Default values are worked out once, when the def line runs, not each time the function is called. For numbers and strings that never matters. For a list, it causes one of Python's most famous bugs:

def add_item(item, basket=[]):
    basket.append(item)
    return basket

print(add_item("apple"))
print(add_item("pear"))

The second call still has the apple in it, because both calls share the same default list. The fix is to use None as the default and make a fresh list inside:

def add_item(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

print(add_item("apple"))
print(add_item("pear"))