Add utility function to mark RunPython operations as noops

[Markus Holtermann]

When adding a field to a model and propagating new values with a RunPython operation in migrations, there is often no reason for a backwards function to call. Same goes for the other direction.

However, if no backwards callable is specified, the respective migration isn't reversible. And the forwards callable is required.

I propose to add a class attribute that effectively provides a no-op but makes the code a bit more expressive:

class RunPython(Operation):
    # ...
    noop = lambda apps, schema_editor: None

Hence the migration could look like

from django.db import migrations

def forwards(apps, schema_editor):
    # Do some stuff

class Migration(migrations.Migration):
    operations = [
        # Current way
        migrations.RunPython(forwards, lambda apps, schema_editor: None)
        # New approach
        migrations.RunPython(forwards, migrations.RunPython.noop)
    ]

[Loic Bistuer]

As discussed on IRC we should do the same thing for RunSQL.

An empty string '' should work well to represent a provided no-op query but we can also alias it to RunSQL.noop for consistency with RunPython.

[Markus Holtermann]

PR: ​https://github.com/django/django/pull/3859

[Tim Graham <timograham@…>]

In c8bac4b556cf4716dc9003e5da48060cb72ba7cb:

Fixed #24098 -- Added no-op attributes to RunPython and RunSQL

Thanks Loïc Bistuer and Tim Graham for the discussion and review.