KITFORMA · EDITORIAL ANSWER
Why does a Python function remember items from the previous call?
KitForma editorial guide. A mutable default argument is created when the function is defined, not each time it runs. A default list can therefore become accidental shared state.
Step-by-step answer
A mutable default argument is created when the function is defined, not each time it runs. A default list can therefore become accidental shared state.
Use
def collect(item, bucket=None): and create bucket = [] only when bucket is None; then append and return it. Call the function twice without a bucket and confirm the returned lists are different objects. Also pass an explicit empty list and verify that it is reused intentionally. Do not use bucket = bucket or []: that silently replaces a caller’s empty list. A deliberately shared cache needs a separate documented lifetime and concurrency policy.
Minimal example:
def collect(item, bucket=None):
if bucket is None:
bucket = []
bucket.append(item)
return bucket
a, b = collect("a"), collect("b")
assert a == ["a"] and b == ["b"] and a is not b
owned = []
assert collect("x", owned) is ownedSources and verification
Sources checked:
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