UpdatedAug 4, 2026

Mutable Default Arguments

CONCEPTS:Python Mutable Default Arguments

Question Variations

"Why does a list default argument retain values between calls?"
"How would you safely define an optional list parameter in Python?"
"Are immutable default arguments affected by this behavior?"
"When is shared function-level state appropriate?"

Why This Is Asked

This question tests whether you understand Python’s evaluation model rather than only its syntax. It also reveals whether you can spot a subtle source of state leakage in APIs and write a safe alternative.

Key Concepts

Default argument expressions are evaluated once, when the def statement runs.
A mutable default object is reused by every call that omits the argument.
Use None as a sentinel and create a new mutable object inside the function.
Intentional shared defaults are rare and should be made explicit instead.

Question Variations

“Why does a list default argument retain values between calls?”
“How would you safely define an optional list parameter in Python?”
“Are immutable default arguments affected by this behavior?”
“When is shared function-level state appropriate?”

Answers by Technology

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Expected Answer (Python 3.14)

Python evaluates default values when the function is defined, not each time it is called. A list, dictionary, or set used as a default is therefore one shared object. Mutating it in one call changes what later calls observe.

Use None as the default and construct the list inside the function.

def add_tag(tag, tags=None):
    if tags is None:
        tags = []
    tags.append(tag)
    return tags

assert add_tag("python") == ["python"]
assert add_tag("testing") == ["testing"]

Why It Matters

This bug can cause requests, users, or test cases to leak state into one another. The failure is especially confusing because it depends on call history and may not appear in isolated tests.

Common Mistakes

Using [] or {} as an optional parameter default: The container is shared by calls that omit it.
**Writing if not tags: instead of if tags is None: An intentionally supplied empty list is valid and should not be replaced.
Assuming every default is unsafe: Immutable defaults such as strings, tuples containing immutable values, and integers cannot be mutated in place.

Follow-up Questions

When is the default expression evaluated? (Answer: Once, when the function definition executes.)
Does a tuple default always avoid shared-state issues? (Answer: Only when its contents are immutable; it may contain mutable objects.)