Avoid calling SchemaEditor.effective_default() if not necessary.

[Shipeng Feng]

Example:

def old_default_code():
    return 'old'

def new_default_code():
    # running something that is stateful, for example, increase a database counter

class Student(models.Model):
    code = models.CharField(default=new_default_code)

We change the default callable object, and run manage.py migrate, ideally new_default_code shouldn't be
called, however, we are getting the model field default value no matter whether we need it or not:

​https://github.com/django/django/blob/master/django/db/backends/base/schema.py#L679

I'd love to send a pull request for this if necessary.

[Mariusz Felisiak]

Agreed, we can add this small optimization, i.e. avoid calling SchemaEditor.effective_default() if we don't change a field to a non-nullable.

[Shipeng Feng]

​PR

[GitHub <noreply@…>]

In 8fe2447a:

Fixed #31392 -- Avoided unnecessary SchemaEditor.effective_default() calls when altering a field.