Python Default Arguments

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Default Arguments

Defaults make APIs ergonomic—learn how they evaluate and how to avoid the infamous mutable-default bug.

Review syntaxSee best practices

Syntax

def connect(host="localhost", port=5432):
    ...

Defaults evaluate once at function definition time, not on every call.

Mutable defaults warning

def append_item(item, items=[]):
    items.append(item)
    return items

Each call shares the same list. Fix it by using None sentinel:

def append_item(item, items=None):
    if items is None:
        items = []
    items.append(item)
    return items

Ordering rules

Positional-only → positional/keyword → varargs (*args) → keyword-only → **kwargs. Defaults must follow required parameters.

Next up in your learning path

Argument types

Combine defaults with keyword-only args.

Companion

Return values

Document defaults with type hints.

Reference

Function pillar

See defaults in the bigger context.

Guide