python check if a method is called without mocking it away

You can set the Mock.side_effect to be the original method.

from unittest.mock import MagicMock

class A():
    def tmp(self):
        print("hi")

def b(a):
    a.tmp()

a = A()
a.tmp = MagicMock(side_effect=a.tmp)
b(a)
a.tmp.assert_called()

When side_effect is a function (or a bound method in this case, which is a kind of function), calling the Mock will also call the side_effect with the same arguments.

The Mock() call will return whatever the side_effect returns, unless it returns the unnittest.mock.DEFAULT singleton. Then it will return Mock.return_value instead.