Mutable Default Arguments Are Shared Between Calls in Python

Default argument values are evaluated once, when the def statement runs, not on every call. A mutable default such as [] is therefore the same object on every call, and changes to it pile up.

CommonplaceJun 3, 2024Source

>>> def foo(s=[]):
...     s.append(1)
...     return s
...
>>> foo()
[1]
>>> foo()
[1, 1]

The usual fix is a None sentinel, creating the fresh object inside the function:

def foo(s=None):
    if s is None:
        s = []
    s.append(1)
    return s

The defaults live on the function object, in foo.__defaults__, which is why they persist between calls.
Immutable defaults (0, "", None, tuples) are safe: they can’t be changed in place.