Invalid migrations caused by incorrect serialization of functools.partial()

[Nick Pope]

So this one was interesting... I ended up with a migration that contained something like this (reduced to a trivial example):

models.CharField(default=functools.partial(int, *('123', ), **None), ...)

The expected output should have been:

models.CharField(default=functools.partial(int, *('123', ), **{}), ...)

It boils down to the following issue:

# Python 2.7.6: Empty args working, empty keywords broken.
>>> import functools
>>> print(functools.partial(int, base=10).args)
()
>>> print(functools.partial(int, '123').keywords)
None

# Python 2.7.14: Empty args working, empty keywords working.
>>> import functools
>>> print(functools.partial(int, base=10).args)
()
>>> print(functools.partial(int, '123').keywords)
{}

Scouring the release notes for Python, I found the following:

"The keywords attribute of functools.partial is now always a dictionary."

This was resolved in 2.7.10, 3.4.4 & 3.5.0.

As Django 2.1 will only support Python 3.5 or later, this only needs to be backported to 1.11 and 2.0 and need not be applied to master.

Pull request incoming...

[Tim Graham]

I'm not sure if this merits fixing considering functools.partial support was added in Django 1.9 so this isn't a new or common (apparently) regression. Also, regarding Python support, Django's policy is "We highly recommend and only officially support the latest release of each series." Can you workaround it in your project?

[Nick Pope]

Indeed - I know of the policies, it's just an awkward problem. I'm not sure... It might be possible to force everything through as keyword arguments because args is unaffected.

I've updated the ​PR anyway so that it has a proper test...

[Tim Graham]

How about adding **{} to your code?

>>> print(functools.partial(int, '123', **{}).keywords)
{}

[Nick Pope]

Cool. Nice hack!

I've just tried changing things to always have at least one keyword argument anyway.

[Tim Graham]

All right, I don't think it's worth adding a workaround to Django then.