FresherFresher (0–1 yrs)CodingPythonTCSInfosysWipro

mutable default argument pitfall

Why is `def f(a=[])` dangerous in Python? Show the bug and the correct pattern.

Answers use simple, clear English.

Quick interview answer

Default args are evaluated once at function definition. A mutable default is shared across calls — appends accumulate. Correct: use None sentinel and create a new list inside.

Detailed answer

Default args are evaluated once at function definition. A mutable default is shared across calls — appends accumulate. Correct: use None sentinel and create a new list inside. Same for dict/set defaults.

Real example & use case

Helper that accumulates options accidentally shares list between requests.

Pros & cons

Pros of sentinel: fresh mutable each call. Cons: slightly more verbose.

Code example

# bad
def add_bad(x, bucket=[]):
    bucket.append(x)
    return bucket

# good
def add_good(x, bucket=None):
    if bucket is None:
        bucket = []
    bucket.append(x)
    return bucket

Practice code · python (view only · no execution)

# bad
def add_bad(x, bucket=[]):
    bucket.append(x)
    return bucket

# good
def add_good(x, bucket=None):
    if bucket is None:
        bucket = []
    bucket.append(x)
    return bucket

#python#pitfalls#defaults