mock_instance_method.py

class Foo():
    message = 'foo'
    
    def bar(self):
        print('bar')

from unittest.mock import patch

def baz(self):
    print(self.message)

@patch.object(Foo, 'bar', autospec=True, side_effect=baz)
#                         ^^^^^^^^^^^^^
#                         this is essential
def test(mock_bar):
    foo = Foo()
    foo.bar()
    
    assert mock_bar.call_count == 1

test()