I was able to use 
`alembic --autogenerate` for when adding / removing columns.

However, when I wanted to modify for example a "url" column from 200 characters to 2000 characters, it doesn't detect the change.

How can I make Alembic (using SQLAlchemy), detect changes and autogenerate scripts to my model's "sizes" of various columns and create "alter\_column" commands for PostgreSQL ??

Edit:

Why doesn't alembic automatically add:

```
op.alter_column('mytable', 'url', type_=sa.String(2000), existing_type=sa.String(length=200), nullable=True)

```