JuniorCodeVery common

What does this mutable default argument print?

Predict the output of these two calls.

Python

def add(item, bucket=[]):
    bucket.append(item)
    return bucket

print(add(1))
print(add(2))

Determine the output and explain.

[1] then [1, 2]. The default [] is created once, at function-definition time, and reused across calls, so state leaks between them. Fix with a sentinel: def add(item, bucket=None): bucket = [] if bucket is None else bucket.

Thinking default arguments are re-evaluated on each call
Expecting a fresh list per call
Not knowing the None-sentinel fix

Why are default arguments evaluated at definition time rather than at call time?
How exactly does the None sentinel pattern fix this?

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