simplify_regex should remove redundant escape sequences outside groups

[Cristi Vîjdea]

django.contrib.admindocs.views.simplify_urls should clean up escapes found outside path parameters. Otherwise, broken URLs with backslashes can be generated and displayed.

This is readily apparent with Django 2's path(), which aggresively escapes everything outside a <parameter> specifier, resulting in a urlpattern with backslash-escaped forward slashes:

>>> simplify_regex(r"^(?P<sport_slug>\w+)/athletes/(?P<athlete_slug>\w+)/$")
'/<sport_slug>/athletes/<athlete_slug>/'
>>> simplify_regex(r"^(?P<sport_slug>\w+)\/athletes\/(?P<athlete_slug>\w+)\/$")
'/<sport_slug>\\/athletes\\/<athlete_slug>\\/'

The second example is what path() would generate in urlpatterns.

You can, for example, see this issue affecting django-rest-framework ​here.

[Oliver Cleary]

I have a PR for this ticket, however I am not sure it really needs to be fixed.

The referenced DRF ticket was resolved by changing the usage of the simplify_regex function to match the usage by Django, which is to pass in the paths route string directly, rather than using it's generated regex pattern.

Additionally, the given example does not seem to be correct as path does not escape forward slashes.

>>> path('<slug:sports_slug>/athletes/<slug:athletes_slug>/', lambda: None).pattern.regex.pattern
'^(?P<sports_slug>[-a-zA-Z0-9_]+)/athletes/(?P<athletes_slug>[-a-zA-Z0-9_]+)/$'

Testing the example in the DRF ticket however does exhibit the issue:

>>> path('^api/token-auth/', lambda: None).pattern.regex.pattern
'^\\^api/token\\-auth/$'
>>> simplify_regex(r'^\^api/token\-auth/$')
'/\\api/token\\-auth/'

With the fix in the PR the special characters are unescaped, and the ^?$ are only stripped if not escaped.

[Carlton Gibson]

Thanks for the patch Oliver.

... I am not sure it really needs to be fixed.

I think this is right.

As with the DRF issue, calling str() on path.pattern before passing to simplify_regex() is the correct usage, and ​it is what `admindocs` does.

Here using ​the original example from the DRF issue:

>>> from django.urls import path
>>> from django.contrib.admindocs.views import simplify_regex
>>> p = path('^api/token-auth/', lambda: None)  # Not sure about the `^`, but either way…
>>> p.pattern
<django.urls.resolvers.RoutePattern object at 0x10acd49d0>
>>> p.pattern.regex.pattern
'^\\^api/token\\-auth/$'
>>> simplify_regex(p.pattern.regex.pattern)  # Wrong usage. 
'/\\api/token\\-auth/'
>>> str(p.pattern)
'^api/token-auth/'
>>> simplify_regex(str(p.pattern))  # Correct usage. 
'/api/token-auth/'