Mutable default argument bug

Question

Accepted Answer

The default list [] is created once when the def statement runs. Every call that omits lst shares that same list object, so items accumulate across calls. The first call appends 1 and returns [1]. The second call appends 2 to the same list and returns [1, 2]. The fix is def append_to(item, lst=None) with lst = [] inside the function body.