Make django.core.validators only load Pillow if needed

[Collin Anderson]

Before #21548, Pillow was only loaded if needed. It would be nice if this were still true.

[Collin Anderson]

​https://github.com/django/django/pull/9619

[Tim Graham]

Can you elaborate on the motivation for this?

[Collin Anderson]

A few advantages:
If for some reason you can't or have trouble installing pillow, everything else will work fine. It makes django a bit more "lightweight" to have the dependency be optional.

Loading Pillow increases memory usage by 5% (2mb) (in my quick testing) for a runserver on an empty project:
37608kb without loading pillow
39768kb with loading pillow

Also, doesn't matter quite as much, but running "check" also takes about 5% longer (25ms) on an empty project for me when including pillow:
360-400ms without loading pillow
393-424ms with loading pillow

I should also note that pretty much every other spot in the codebase tries to make the Pillow dependency optional by not making the import at the top of the file.

[Loic Bistuer]

+1 to making Pillow truly optional.

[Tim Graham]

Isn't it already "optional" with the try/except ImportError? I think the idea you're proposing is to import it lazily (which could help if it's installed but unused) which makes sense to me.

[Collin Anderson]

Good point. The installed but unused case is what I'm running into.

[Tim Graham <timograham@…>]

In 46b3e3f:

Fixed #29065 -- Made django.core.validators only load Pillow if needed.