render_table tag overwrites "table" variable from outer context

[thunderk]

The render_table tag binds its first argument (the Table object) to a table variable, which is then used in child template for rendering.
The problem is that this seems to overwrite any previous table variable set in the parent context.

Here is a small example, with a django-tables2 Table object in mytable template variable:

{% with "My poor variable" as table %}
<p>{{ table }}</p>
{% render_table mytable %}
<p>{{ table }}</p>
{% endwith %}

The output of this template is:

My poor variable
[the mytable rendered]
<django_tables2.Table object at 0x...>

Tested with version 1.17.1

[jieter]

The comments here suggest the added entry is popped off. Maybe something changed in how django handles this and there clearly is no test checking for it.

I don't think we really need the original context, except for request, so we might be able to use a fresh context. It would really help if you are able to provide a failing test case for the test suite to solve this bug.

[jieter]

@thunderk I tried reproducing the issue with this test, but it seems to pass without changes.

    def test_does_not_touch_context(self):
        '''
        Make sure the tag does not change the context of the template the tag is called from
        https://github.com/jieter/django-tables2/issues/547
        '''

        table = CountryTable([], template_name='dummy.html')
        template = Template(
            '{% load django_tables2 %}'
            '{% with "foo" as table %}{{ table }} {% render_table mytable %} {{ table }}{% endwith %}'
        )

        html = template.render(Context({
            'request': build_request(),
            'mytable': table,
        }))

        self.assertEqual(html, 'foo dummy template contents\n foo')

Can you spot the difference between your case and my test?

[jieter]

Fyi, this is a commit with change which would fix this problem. But the test also passes without the fix.
d1d95a6

[thunderk]

Sorry about the delay, but I'm struggling to reproduce the bug on a small case (it happens in our quite large app, with several levels of template inheritance and custom tags).
In our case, it happens when rendering a table with a TemplateColumn. Removing this column fixes the behavior. But the template used in the column does not bind a 'table' variable, so I don't see why it should interfere.
I will still try to provide you with a simple example.

[thunderk]

Ok, got it, the table has to be non-empty to trigger the bug:

    def test_does_not_touch_context(self):
        '''
        Make sure the tag does not change the context of the template the tag is called from
        https://github.com/jieter/django-tables2/issues/547
        '''
        class TestTable(Table):
            col = TemplateColumn(template_name='test_template_column.html')
        table = TestTable([{}])
        template = Template(
            '{% load django_tables2 %}'
            '{% with "foo" as table %}{{ table }} {% render_table mytable %} {{ table }}{% endwith %}'
        )

        html = template.render(Context({
            'request': build_request(),
            'mytable': table,
        }))

        self.assertEqual(html[:4], 'foo ')
        self.assertEqual(html[-4:], ' foo')

[jieter]

@thunderk Can you verify #550 solves your use case?

pip install https://github.com/jieter/django-tables2/archive/prevent-context-mutation.zip

[thunderk]

@jieter Yes, case solved ! Thank you.

[jieter]

released 1.21.0