How to test code paths that require pytest dependencies *NOT* be installed?

[posita]

I have a package that will opportunistically take advantage of typing-extensions when running in Python 3.7 if available, but will operate at reduced functionality without it. I would like to be able to run my entire battery of tests to ensure they all pass both with and without typing-extensions.

However, as far as I can tell, pytest sweeps in typing-extensions for Python 3.7. (More specifically, it requires importlib-metadata, which requires typing-extensions.)

How does one go about testing code paths that are only executed running an entire suite of tests when typing-extensions is unavailable in that environment?

Consider: I have a core component A in my project proj. Pretty much everything in proj depends on A. Under the covers I have two different implementations of A: AwithB if dependency B is found, and a reduced-functionality stand-in AwithoutB if it is absent. (B is an optional dependency of proj.) How do I go about running all the tests I have for proj if B is always swept in by pytest?

I feel like I can't be the first person to encounter or think about this.

UPDATE: Could this be done using something akin to a context manager that temporarily sets sys.modules['typing_extensions'] to None (e.g., something like this). Maybe I can configure that via conftest.py or a hook wrapper?

[RonnyPfannschmidt]

More context is needed to give a good answer, is this a tool, or just about type specs

[posita]

This is an expression of an unmet need. It's an observation about the limitations of pytest (i.e., inclusion of its own dependencies in the testing environment that are inseparable from those available to the code being tested) and how that fights with my particular requirement that one such dependence very much not be present in the testing environment.

This is a question about practices for addressing that shortcoming. I have no opinion about the form of a solution. I'm open.

[q0w]

Check this: https://youtu.be/PXu3KCMT3l4

[posita]

That's actually a pretty cool trick, but that how does one mock the absence of a dependency for the entire test suite?

[posita]

I suppose one answer is to refactor the implementation to enable something like dependency injection for the with- and without-optional dependency cases. Then I suppose you could use some environment-based configuration (e.g., an environment variable or some other initialization machinery) to override injecting the set of dependencies you're trying to test. But this isn't always practical, especially when inheriting a large codebase.