Re-number Django migrations during a rebase, use like `$ git rebase -i devel --exec "python3 rebase_migrations.py"`

rebase_migrations.py

from collections import defaultdict
from pathlib import Path
import re
import subprocess


changed_files = subprocess.check_output("git diff-tree --no-commit-id --name-status -r HEAD", shell=True)
changed_status = [line.split('\t') for line in changed_files.decode('utf-8').rstrip('\n').split('\n')]
changed_migrations = {fname for status, fname in changed_status
                      if '/migrations/' in fname and Path(fname).name[0].isdigit() and status != 'D'}
if changed_migrations:
    print("Changed migrations: {}".format(changed_migrations))

migrations = defaultdict(list)
for m in sorted(changed_migrations):
    migrations[Path(m).parent].append(m)

for d, S in migrations.items():
    existing_migrations = sorted(
        f for f in Path(d).iterdir()
        if f.is_file() and f.name[0].isdigit() and f.name.endswith('.py') and str(f) not in S
    )
    existing_numbers = [int(f.name[:4], base=10) for f in existing_migrations]

    for m in S:
        migration = Path(m)
        num = int(migration.name[:4], base=10)

        if num in existing_numbers:
            with open(m) as f:
                contents = f.read()
            print('git rm', m)
            subprocess.check_output(['git', 'rm', m])

            updated = re.sub(
                r'{:04d}_[a-zA-Z0-9_]*'.format(num - 1),
                max(existing_migrations).name[:-3],
                contents
            )

            migration = d / '{:04d}{}'.format(max(existing_numbers) + 1, migration.name[4:])
            with migration.open('w') as f:
                f.write(updated)

            print('git add', str(migration))
            subprocess.check_output(['git', 'add', str(migration)])

        existing_migrations.append(migration)
        existing_numbers.append(num)

subprocess.check_output("git commit --amend --no-edit", shell=True)