Can't catch mocked exception because it doesn't inherit BaseException

Question

I\'m working on a project that involves connecting to a remote server, waiting for a response, and then performing actions based on that response. We catch a couple of diff

Answer 1

For those of us who need to mock an exception and can't do that by simply patching head, here is an easy solution that replaces the target exception with an empty one:

Say we have a generic unit to test with an exception we have to have mocked:

# app/foo_file.py
def test_me():
    try:
       foo()
       return "No foo error happened"
    except CustomError:  # <-- Mock me!
        return "The foo error was caught"

We want to mock CustomError but because it is an exception we run into trouble if we try to patch it like everything else. Normally, a call to patch replaces the target with a MagicMock but that won't work here. Mocks are nifty, but they do not behave like exceptions do. Rather than patching with a mock, let's give it a stub exception instead. We'll do that in our test file.

# app/test_foo_file.py
from mock import patch


# A do-nothing exception we are going to replace CustomError with
class StubException(Exception):
    pass


# Now apply it to our test
@patch('app.foo_file.foo')
@patch('app.foo_file.CustomError', new_callable=lambda: StubException)
def test_foo(stub_exception, mock_foo):
    mock_foo.side_effect = stub_exception("Stub")  # Raise our stub to be caught by CustomError
    assert test_me() == "The error was caught"

# Success!

So what's with the lambda? The new_callable param calls whatever we give it and replaces the target with the return of that call. If we pass our StubException class straight, it will call the class's constructor and patch our target object with an exception instance rather than a class which isn't what we want. By wrapping it with lambda, it returns our class as we intend.

Once our patching is done, the stub_exception object (which is literally our StubException class) can be raised and caught as if it were the CustomError. Neat!