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Default parameters

parameters

Parameters may have a default value. A mutable object as a default is a trap! Use the None pattern instead.

Syntax

def f(x, y=10): ...

A default value is evaluated once, when the function is defined — not on every call. So the same list or dict survives between calls and accumulates changes; the None pattern (y=None, then if y is None: y = [] inside) builds a fresh object each time.

Examples

def greet(name='World'):
    return f'Hello, {name}!'
print(greet())  # → Hello, World!
print(greet('Alice'))  # → Hello, Alice!

# Ловушка: изменяемый default
def bad(lst=[]):
    lst.append(1)
    return lst
print(bad())  # → [1]
print(bad())  # → [1, 1] — тот же список!

# None-паттерн (правильно)
def good(lst=None):
    if lst is None:
        lst = []
    lst.append(1)
    return lst
print(good())  # → [1]
print(good())  # → [1]

def connect(host='localhost', port=5432):
    return f'{host}:{port}'
print(connect())  # → localhost:5432
print(connect(port=3306))  # → localhost:3306

See also

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