Make mock library available for testing in Django

[Claude Paroz]

This question was raised while working on #21523.
unittest.mock is available in Python 3.3+. Backports for previous Python versions are available from ​https://pypi.python.org/pypi/mock.

Basically the main options are:

1. Vendor the mock library backport in Django and conditionally import that backport (we used to have that for unittest2).

2. Use a strategy similar to the selenium dependency, that is having a test class or a mixin class explicitly used when we are using mock functionality in a test, and that class would skip the test when mock is not available.

3. Have the mock library as a hard dependency for our test suite, checking its availability in runtests.py for Python < 3.3 with an appropriate message.

[Collin Anderson]

I assume it's not possible or extremely complicated to write the test without mock? It seems to me #1 is the best option if we do use mock. It would be great if the tests just worked.

[Claude Paroz]

mock's utility goes beyond its potential usage in the mentioned ticket. It might certainly be useful in several other cases in the current Django test suite.

I've purposefully refrained from giving any personal preference in the ticket description, but experience has told us that vendoring should be a last-resort solution. My preference goes to 3., while 2. might be interesting in the sense that it might be useful for some external reusable Django apps.

[Collin Anderson]

I think it's great that you can just download Django and ./runtests.py works. I know pip and venv have made dependencies much easier, but I think there are still some really great benefits to making dependencies as optional as possible. I'm new to testing in general: is it too hard to manually patch things to make a mock by hand?

[Tim Graham]

Undecided on the approach, but definitely would like this.

[Tim Graham]

​PR is ready for review. I only updated one place to use mock, I'm sure there are others that could make use of it, but I'll create a separate ticket to audit for them after this goes in.

[Tim Graham <timograham@…>]

In 82e4f956e3faa4a48fe3c90e02673c1a916d6943:

Fixed #23289 -- Added mock as a test dependency.