Python Gotcha : Mutable Default Arguments ( Python’s default arguments are evaluated once when the function is defined, not each time the function is called. This means that if you use a mutable default argument and mutate it, you will and have mutated that object for all future calls to the function as well.)

mutable_default_args.py

def append_to(element, to=[]):
    to.append(element)
    return to


my_list = append_to(12)
print(my_list)

my_other_list = append_to(42)
print(my_other_list)


# expected output : [12] [24]
# actual output : [12] [12, 42]