CODEWITHSUNDEEP
pythonpython-importimporterror
Ark-kun
Ark-kun

Reputation: 6812

Making imports work when creating "alias" for a package

Real goal: I have a module that is common between two packages (say, bar and bar2). I want to use exact same test files for both cases so I want to change the test imports to not name the package explicitly. (Why? This can be useful during the process of extracting modules from a mega-package into separate packages.)

My idea was to add another module that imports a particular package and provides an "alias" for it. It almost worked, but I got a problem.

Initially I had:

# test.py:
from bar import some_function

If I do nothing magical, there will be two versions of test.py: one with from bar import some_function and another with from new_bar import some_function. I want to avoid this and have the test code files remain the same.

After I added indirection:

#foo.py:
import bar as baz

#test.py:
from .foo import baz  # Works!
from .foo.baz import some_function  # ModuleNotFoundError: No module named 'cur_dir.foo.baz'; 'cur_dir.foo' is not a package

I can make foo a package:

#foo/__init__.py:
import bar as baz

#test.py:
from .foo import baz  # Works!
from .foo.baz import some_function  # ModuleNotFoundError: No module named 'cur_dir.foo.baz'

The error changes a bit, but still remains.

I know that I can work around the problem by writing

# test.py:
from .foo import baz
some_function = baz.some_function

Is there any other way? I want my imports to be "normal".

Is there a way to create an "alias" for a package that can be used with the standard import mechanism?

Upvotes: 0

Views: 319

Answers (1)

MisterMiyagi
MisterMiyagi

Reputation: 51979

The import statement only looks at actual modules and their paths, not at aliases inside the loaded modules. An actual module alias in Python's module registry, sys.modules, is required.

import sys
import os

sys.modules["os_alias"] = os  # alias `os` to `os_alias`
import os_alias               # alias import works now
from os_alias import chdir    # even as from ... import ...

Once a module alias has been added to sys.modules, it is available for import in the entire application.

Note that module aliasing can lead to subtle bugs when submodules of aliased modules are loaded. In specific, if the submodules are not aliased explicitly, separate versions are created that are not identical. This means that any tests based on object identity, including isinstance(original.submodule.someclass(), alias.submodule.someclass), will fail if the versions are mixed.

To avoid this, you must alias all submodules of any aliased package.

Upvotes: 1

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