When you use a mutable object (like a list or dictionary) as a default argument in a Python function, it’s created once when the function is defined, not each time the function is called. This means modifications persist across all future calls.

Here’s the problem:

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

my_list = append_to(12)
print(my_list)  # [12]

my_other_list = append_to(42)
print(my_other_list)  # [12, 42] — unexpected!

The second call returns [12, 42] because both calls share the same default list object.

The solution: Use None as the default and create a new object inside the function:

def append_to(element, to=None):
    if to is None:
        to = []
    to.append(element)
    return to

my_list = append_to(12)
print(my_list)  # [12]

my_other_list = append_to(42)
print(my_other_list)  # [42]

This ensures each call gets its own independent list.

Original source