Installing pytz breaks code already using django's timezone module

[tom.michaelis@…]

pytz is an optional module for Django, used in django.utils.timezone (and possibly elsewhere). The documentation recommends that pytz is installed, but it's not a requirement.

The bug is that after installing pytz, methods in django's timezone module do not present the same interface as before installation. Installing an optional module should not adjust the interface presented.

The following code works differently before and after installation of pytz:

from django.utils import timezone

now = timezone.now()
timezone.make_aware(now, timezone.utc())

Before installing pytz, timezone.make_aware essentially does nothing (the variable is already timezone aware). After installing pytz, timezone.make_aware makes a call to pytz which raises 'ValueError: Not naive datetime (tzinfo is already set)', and causes everything to break.

While I discovered this in django 1.5, I've also been able to replicate the bug in django 1.6, so I am filing the bug report under that.

[Aymeric Augustin]

There are two possible semantics when make_aware receives an aware datetime (or make_naive receives a naive datetime):

• raise an exception
• return the value unchanged

Since Django uses the first one when pytz is installed, and the docs recommend to install pytz, it makes sense to enforce that behavior.

[Aymeric Augustin <aymeric.augustin@…>]

In fa89acf1d0a4adbf0802c1275494f5159a912c9e:

Fixed #22625 -- Normalized make_aware/naive errors.

Also added tests for is/make_aware/naive.

Thanks Tom Michaelis for the report.

[Aymeric Augustin <aymeric.augustin@…>]

In 6f8301d6cd3815409c7c2c8468dbfd239d39f468:

[1.7.x] Fixed #22625 -- Normalized make_aware/naive errors.

Also added tests for is/make_aware/naive.

Thanks Tom Michaelis for the report.

Backport of fa89acf from master

[Aymeric Augustin <aymeric.augustin@…>]

In 1109ebd7b373e14b68616346ea5dd3e89a8ae2fe:

Optimized make_aware/naive by removing redundant checks. Refs #22625.

Also added tests with pytz and removed misplaced tests.

[Aymeric Augustin <aymeric.augustin@…>]

In 62de03ee337e085dbaa9d94a43a2aedbd3cdef84:

[1.7.x] Optimized make_aware/naive by removing redundant checks. Refs #22625.

Also added tests with pytz and removed misplaced tests.

Backport of 1109ebd from master