Crash in 1.3.x when a cmd2.Cmd method is mocked

[tleonhardt]

If you mock a method within a cmd2 which isn't a command or subcommand method using a MagicMock such as with with mock.patch() or with mock.patch.object(), then things crash because our code thinks it is a subcommand.

Here is a simple unit test that can be added to any of the cmd2 tests files such as test_cmd2.py to reproduce this problem:

class MockMethodApp(cmd2.Cmd):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def foo(self):
        pass


def test_mocked_methods():
    with mock.patch.object(MockMethodApp, 'foo'):
        cli = MockMethodApp()

And there is the test failure traceback:

    def test_mocked_methods():
        with mock.patch.object(MockMethodApp, 'foo'):
>           cli = MockMethodApp()

tests/test_cmd2.py:2398: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/test_cmd2.py:2390: in __init__
    super().__init__(*args, **kwargs)
cmd2/cmd2.py:415: in __init__
    self._register_subcommands(self)
cmd2/cmd2.py:646: in _register_subcommands
    command_func = self.cmd_func(command_name)
cmd2/cmd2.py:2406: in cmd_func
    func_name = self._cmd_func_name(command)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <tests.test_cmd2.MockMethodApp object at 0x10c28d8e0>, command = <MagicMock name='foo.parent_command.split().__getitem__()' id='4498776416'>

    def _cmd_func_name(self, command: str) -> str:
        """Get the method name associated with a given command.
    
        :param command: command to look up method name which implements it
        :return: method name which implements the given command
        """
        target = constants.COMMAND_FUNC_PREFIX + command
>       return target if callable(getattr(self, target, None)) else ''
E       TypeError: getattr(): attribute name must be string

cmd2/cmd2.py:2417: TypeError
=============================================================================================== short test summary info ================================================================================================
FAILED tests/test_cmd2.py::test_mocked_methods - TypeError: getattr(): attribute name must be string

[tleonhardt]

Its important to note that this can be fixed on the test side by changing the MagicMock to use spec=True like so:

def test_mocked_methods():
    with mock.patch.object(MockMethodApp, 'foo', spec=True):
        cli = MockMethodApp()

But we should do something on the cmd2 side to help with this, I'm just not 100% sure what.

At a minimum, we should have something like a "Testing cmd2 applications" section in our documentation with recommended best practices where we warn about this sort of thing. But that might not be very discoverable. So an improved exception message would also be good.