Mock an `ImportError` with pytest

monkeypatch builtins importlib

Source

• https://stackoverflow.com/a/2481588/12780516

• use the (already defined) pytest monkeypatch fixture to temporarily modify the state of the builtins module and the sys.modules dict
• define a custom import function that raises ImportError when a certain module (in this case named labbcan) is requested to be imported
• force a reload of the module that imports the module that should be mocked to be not existent (in this case cetoni_qtro_models.linear_scaling)
  → this reload will in turn try to import labbcan again which will fail with an ImportError

import builtins
import importlib
import sys
from collections.abc import Mapping, Sequence
from types import ModuleType
 
import pytest
 
realimport = builtins.__import__
 
 
def no_labbcan_import(
    name: str,
    globals: Mapping[str, object] | None = None,  # noqa: A002
    locals: Mapping[str, object] | None = None,  # noqa: A002
    fromlist: Sequence[str] = (),
    level: int = 0,
) -> ModuleType:
    if name.startswith("labbcan"):
        raise ImportError
    return realimport(name, globals, locals, fromlist, level)
 
 
def test_import_error(monkeypatch: pytest.MonkeyPatch) -> None:
    with monkeypatch.context() as m:
        m.setattr(builtins, "__import__", no_labbcan_import)
        m.delitem(sys.modules, "cetoni_qtro.models.linear_scaling")
 
        importlib.reload(cetoni_qtro.models)
        from cetoni_qtro.models.linear_scaling import LinearScaling
        # test what ever you like without labbcan being imported if the reloaded module already
        # handles the `ImportError` gracefully
        
        # or
        with pytest.raises(ImportError):
            importlib.reload(cetoni_qtro.models)
            from cetoni_qtro.models.linear_scaling import LinearScaling