Mutable Default Function Arguments


def func(val, arg1=[], arg2={}):
    arg2[val] = str(val)
    print("arg1:", arg1)
    print("arg2:", arg2)

What does the function print on the call to func('c')?

Expected Result

arg1: ['c']
arg2: {'c':'c'}

Actual Result

arg1: [1, 2, 'c']
arg2: {1: '1', 2: '2', 'c': 'c'}


Default function arguments in Python are persistent. The function is created once and then called multiple times. The default arguments are mutable, so despite being persistent they can be overwritten.

If your intention is to have the default values created fresh each time, then one option would be to give the default argument a value of None and then use an if check to set it to what is wanted:

def func(val, arg1=None, arg2=None):

    # Set the arguments to the desired defaults here
    if arg1 is None:
        arg1 = []
    if arg2 is None:
        arg2 = {}

    arg2[val] = str(val)

This would give the original expected output of:

arg1: ['c']
arg2: {'c':'c'}