#!/bin/env python
# Default arguments are evaluated just once and in a static scope
# To avoid sharing of the default value between subsequent calls, a local definition for a non-existing arg can be used:
def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

print (f(1))
print (f(2))

# this will print
#  [1]
#  [2]