Refs #23130 -- Allowed BooleanField to be null=True

[coilysiren]

https://code.djangoproject.com/ticket/23130

Picks up were @timgraham left off in #8467

refs:

• Fixed #23130 -- BooleanField should not override 'blank' if choices are specified #3025
• https://code.djangoproject.com/ticket/22282
• https://code.djangoproject.com/ticket/26168
• https://code.djangoproject.com/ticket/22282

details:

• BooleanField model field can take null=True
• NullBooleanField is now just BooleanField(null=True)
• depreciates NullBooleanField (this decision was made in @timgraham's PR)
• normalizes boolean type casting, and incorporates distutils.util.strtobool
• collects all the different boolean casting special cases into a single file django/utils/typecasting.py

still TODO:

• there's at least a dozen tests failing <<< resolved
• determine if this is a large enough PR to get added to AUTHORS ^_^ <<< call: no. the PR is much simpler without the typecasting.py change
• check for NullBooleanField isinstance calls to turn into null=True
• typecasting.py entrypoint functions need better names
• evaluate typecasting.py nullable=[ True | False ] handling
• add a few more cases to test_typecasting.py

[coilysiren]

✔️ tests passing

Solve the issue with Oracle check constraints. I think the appropriate check constraint for BooleanField(null=True) won't be generated because Field.get_internal_type() returns BooleanField rather than NullBooleanField. A possible fix is...

Implemented that fix! But also I feel this becomes less relevant if #9018 is merged first

As needed, continue adding tests for BooleanField(null=True) where NullBooleanField is tested.

So ^ is the last thing to do

Although I'm not sure to what extent adding those tests would be overkill, since NullBooleanField is now just

https://github.com/lynncyrin/django/blob/254d3149cb50016ae6e0746488b1042f5a05d8ec/django/forms/fields.py#L742-L748

class NullBooleanField(BooleanField):
    """
    A BooleanField that forces an optional NullBooleanSelect widget
    """

    def __init__(self, **kwargs):
        super().__init__(null=True, **kwargs)

[timgraham]

1. Yes, I think we can omit the change to get_internal_type() considering Unified NullBooleanField's and BooleanField's check constraints on Oracle. #9018.
2. I don't think the form field changes to put NullBooleanField's logic in BooleanField is much of a simplification. Let's focus on the model field and leave the form fields alone for now.
3. The idea with the additional tests is about the model field. For example, in tests/bulk_create/models.py there's a usage of NullBooleanField. A similar test for BooleanField(null=True) should be added there.

[coilysiren]

omit the change to get_internal_type() considering #9018

✔️

focus on the model field and leave the form fields alone for now

✔️

additional tests [for] the model field

WIP. I added the bulk_create one (it was obvious what to do there). I still need to appraise what adequate tests for be for:

• postgres_tests
• test_promises
• serializers
• test_booleanfield
• managers_regress
• generic_relations_regress
• expressions_case
• annotations

[coilysiren]

In case I forget to make them, some optimization ticket ideas spawned from this PR are:

• make all boolean type casting utilize distutils.util.strtobool, and collect form field and model field special cases into a single boolean type casting file
• make NullBoolean model field BooleanField(null=True)
• make NullBoolean form field BooleanField(null=True)

[timgraham]

I incorporated some of these updates in #8467 which should be ready for review now. Thanks!