on settings.TEMPLATES.
+    """
+    def get_template(self, template_name):
+        return get_template(template_name)