render_to_string without providing a dictionary still puts a dictionary into the context

[Keryn Knight <django@…>]

given the following scenario:

>>> from django.template.loader import render_to_string
>>> from django.template import Context
>>> render_to_string('file.extension', context_instance=Context())
>>> # or ...
>>> render_to_string('file.extension')

where a dictionary is explicitly not being provided, the final context given to the template when render() is called will have {} as the last of the .dicts

The reason is because if ​none is provided, a new dict is created, and then it is provided to the context instance (​1, ​2) without checking the dict's len()

Marking as cleanup/optimization because it doesn't cause any problems (the only one being: it appears in debug_toolbar), but providing the ticket at least means the option is there to change it, and a history of why not to change it exists if wontfix'd (which may be the best course of action, on balance, but that's not for me to say)

[Baptiste Mispelon]

Hi,

I'm +0 on this but I think it'd be easier to discuss over a concrete proposal so I'll accept the ticket.

Any proposal should not add too much complexity and should aim at remaining as backwards-compatible as possible.

Thanks

[Keryn Knight <django@…>]

The patch, such as it is,

• is in this branch ​https://github.com/kezabelle/django/tree/bugfix/ticket_21741
• diff against master is ​https://github.com/kezabelle/django/compare/bugfix;ticket_21741
• the commit in question is currently ​https://github.com/kezabelle/django/commit/b2f6a4fa7049e888465a50394f038f6c7a7d31f4

All tests pass, but there's no test to evidence the proposed fix works as intended, because I'm not sure how to write such a unit test - the function is a black box - I can't verify the context given to the template is of the correct length, because the return value is obviously a string. Any ideas on how one might test the context for correctness in this scenario would be gratefully received.

[Baptiste Mispelon]

I guess it'd be possible to write a custom template tag that outputs info about the context and use it, but that might be a bit complicated.

Another way would be to create a custom Context and pass it as context_instance. That way you could make sure that no extra layer is added to it.
The downside of this is that you're only testing half of the change but other than that, it's quite simple to implement.

[Baptiste Mispelon <bmispelon@…>]

In 7e1376c2b0ea9e02e61c1c764f02cd40c6f7e849:

Fixed #21741 -- Fixed render_to_string to stop pushing empty dictionaries to its Context

Thanks to kezabelle for the report and original patch
and to numerodix for his improved patch.