Fixed #23891 -- Moved deprecation of IPAddressField to system check framework.

[timgraham]

No description provided.

[MarkusH]

If we want to recommend this way for 3rd party apps as well, I suggest we offer a simple API, where e.g. setting a field class attribute will automatically raise a warning or error.

class Field(...):
    deprecated_for_migrations = False
    removed_for_migrations = False

    def check(...):
        # ...
        errors.extend(self._check_migration_availability(...))
        # ...

    def _check_migration_availability(self):
        if self.__class__.removed_for_migrations:
            return [
                checks.Error(
                    'The field %s has been removed except for support in historical migrations.' % self.__class__.__name__,
                    hint=None,
                    obj=self,
                    id='fields.EXXX',
                )
            ]
        elif self.__class__.deprecated_for_migrations:
            return [
                checks.Warning(
                    'The field %s has been deprecated. Support for it (except in historical migrations) will be removed according to the deprecation timeline' % self.__class__.__name__,
                    hint=None,
                    obj=self,
                    id='fields.WXXX',
                )
            ]

class IPAddressField(...):
    deprecated_for_migrations = True

[MarkusH]

The check id should be added to docs/ref/checks.txt

[timgraham]

Markus, can we make your suggestion the subject of ticket #23861 then? That seems like a new feature and I am not sure it requires a backport to 1.7.

I wasn't sure about the WXXX codes which is why I didn't document it yet. It seems nice to be able to have a unique code for each field so they can be silenced if desired, rather than one code for all deprecations, but this seems possible only if we use the model field class name to generate the code, in which case we'll have to depart from the usual numbering scheme.

[MarkusH]

I missed the fact you want to backport this to 1.7. I that case, yes, this could be part of #23861 if it makes sense.

[timgraham]

Markus, updated #23891 per your comments and added a sketch for how #23861 might look. Will merge the former and continue work on the latter if all looks okay..

[MarkusH]

Could you wrap this line after the comma.

[MarkusH]

Looks good to me, Tim.