How to setup return_value for a function call with specific signature?

Question

I have some code like this

class MyClass:
    def my_function(self):
        ....
        return lst

I want to test some other functions calling my_function. I can't figure out how to set the return_value for the call of function with specific signature. Is there any way to do that? This can be done very easily with Moq in .Net though...

This is what I tried but it doesn't work.

with patch("MyClass.my_function") as my_mock:
    my_mock(1, 2).return_value = [
        {
            "id" : "A"
        }
    ]

    #call the function...
    #assert...

Answer 1

my_mock(1, 2).return_value = value

What you are doing with that line is actually setting the return_value for another mock object returned by call to my_mock. The value you've assigned there will be returned if you execute m()(), not m(1, 2) as you probably expected.

return_value doesn't depend on the call signature and should be used like this:

>>> my_mock.return_value = 'foobar'
>>> my_mock()
'foobar'
>>> my_mock(1, 2)
'foobar'

If you want the return value to depend on call arguments, you should use side_effect:

>>> def mock_func(*args):
...     if args == (1, 2):
...         return 'foo'
...     else:
...         return 'bar'
...
>>> my_mock.side_effect = mock_func
>>> my_mock(1, 2)
'foo'
>>> my_mock()
'bar'

BTW, I prefer to patch methods with patch.object. I don't think patch("MyClass.my_function") will work because patch() requires target to start with package/module name.