PoC of an interesting buggy behavior in Python.

large.py

#!/usr/bin/env python2
# -*- coding: utf-8 -*-

"""
PoC of an intresting buggy behavior in Python.
"""

from pprint import pprint


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 copy(self):
        return Mutable(attr=self.attr)


class Meta:
    def __init__(self, mutable=None):
        self.mutable = Mutable() if mutable is None else mutable

    def __hash__(self):
        return hash(repr(sorted(vars(self).items())))

    def __repr__(self):
        return "<%s.%s object at %s - %s>" % (
            self.__class__.__module__,
            self.__class__.__name__,
            hex(id(self)),
            self.mutable,
        )

    def change(self):
        self.mutable.change()

    def copy(self):
        return Meta(
            mutable=self.mutable,
        )

    def deepcopy(self):
        return Meta(
            mutable=self.mutable.copy(),
        )


def check(d):
    # print(d)
    for key in d:
        print(key in d)


def main():
    # dic = dict()
    # mut = Mutable()
    # for i in range(2):
    #     key = Meta(mutable=mut)
    #     extended_key = key.copy()
    #     extended_key.mutable.attr = i + 1
    #     dic[extended_key] = key
    #
    #     check(dic)

    # dic = dict()
    # mut = Mutable()

    # for i in range(2):
    #     key = Meta(mutable=mut)
    #     extended_key = key.deepcopy()
    #     extended_key.mutable.attr = i + 1
    #     dic[extended_key] = key

    #     check(dic)

    dic = dict()
    key = Mutable()
    dic[key] = "value"
    check(dic)
    key.change()
    check(dic)


if __name__ == "__main__":
    main()

small.py

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

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


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()