Patching imports with monkeypatch #945

I have some tests where I need to patch importing of modules, to test e.g. if a module behaves properly when some optional dependency couldn't be imported.

I already have something to monkeypatch builtins.__import__ and importlib.import_module, in my testsuite, but I thought it might be a good addition to pytest as it's not trivial to get right, and there are differences between Python 2 and 3.

Proposed API:

• New monkeypatch.import(name, obj, module=None) method
  • name: The name of the modified import
  • obj: The object which should get imported instead, or ImportError to raise an ImportError on import
  • module: If given (as a dotted string path), importlib.import_module and maybe others are patched as well for this module.

Caveats:

• import is patched globally for the duration of the test, not only in the given module - the module parameter is only for importlib.import_module and similar things.
• Making ImportError for obj special cased might be weird? But I can't think of a better API.

What do you think? Would this be something which is generally useful or should I keep it in my own testsuite?