>>> def f(number, constants, step=0):
...     if step < 2:
...         return f(number + constants[step], constants, step + 1)
...     else:
...         return number
... 
>>> f(0, [1, 2])
3
>>> f(0, [1, 2, 3])
3
>>> f(0, [3, 4, 5])
7
>>>