Two Objects Created from the Same Class, isinstance = False

Question

I'm trying to create some unit tests for some code here at work.

The code takes in an object and based on information within that object imports a specific module then creates an instance of it.

The test I am trying to write creates the object and then I check that it is an instance of the class I expect it to import. The issue is the isinstance check is failing.

Here is what my test looks like.

import importlib
from path.to.imported_api import SomeApi

api = importlib.import_module("path.to.imported_api").create_instance()  # create_instance() is a function that returns SomeApi().

assert isinstance(api, SomeApi)  # This returns false, but I am not sure why.

Answer 1

The reason for the difference is, that whereas both objects refer to the same module, they get different identifiers as you load a new module and bypass sys.modules. See also the explanation here: https://bugs.python.org/issue40427

A hack might be to compare the name:

assert isinstance(api.__class__.__name__, SomeApi.__name__)

Answer 2

There are a few things that could cause that: So first, it could be that the api is just returning something that looks like SomeApi(). Also it coud is be that SomeApi is overwriting isinstance behaviour.