Example of python's mutable arguments being mutated

py_mutable_args.py

>>> def make_a_list(val1 = 0, val2 = 0, lst = []):
...    lst.append(val1)
...    lst.append(val2)
...    return lst

>>> # you need to know that you're modifying base_list
>>> base_list = [1, 2, 3, 4]
>>> extend_list = make_a_list(5, 6, base_list)
>>> print(extend_list)
[1, 2, 3, 4, 5, 6]

>>> # so it's now the same as extend_list
>>> print(base_list)
[1, 2, 3, 4, 5, 6]

>>> # good luck if you don't know it's referenced
>>> # in this case lst = [] is defined at 
>>> # function definition and is mutable
>>> my_list = make_a_list(3, 4)
>>> print(my_list)
[3, 4]

>>> other_list = make_a_list(7, 8)
>>> print
[3, 4, 7, 8]