I need to use a utility function that exists in an external module I do not control. The module has many imports that are not used by the specific function I care about. I don't want to install those unnecessary dependencies (and especially not bloat my packages dependency list with the module).

It doesn't seem possible to use the function without the imports. I wondered if it's possible to mock the module? I had briefly hoped this would work:

""" module_a """
import unused_module

def f():
  return something_of_value


""" module_b: needs to use module_a.f() but doesn't have unused_import installed. """
import mock_module as unused_module
global unused_module
import module_a

module_a.f()

But I couldn't inject unused_module into the modules setup. Is there a workaround I could use here?

CodePudding user response：

What about this? We're exploiting the module caching system, where modules only get imported once and get stored in sys.modules which is a dict.

utili.py (which I should have called module_a.py)

""" module_a """
import unused_module

def f():
  return "something_of_value"

Then the client code that needs to use your function.

user.py

import sys

#we're not really doing anything here, just 
#slotting in a fake entry into sys.modules 
#so that it wont bother to "reimport" unused_modules
sys.modules["unused_module"]  = 1

from utili import f

print(f"{f()=}")

output of executing user.py:

python user.py
f()='something_of_value'

And, to be clear, I don't have an actual unused_module anywhere, so the import will fail if utili.py is called first.

py utili.py
Traceback (most recent call last):
  File "/Users/myuser/explore/test_384_mockmo/utili.py", line 2, in <module>
    import unused_module
ModuleNotFoundError: No module named 'unused_module'

Things will also not go well if any code tries to use unused_module so your f definitely can't require its actual existence. Ditto for any code that gets executed at the module load of utili.py. You could however assign a mock of some sorts instead of the litteral 1.