{% extends %} tag causes AttributeError if origin is None and path is relative

[Yogya Chugh]

So while working on a ticket, I came across a strange behaviour that whenever I pass a relative path like './goa.html' to the extends tag that leads to an Attribution error !
I have attached a test case which will pass if error is produced ! You can check it out !
So, I tried to find the reason for this and found:-
When we separately create a Template and Context class along with calling the render method of Template, the error is produced.

This happens because in such a case the origin for the template is None hence, setting it to '<unknown source>' due to this code:-

#django/template/base.py
if origin is None:
    origin = Origin(UNKNOWN_SOURCE)

Here, UNKNOWN_SOURCE is global variable with value "<unknown source>"

So this ain't an issue normally but when we use {% extends %} tag, it makes use of a function construct_relative_path which is passed the template name from origin !

The function construct_relative_path performs an lstrip method on the template leading to error in case the template's origin is "<unknown source>". Because, origin.template_name is None. And so does the lstrip method produces error .

Code Snippet:-

#django/template/loader_tags.py
new_name = posixpath.normpath(
        posixpath.join(
            posixpath.dirname(current_template_name.lstrip("/")),
            new_name,
        )
    )

The Error log:-

Internal Server Error: /
Traceback (most recent call last):
  File "C:\Users\Lenovo\AppData\Roaming\Python\Python313\site-packages\django\core\handlers\exception.py", line 55, in inner
    response = get_response(request)
  File "C:\Users\Lenovo\AppData\Roaming\Python\Python313\site-packages\django\core\handlers\base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\Lenovo\OneDrive\Desktop\djangoplz\new\trial\views.py", line 9, in index
    temp = Template(temp_string)
  File "C:\Users\Lenovo\AppData\Roaming\Python\Python313\site-packages\django\template\base.py", line 154, in __init__
    self.nodelist = self.compile_nodelist()
                    ~~~~~~~~~~~~~~~~~~~~~^^
  File "C:\Users\Lenovo\AppData\Roaming\Python\Python313\site-packages\django\template\base.py", line 196, in compile_nodelist
    nodelist = parser.parse()
  File "C:\Users\Lenovo\AppData\Roaming\Python\Python313\site-packages\django\template\base.py", line 518, in parse
    raise self.error(token, e)
  File "C:\Users\Lenovo\AppData\Roaming\Python\Python313\site-packages\django\template\base.py", line 516, in parse
    compiled_result = compile_func(self, token)
  File "C:\Users\Lenovo\AppData\Roaming\Python\Python313\site-packages\django\template\loader_tags.py", line 293, in do_extends
    bits[1] = construct_relative_path(parser.origin.template_name, bits[1])
              ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Lenovo\AppData\Roaming\Python\Python313\site-packages\django\template\loader_tags.py", line 258, in construct_relative_path
    posixpath.dirname(current_template_name.lstrip("/")),
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'lstrip'

This error is produced on both 5.1.7 version and on the dev too !

I would love to work on this ! Please do review

[Sarah Boyce]

Thank you, replicated. I think this should be raising a TemplateDoesNotExist error instead similar to if the path was not relative

tests/template_tests/syntax_tests/test_exceptions.py

@@ -1 @@
-from django.template import TemplateDoesNotExist, TemplateSyntaxError
+from django.template import TemplateDoesNotExist, TemplateSyntaxError, Context, Template
 from django.test import SimpleTestCase
 
 from ..utils import setup
@@ -14 +14 @@
         with self.assertRaises(TemplateDoesNotExist):
             self.engine.render_to_string("exception01")
 
+    def test_template_does_not_exist(self):
+        for template_name in ["nonexistent.html", "./nonexistent.html"]:
+            with self.subTest(template_name=template_name):
+                template = Template(f"{{% extends '{template_name}' %}}")
+                with self.assertRaises(TemplateDoesNotExist):
+                    template.render(Context({}))
+
     @setup({"exception02": "{% extends nonexistent %}"})
     def test_exception02(self):

[Yogya Chugh]

Yup ! I will add a check condition for None template name in construct_relative_path and add test case for when the template do exist !

[Sarah Boyce <42296566+sarahboyce@…>]

In 7164f08:

Fixed #36271 -- Raised TemplateSyntaxError when using a relative template path with an unknown origin.