How to include conditional installs -- namely for backports

[thorwhalen]

Let's start with a concrete example.

It often happens that I want to use from importlib.resources import files, but that only exists in 3.9+.

In my code, I end up doing:

try:
    from importlib.resources import files  # ... and any other things you want to get
except (ImportError, ModuleNotFoundError):
    from importlib_resources import files  # pip install importlib_resources

But I feel shame when copy/pasting this code everywhere. Additionally, it still doesn't solve my problem "completely".
The question is: Do I include importlib_resources, a third-party backport of importlib.resources, in my setup.cfg install_requires? If I don't, my code will fail on python < 3.9. If I do include it, I'm overkilling my dependencies.

Ideally, I'd like a solution where I can just do

from importlib_resources import files

in my code, and trust that the setup.cfg took care of doing two things:

• telling the install to only do a pip install importlib-resources if python version < 3.9
• telling it that importlib_resources is an alias for importlib.resources if in 3.9+, and of importlib_resources (the pip install backport) if not.

Trying to solve the first problem only, I did this.

install_requires =
    dol
    importlib_resources; python_version < '3.9'

But it doesn't condition the install at all (it's installed on both 3.8 and 3.10!).

Not sure how to even start with the second "aliasing" desire.

[thorwhalen]

Posted the question to this stackoverflow question.

[thorwhalen]

In the first stackoverflow comment, the responder says we can't get rid of the need of the "conditional import" code, and mentions this pattern as an alternative to do it:

if sys.version_info >= (3, 8):
    import importlib.metadata as importlib_metadata
else:
    import importlib_metadata

It has its merits: We'd be more explicit there.
But what I don't like about it is that it has more error-prone "surface".
We could make a mistake in the version threshold or the comparison operator (>=, vs >, vs <=, vs <),
and we'd reuse it via copy paste, we'd have to make sure to edit the version too.

In the try-except block we only have to change the import statements.

[thorwhalen]

Unless we put it in a often used package, like i2, there's no real point in encompassing the pattern in a function, but since I was at it, I wrote a function for this and put it in wads.util.

Here's the code without the docs:

def find_obj(*module_and_obj_names):
    module_and_obj_names = list(map(_ensure_pair, module_and_obj_names))
    for module_name, obj_name in module_and_obj_names:
        try:
            return import_obj(module_name, obj_name)
        except (ImportError, ModuleNotFoundError, AttributeError):
            continue
    from_import_statements = "\n\t" + "\n\t".join(
        map("from {} import {}".format, *zip(*module_and_obj_names))
    )
    raise ImportError(f"All of these import attempts failed:{from_import_statements}")

def import_obj(module_name: str, obj_name: str):
    module = __import__(module_name, fromlist=[obj_name])
    obj = getattr(module, obj_name)
    return obj

def _ensure_pair(x):
    if isinstance(x, str):
        x = x.split()
    assert len(x) == 2, f"should be a pair of strings: {x}"
    return x

Example usage:

files = find_obj('importlib.resources files', 'importlib_resources files')