Python problem 16 of 30

The default argument trap

Python
Medium

What does this print, and why? How would you fix it?

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

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

Show hint

Default values are evaluated once, when the def line runs, not on every call.

Show solution

# Prints:
# [1]
# [1, 2]

def add(item, bucket=None):
    if bucket is None:
        bucket = []
    bucket.append(item)
    return bucket

The same list object is shared by every call that uses the default, so it keeps growing. Use None as the default and create a fresh list inside the function.

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