Add exclude_modules option for _collect_instances

[srohr]

This PR adds the ability to exclude specified import paths from calls to collect_instances, collect_subclasses or walk_modules. This might be desirable if some module (or its submodules) contain imports of libraries you only have installed in a dev configuration, or if you have large/slow modules to walk.

[olirice]

thanks for the PR!

I can definitely see how excluding modules would be useful in practice and your implementation looks solid, but I'd prefer to keep these functions as-is

My hesitation is mostly with the interface of a list of stringified module names as an argument. It makes sense since we're trying to avoid importing those modules but I'm not sure it'd be obvious to users without tracking down the test case.

Since the function is the experimental module and generic (not coupled to alembic_utils) keeping it in a separate module feels like the right choice

[DanCardin]

Insofar as this function is useful in the first place (which i definitely think it is!) for finding alembic-utils objects (like views defined throughout a set of database models), the current implementation suffers from lack of flexibility.

If, for whatever reason you cannot import a module or package in the greater context of your root package. Say, like @srohr said, due to dev-deps or optional deps. or maybe side-effects, the mutation of the MetaData as you import models, comes to mind). But whatever the reason, your only options are:

• change your organization strategy, colocate all alembic-utils-defined objects in one root location (maybe fine, but kind of annoying)
• use a series of calls for known-okay paths. (at which point it kind of defeats the purpose; you might as well individually import them)

• fork the implementation (sad)

  Truly, I think an explicit exclusions list is the most direct and least ambiguous way of solving the problem.

But an alternative that comes to mind is you might instead

try:
    module = importlib.import_module(module_import_path)
except ImportError:
    continue
else:
    yield module

This seems less ideal to me, but is really the only other obvious solution to the problem.