LocaleMiddleware inspects language from URL path in a backwards incompatible way

[Anssi Kääriäinen]

As reported by Ryan Kaskel in ​this django-dev thread, the LocaleMiddleware inspects the user's language from the URL path in a backwards incompatible way.

Kaskel had /sl/ as a prefix for his URL path, and due to that Slovak was activated as the language to use. He did not use the new i18n_patterns method in his patterns.

The relevant commit is this: r16405

There is this warning added to the docs:

    Ensure that you don't have non-prefixed URL patterns that might collide 
    with an automatically-added language prefix. 

An user doesn't have much chance to dodge this issue: suggesting change of his existing URLs doesn't really fly.

I think the only sane fix is to detect if the user is using i18n_patterns in his root urls.py. If he isn't, no language introspection should be done based on the URL path. I have attached a patch with tests for this. My reading of the docs suggests no changes are needed there.

I do think this is backwards incompatible and should be fixed in one way or another before 1.4. Marking this as Release Blocker due to this not really being my ticket, but a confirmation of Kaskel's findings.

[Jannis Leidel]

In [17547]:

Fixed #17720 -- Stopped the LocaleMiddleware from overeagerly using the request path for language activation if it's actually not wanted. Thanks to Anssi Kääriäinen for the initial patch.

[Ryan Kaskel]

I've opened a ticket for a docs change related to this patch: #17723.