What does RunPython.noop() do? - django

Question

In the documentation it says,
"Pass the RunPython.noop method to code or reverse_code when you want the operation not to do anything in the given direction. This is especially useful in making the operation reversible."

Answer 1

Sometimes it is possible that you want to revert a migration. For example you have added a field, but now you want to bring the database back to the state before the migration. You can do this by reverting the migration [Django-doc] with the command:
python3 manage.py migrate app_name previous_migration_name
Then Django will look how it can migrate back the the previous_migration_name and perform the operations necessary. For example if you renamed a field from foo to bar, then Django will rename it from bar back to foo.
Other operations are not reversible. For example if you remove a field in the migration, and that field has no default and is non-NULLable, then this can not be reversed. This makes sense since the reverse of removing a field is adding a field, but since there is no value to take for the existing records, what should Django fill in for that field that is recreated for the existing records?
A RunPython command is by default not reversible. In general in computer science one can not computationally determine the reverse of a function if any exists. This is a consequence of Rice's theorem [wiki]. But sometimes it is possible. If we for example constructed a migration where we incremented a certain field with one, then the reverse is to decrement all the fields with one, for example:
from django.db.models import F
from django.db import migrations
def forwards_func(apps, schema_editor):
MyModel = apps.get_model('my_app', 'MyModel')
db_alias = schema_editor.connection.alias
MyModel.objects.using(db_alias).all().update(
some_field=F('some_field')+1
])
def reverse_func(apps, schema_editor):
MyModel = apps.get_model('my_app', 'MyModel')
db_alias = schema_editor.connection.alias
MyModel.objects.using(db_alias).all().update(
some_field=F('some_field')-1
])
class Migration(migrations.Migration):
dependencies = []
operations = [
migrations.RunPython(code=forwards_func, reverse_code=reverse_func),
]
But sometimes it is possible that a (data)migration does nothing when you migrate it forward, or more common when you migrate it backwards. Instead of each time implementing an empty function, you can then pass a reference to noop, which does nothing:
from django.db.models import F
from django.db import migrations
def forwards_func(apps, schema_editor):
# … some action …
pass
class Migration(migrations.Migration):
dependencies = []
operations = [
migrations.RunPython(code=forwards_func, reverse_code=migrations.RunPython.noop),
]