#!/bin/env python
# Default arguments
# From: https://docs.python.org/3/tutorial/controlflow.html#defining-functions
# NB: The default value is evaluated only once!

def f(a, L=[]):
    L.append(a)
    return L

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

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