When you use Python optional arguments you should know that the default value you set is static, it is set at the time the function is defined, and does not change after that. If you a mutable type as the default value, you need to be careful.
Ex:
>>> def enlist(n, lst=[]):
... lst.append(n)
... return lst
...
>>> enlist(3)
[3]
>>> enlist(4)
[3, 4]
>>> enlist(5)
[3, 4, 5]
>>> enlist(6)
[3, 4, 5, 6]
Everytime you call the enlist function, the list gets bigger, because lst is never reset to the empty list. It is set to the empty list only once, when enlist was defined. After that it references the same instance everytime you don't supply a lst argument.