Beware of mutable default arguments

The default arguments are set upon function definition. Changing mutable arguments inside a function has a side effect when working with default values, for example:

def my_list(x1, x2 = []):
    x2.append(x1)
    return x2
my_list(1)  # returns [1]
my_list(2)  # returns [1,2]

Recall, lists are mutable objects.