PoC of an interesting buggy behavior in Python

mutable-dict-keys.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

"""
PoC of an interesting buggy behavior in Python. Also works in python2.

$ ./mutable-dict-keys.py
{Mutable(0): 'value'} is consistent
{Mutable(1): 'value'} is not consistent
"""


class Mutable():
    __slots__ = ['attr']

    def __init__(self, attr=0):
        self.attr = attr

    def __hash__(self):
        return hash((self.attr))

    def __repr__(self):
        return "%s(%s)" % (
            self.__class__.__name__,
            self.attr,
        )

    def change(self):
        self.attr += 1


def check(d):
    for key in d:
        if key not in d:
            print("%s is not consistent" % d)
            return False

    print("%s is consistent" % d)
    return True


def main():
    dic = dict()
    key = Mutable()
    dic[key] = "value"
    check(dic)          # Return True
    key.change()
    check(dic)          # Return False


if __name__ == '__main__':
    main()