[issue44596] Operand coercion breaks MappingProxyType encapsulation

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

It is a duplicate of issue43838.

--
nosy: +serhiy.storchaka
resolution:  -> duplicate
stage: needs patch -> resolved
status: open -> closed
superseder:  -> There is a way to access an underlying mapping in 
MappingProxyType

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[issue44596] Operand coercion breaks MappingProxyType encapsulation

[Nick Coghlan]

New submission from Nick Coghlan :

The fast locals proxy implementation for PEP 558 used the existing 
MappingProxyType implementation as a starting point.

This meant I noticed the following code in the mapping proxy comparison 
implementation:

==
static PyObject *
mappingproxy_richcompare(mappingproxyobject *v, PyObject *w, int op)
{
return PyObject_RichCompare(v->mapping, w, op);
}
==

Seeing that code lead to trying the following experiment:

==
>>> from types import MappingProxyType
>>> d = {}
>>> proxy = MappingProxyType(d)
>>> class Sneaky:
... def __eq__(self, other):
... if other is d:
... raise RuntimeError("Broke encapsulation")
... 
>>> proxy == Sneaky()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 4, in __eq__
RuntimeError: Broke encapsulation
==

The new `__or__` implementation has a corresponding loophole:


>>> class SneakyOr:
... def __or__(self, other):
... if other is d:
... raise RuntimeError("Broke encapsulation")
... def __ror__(self, other):
... return self.__or__(other)
... 
>>> proxy | SneakyOr()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 6, in __ror__
  File "", line 4, in __or__
RuntimeError: Broke encapsulation


Delegating these operations to the underlying mapping via the abstract 
operation layer means that the underlying mapping gets exposed to the operand 
coercion machinery and can hence be accessed by the other operand.

--
messages: 397242
nosy: ncoghlan
priority: normal
severity: normal
stage: needs patch
status: open
title: Operand coercion breaks MappingProxyType encapsulation
type: behavior
versions: Python 3.11

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