Python: this list grows by ITSELF?!PRO

A default list is made once, so every call shares it. The box=None fix.

# This list grows by ITSELF ?! — a default list is made ONCE, so every call shares it.
def add(item, box=[]):
    box.append(item)
    return box

print(add(1))   # [1]
print(add(2))   # [1, 2] ?!
print(add(3))   # [1, 2, 3] ?!

# Fix: default to None, make a new list inside
def add_fixed(item, box=None):
    if box is None:
        box = []
    box.append(item)
    return box

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