Make CharField(blank=True) not be considered nullable

[Naddiseo]

The documentation on blank says that it "will allow the entry of an empty value", which for a string is just a 0-length string. This patch allows CharField(blank=True,...) to no longer be considered Optional.

closes #38

[Naddiseo]

Hmm, from my understanding, the django test that failed /should/ fail type checking...
@mkurnikov, not sure what to do about that one.

[mkurnikov]

https://github.com/django/django/blob/master/django/db/models/fields/__init__.py#L620
https://github.com/django/django/blob/master/django/core/validators.py#L13

I don't know.

From my understanding of the code, blank=False should disable nullability, if null=True. But there's nothing about blank=True.
You can just add this line to IGNORED_ERRORS for this specific module.

[Naddiseo]

Hmm, interesting. However, allowing a field that doesn't have null=True to be None can cause the database to throw an IntegrityError, eg:

class MyModel(models.Model):
   field = models.CharField(max_length=1, blank=True)
MyModel(field=None).save()
# raises IntegrityError (in mysql): (1048, "Column 'field' cannot be null")

[mkurnikov]

I think blank=True should give nullability in the __init__ method, and fail in create(). And reveal_type should have non-optional type.
So, your PR is almost right, could you add special casing in extract_expected_types similar to https://github.com/mkurnikov/django-stubs/blob/master/mypy_django_plugin/transformers/init_create.py#L169
and add test for that?

Then, there will be no need for additional line in IGNORED_ERRORS, and behaviour you mentioned will be achieved.

[Naddiseo]

Let me know if you want me to format it differently, or move the test cases into say model_init.test or nullable_fields.test, but I think code-wise it should do what we're expecting.

[mkurnikov]

Looks good! The only thing, could be make two tests out of those three - one for blank=True, null=False, and one for blank=True, null=True?
Tests are very slow, trying to keep their numbers low.

[mkurnikov]

Thanks!