How to extend Class instance

Question

MyClass is defined in module.py. There is no way we can modify it. But we do know the Class definition looks like this:

class MyCla         


        

Answer 1

This is an alternative to wim's answer that also involves monkey-patching. However, it does it through functionality provided by unittest.mock. The advantage of this approach is that a context manager is used to automatically apply and remove the patch within a limited scope:

from unittest import mock

# This class would be defined in some third-party library
class MyClass:
    def method(self, msg):
        print('from method:', msg)


def function(msg):
    print('from function:', msg)


old_method = MyClass.method


def new_method(self, msg):
    old_method(self, msg)
    function(msg)


# The patch is only applied within this scope
with mock.patch.object(MyClass, 'method', new_method):
    foo = MyClass()
    foo.method('message with patched')

# By this point MyClass is "back to normal"
print('---')
foo.method('message with original')

Output

from method: message with patched
from function: message with patched
---
from method: message with original