foo.py

class Foo:
  def a(self):
    ...

  def b(self):
    ... #this doesn't return anything, but has side effects that are important, but expensive to poll for

class UsesFoo():
  def __init__(self,foo):
    self.foo = foo

  def run(self):
    self.foo.a()
    if some_condition:
      self.foo.b()

#in test code
from testing.mock import patch
def test_b_not_called():
  with patch.object(Foo,"b"):
    foo = Foo()
    u = UsesFoo(foo)
    u.run()
    foo.b.assert_not_called()