Functional programming & distributed systems blog

Yuriy Netesov on programming and stuff

Python default params trap

Intro

foo.py:

def foo(bar=[]):
        bar.append('bar')
        print bar

    foo()
    foo()
    foo()

output:

['bar']
['bar', 'bar']
['bar', 'bar', 'bar']

Expalation

This is slightly not expected result can be fully explained by the fact that bar got as default parameter a reference to a list, not list itself. So each time we call a foo we deal with the same list. When interpreted came to a function definition it executed a code bar=[]. And then this code is not executed anymore.

Prof

Lest change our foo.py a bit and then execute it:

def make_a_list():
    print 'making list'
    return []


def foo(bar=make_a_list()):
    bar.append('bar')
    print bar

foo()
foo()
foo()

output:

making list
['bar']
['bar', 'bar']
['bar', 'bar', 'bar']

As we see - makealist was called only once when compiler come to a point of function definition.