Test Recursive Python Function

I have a recursive function that I'm looking to test, however I'm having difficulty limiting the recursive call during testing. For example, below is a simple example function of a receive function that calls a bool_function(n) to check if it should break the recursive loop.

def factorial(n):
  if bool_function(n):
      return 1
  else:
      return n * factorial(n-1)

What would be the best way to test or mock bool_function(n) so that it is true for the first iteration and false for any call after?