Arguments get passed to functions as references. When the function is called the values of the arguments are copied to the parameter variables. For lists and dictionaries a copy of the reference is used for the parameter.

def eggs(someParameter):
    someParameter.append('Hello')

spam = [1, 2, 3]
eggs(spam)
print(spam)

i[akraker@localhost python]$ python passingReference.py
[1, 2, 3, 'Hello']

Because someParameter and spam both use a copy of the reference, the list is modified.

Note, forgetting this can be the source of bugs.