Template variable "perms" single-attribute lookup does not work as documented

[Meiyer]

The ​documentation states that the template variable perms may be used with single-attribute lookup {{ perms.foo }} to check for module-level permissions, proxying to User.has_module_perms.
This is not the case in reality. Using single-attribute lookup causes the tag to output all permissions the user has in the module foo, as text.

This is caused by django.contrib.auth.context_processors.PermWrapper’s method

    def __getitem__(self, app_label):
        return PermLookupDict(self.user, app_label)

which in turn invokes the PermLookupDict’s method

    def __repr__(self):
        return str(self.user.get_all_permissions())

The behaviour is different from the {{ "foo" in perms }} construct, which resolves to PermWrapper’s __contains__ that correctly casts the operation to a boolean, causing the PermLookupDict’s method

    def __bool__(self):
        return self.user.has_module_perms(self.app_label) 

to be used instead.

This issue goes back to at least version 1.6 and forward through all versions up to 1.11 (and apparently the single-attribute lookup feature is not used by anyone because this issue went unreported for at least four years).

[Tim Graham]

I confirmed the same behavior as far back as Django 1.4. From a quick glance, I don't see a way to change the code to conform to the documentation, so I think it's best to correct the docs based on the current behavior.

[Simon Charette]

Tim, I think the confusion here is that {{ perms.foo }} should be checked against with a conditional statement (e.g. {% if perms.foo %}) to work.

The behavior reported here is expected, trying to use {{ boolean_var }} won't perform any check by itself and result in bool.__repr__ just like {{ perms.foo }} results in PermLookupDict.__repr__.

I think the documentation is just a bit misleading and should be using {% if perms.foo %} to exemplify its usage.

[Botond Béres]

Added a ​PR where I've tried to replace and rephrase the incorrect/misleading examples.

[Tim Graham <timograham@…>]

In 51d7feff:

Fixed #28131 -- Corrected examples of using attribute lookups on the "perms" template variable.

[Tim Graham <timograham@…>]

In 41009788:

[2.0.x] Fixed #28131 -- Corrected examples of using attribute lookups on the "perms" template variable.

Backport of 51d7feff872e74d5a53479f62163d5e0024b00ed from master