Template error raised in an {% extends %} child template shows incorrect source location on debug page

[Matt Westcott]

If an error occurs in the processing of a template tag on a template that extends another via {% extends %}, the "Error during template rendering" section of the debug page incorrectly shows an excerpt from the parent template rather than the child.

To reproduce:

base.html:

{% block body %}{% endblock %}

home.html:

{% extends "base.html" %}

{% block body %}
    {% include "does_not_exist" %}
{% endblock %}

views.py:

from django.shortcuts import render

def home(request):
    return render(request, 'home.html')

The resulting error page displays:

Error during template rendering
In template /Users/matthew/Development/tbx/wagtail/debug/brokendebug/core/templates/base.html, error at line 0

does_not_exist
1	{% block body %}{% endblock %}
2	

instead of the expected error output:

Error during template rendering
In template /Users/matthew/Development/tbx/wagtail/debug/brokendebug/core/templates/home.html, error at line 4

does_not_exist
1	{% extends "base.html" %}
2	
3	{% block body %}
4	    {% include "does_not_exist" %}
5	{% endblock %}
6	

Bisecting shows that this bug was introduced in #27956 (e643ba8bcf0b1da517cbab689ac157ee031202a3), which was a fix for the opposite case: an error occurring on the parent template. (In the above test case, moving the line {% include "does_not_exist" %} to base.html turns it into a test case for #27956, and fails on pre-e643ba8bcf0b1da517cbab689ac157ee031202a3 revisions as expected.)

[Ankur Jain]

When the parser runs then it appends all the child template nodes to the parent template nodes and hence when template not found error comes then it's reported from the parent html instead of the child html.

[Simon Charette]

#29740 was a duplicate.

[Michael F. Lamb]

Replying to Ankur Jain:
Hi, I'm interested in working on this bug. Are you still working on it, Ankur?

[pascal chambon]

Error confirmed on Django2.2.1, it makes the debug page much less helpful in case of TemplateError, since most websites inherit their templates from some kind of "base.html" skeleton.

[Simone Lazzaris]

Bug still present in 2.2.3; if you (like me) use a tree-structured number of small template files it is very VERY annoying as you have to guess where the bug you're chasing is between the various parts.

[Mariusz Felisiak]

Simone, patch is welcome.

[Min ho Kim]

I'm deassigning myself for now so someone else can have a go until I can come back in the future.

[Ashutosh Sharma]

I couldn't find a way to fix this bug. Deassigning so someone else can have a try.

[Mariusz Felisiak]

#31478 was a duplicate.

[cammil]

Think I have an idea how to fix this. Going to do some more testing to make sure.

[cammil]

I've created a PR: ​https://github.com/django/django/compare/main...cammil:ticket_27694

Does anyone have any comments or feedback?

[Mariusz Felisiak <felisiak.mariusz@…>]

In 313c3d1:

Fixed #28935 -- Fixed display of errors in extended blocks.

Get the template that caused the exception and get the
exception info from that template, using the node that
caused the exception.