Regression: can't reuse a variable name in a {% for %} loop

[timgraham]

Hi, I bisected the introduction of some failures in Django's test suite to 1205e64:

$ ./tests/runtests.py forms_tests.widget_tests.test_multiplehiddeninput.MultipleHiddenInputTest.test_render_attrs

Traceback (most recent call last):
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/tests/forms_tests/widget_tests/test_splithiddendatetimewidget.py", line 21, in test_render_value
    '<input type="hidden" name="date_0" value="2007-09-17" />'
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/tests/forms_tests/widget_tests/base.py", line 23, in check_html
    output = widget.render(name, value, attrs=attrs, renderer=self.jinja2_renderer, **kwargs)
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/django/forms/widgets.py", line 222, in render
    return self._render(self.template_name, context, renderer)
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/django/forms/widgets.py", line 227, in _render
    return mark_safe(renderer.render(template_name, context))
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/django/forms/renderers.py", line 32, in render
    return template.render(context, request=request).strip()
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/django/template/backends/jinja2.py", line 81, in render
    return self.template.render(context)
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/tests/.env/lib/python3.6/site-packages/jinja2/asyncsupport.py", line 76, in render
    return original_render(self, *args, **kwargs)
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/tests/.env/lib/python3.6/site-packages/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/tests/.env/lib/python3.6/site-packages/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/tests/.env/lib/python3.6/site-packages/jinja2/_compat.py", line 37, in reraise
    raise value.with_traceback(tb)
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/django/forms/jinja2/django/forms/widgets/splithiddendatetime.html", line 1, in top-level template code
    {% include 'django/forms/widgets/multiwidget.html' %}
  File "/var/lib/jenkins/workspace/django-security-master/database/postgres/python/python3.6/django/forms/jinja2/django/forms/widgets/multiwidget.html", line 1, in top-level template code
    {% for widget in widget.subwidgets %}{% include widget.template_name %}{% endfor %}
UnboundLocalError: local variable 'l_1_widget' referenced before assignment

Let me know if I can help or provide additional info.

[davidism]

Please include a self-contained example that produces this error.

[timgraham]

Okay, I'm trying to write a test now. Wasn't sure if the bug would be obvious given the traceback.

[timgraham]

I think the issue is the construct {% for widget in widget.subwidgets %} -- the widget name is reused.

[timgraham]

Here's a test:

diff --git a/tests/test_core_tags.py b/tests/test_core_tags.py
index 0438008..6cc7dac 100644
--- a/tests/test_core_tags.py
+++ b/tests/test_core_tags.py
@@ -22,6 +22,10 @@ def env_trim():
 @pytest.mark.for_loop
 class TestForLoop(object):
 
+    def test_simple_reuse(self, env):
+        tmpl = env.from_string('{% for item in item %}{{ item }}{% endfor %}')
+        assert tmpl.render(item=list(range(10))) == '0123456789'
+
     def test_simple(self, env):
         tmpl = env.from_string('{% for item in seq %}{{ item }}{% endfor %}')
         assert tmpl.render(seq=list(range(10))) == '0123456789'

It errors out with UnboundLocalError: local variable 'l_1_item' referenced before assignment.

[ThiefMaster]

While this looks a lot like a bug that should be fixed, is it really a good idea to write such code? Especially considering that while Jinja (probably) gets the scoping right, Python itself doesn't (since it has no block scope):

>>> foo = [1, 2, 3]
>>> for foo in foo:
...     print foo
...
1
2
3
>>> foo
3

[timgraham]

I'm a maintainer of Django and don't have much experience with Jinja2 (and didn't write the template). In our case, I'm not sure what the alternative is:
{% for widget in widget.subwidgets %}{% include widget.template_name %}{% endfor %}
the included template relies on the variable being named widget.