blah.py

def run(payment=payment, **kwargs): # here the trick is to pass the payment instance as default value.
  # Since we have an instance of payment in parameter now, we can inject it in the tests easily.
  # Check that the client payed.
  try:
      contains_payment = payment.contains_payment(_price, request.headers, **kwargs)
  except BadRequest as e:
    return Response(e.description, BAD_REQUEST)
  
  # Actually do stuff
  run_params = request.get_json(silent=False)
  ...
  
  
# In the tests:
def test_payment_method(self):
  
  class FakePayment(object): # That's a Fake. It looks like a Payment, with only the method we care about
    def contains_payment(self):
      return True # You have to write a test that returns False as well!
  
  result = run(payment=FakePayment())
  # (assert somehting on the result)