Make migration generation more deterministic

[Tomas Dobrovolny]

When you have an application and run makemigrations command twice, generated content of migration file is different (order of generated function call parameters etc). Thus, it is very difficult to merge changes from two development branches together.

Workflow
2) run makemigrations
3) move generated migration somewhere (app/migrations/0001_initial.py to /tmp/)
4) run makemigrations again
5) compare generated migration with moved one (diff app/migrations/0001_initial.py with /tmp/0001_initial.py

You got almout all line of code is different.

[Tim Graham]

I can reproduce on Python 3 at least.

With the tutorial, one time I get:

migrations.CreateModel(
            name='Choice',
            fields=[
                ('id', models.AutoField(primary_key=True, verbose_name='ID', auto_created=True, serialize=False)),

Next time:

                ('id', models.AutoField(serialize=False, primary_key=True, verbose_name='ID', auto_created=True)),

I can't think of any drawbacks of sorting the parameters, but I haven't looked into implementation feasibility.

[Tomas Dobrovolny]

Not only fields kwargs in migration but also:

imports

One time:

from django.db import models, migrations
from django.conf import settings
import datetime
import django.db.models.deletion
import django.core.validators
from django.utils.timezone import utc

Next time:

from django.db import models, migrations
from django.utils.timezone import utc
import django.core.validators
import django.db.models.deletion
import datetime
from django.conf import settings

dependencies member of Migration class

One time:

class Migration(migrations.Migration):

    dependencies = [
        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
        ('iso3166', '0002_initialdata'),
        ('auth', '0001_initial'),
    ]

Next time:

class Migration(migrations.Migration):

    dependencies = [
        ('iso3166', '0002_initialdata'),
        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
        ('auth', '0001_initial'),
    ]

and

• options
• and models.ForeignKey, ManyToManyField and OneToOneField kwargs,

[Markus Holtermann]

I'm wondering how to sort the imports. The easiest would be a sorted(import_strings) but that would mean

from django.db import models, migrations
from django.utils.timezone import utc
import datetime

We could make the sorting more intelligent but I don't think that's necessary. After all, this is still auto generated code.

[Aymeric Augustin]

You could use key=lambda stmt: stmt.split()[1] for slightly better results.

[Markus Holtermann]

PR: ​https://github.com/django/django/pull/3943

I don't think this change needs release notes.

[Markus Holtermann <info@…>]

In 7f20041bca43ca33f0a9617793f2af7ab07c3fab:

Fixed #24155 -- Maintained kwargs and import order in migration writer

Thanks Tomas Dobrovolny for the report and Tim Graham for the review.