Modifying nullable columns in Alembic

If you try to add a new column to a table and set it to NOT NULL (or alter a column to be NOT NULL), alembic will bork on:

python manage.py db upgrade

It tells you - rightly - that there are (or will be) existing NULL values in the database for that column. The answer is to edit the migration file to update the column before changing the column definition:

from sqlalchemy.sql import table, column

def upgrade():
    op.add_column('role', sa.Column('role_name', sa.String(length=30), nullable=True))
    role = table('role', column('role_name'))       
    op.execute(role.update().values(role_name=''))       
    op.alter_column('role', 'role_name', nullable=False)