CommandLine should be more easily extendable with custom commands

[mikeroll]

Describe the use case
I would like to be able to plug custom commands that support the specifics of my project's alembic workflows.

A previous discussion suggested that subclassing CommandLine and extending it with custom logic is a good enough solution, and I agree with that. However, the whole initialization of the CLI, including the registration of commands, happens in the single CommandLine._generate_args - which makes it hard to add extra commands without duplicating the whole process or inventing inferior workarounds.

I would suggest to refactor the method and split it into smaller ones, with one of the parts being something called CommandLine._register_command. This would be still called internally to register commands from alembic.command but would also be available to subclasses to register other externally provided functions as commands.

Example Use
I would like to be able to define a subclass like so

class MyCommandLine(CommandLine):
    def _generate_args(self, prog: str | None) -> None:
        super()._generate_args(prog)

        self._register_command(my_custom_command)

which would make my_custom_command immediately available in the cli.

Have a nice day!

[CaselIT]

Hi,

I think it makes sense as a request, will look at the PR

[sqla-tester]

Mikhail Bulash has proposed a fix for this issue in the main branch:

Refactor CommandLine command registration https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/5718