Mock functions in a function dict in python

Let's say I have a class declared like below:

class SampleClass(object):
    def __init__(self):
       self.ops = {
           'key 1': self.function_a,
           'key 2': self.function_b
       }

    def function_a(self):
      print('Inside a')

    def function_b(self):
      print('Inside b')

    def main(self, key):
       self.ops[key]()

Based on the key I get, I choose and execute either of the 2 functions.

I am trying to unit test main function but I am not able to patch either function_a or function_b.

I am writing unit tests in a separate file called test.py.

When I try to patch function_a with :

self.sampleclass = SampleClass()
with patch.object(SampleClass, 'function_a', return_value=None) as test_function:
    self.sampleclass.main('key1')

Control goes into function_a and Inside a is printed.

How does one mock a function that is called from a function dict?