Python gotcha

pdo sw python devel eng

This code:

#!/usr/bin/python3

class Test:
    def __init__(self, items=[]):
        self.items = items

    def add(self, item):
        self.items.append(item)


a = Test()
a.add("foo")
b = Test()
b.add("bar")
print(repr(a.items))
print(repr(b.items))

"obviously" prints:

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

Because the default value of the items argument is a mutable list constructed just once ~~when the code is compiled~~ when the function definition is evaluated, and then reused.

So, in Python, mutable items in default arguments are a good way to get more fun time with debugging.