René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"3.12.0b1 includes backwards incompatible change to operation of `super()`","articleBody":"# Bug report\r\n\r\nThe optimisation to LOAD_SUPER_ATTR introduced by #104270 appears to have introduced a backwards incompatibility.\r\n\r\nThe following code will reproduce the problem:\r\n```\r\nclass MyInstance:\r\n    def __new__(cls, ptr, name, bases, attrs):\r\n        self = super().__new__(cls, name, bases, attrs)\r\n        print(f\"{MyInstance=} {self=} {type(self)=}, {super(MyInstance, type(self))=}\")\r\n        super(MyInstance, type(self)).__setattr__(self, \"ptr\", ptr)\r\n        return self\r\n\r\n    def __setattr__(self, name, value):\r\n        raise Exception()\r\n\r\n\r\nclass MyClass(MyInstance, type):\r\n    def __new__(cls, name):\r\n        self = super().__new__(cls, id(name), name, (MyInstance,), {})\r\n\r\n        return self\r\n\r\n\r\nclass1 = MyClass(\"Class1\")\r\nprint(f\"{class1.ptr=}\")\r\n\r\nclass2 = MyClass(\"Class2\")\r\nprint(f\"{class2.ptr=}\")\r\n```\r\nUnder 3.12.0a7 (and previous stable Python versions going back to at least 3.7), this will succeed, outputting:\r\n```\r\nMyInstance=\u003cclass '__main__.MyInstance'\u003e self=\u003cclass '__main__.Class1'\u003e type(self)=\u003cclass '__main__.MyClass'\u003e, super()=\u003csuper: \u003cclass 'MyInstance'\u003e, \u003cMyClass object\u003e\u003e\r\nclass1.ptr=4364457392\r\nMyInstance=\u003cclass '__main__.MyInstance'\u003e self=\u003cclass '__main__.Class2'\u003e type(self)=\u003cclass '__main__.MyClass'\u003e, super()=\u003csuper: \u003cclass 'MyInstance'\u003e, \u003cMyClass object\u003e\u003e\r\nclass2.ptr=4365761904\r\n```\r\n\r\nUnder 3.12.0b1, it raises an error:\r\n```\r\nMyInstance=\u003cclass '__main__.MyInstance'\u003e self=\u003cclass '__main__.Class1'\u003e type(self)=\u003cclass '__main__.MyClass'\u003e, super()=\u003csuper: \u003cclass 'MyInstance'\u003e, \u003cMyClass object\u003e\u003e\r\nclass1.ptr=4370144336\r\nMyInstance=\u003cclass '__main__.MyInstance'\u003e self=\u003cclass '__main__.Class2'\u003e type(self)=\u003cclass '__main__.MyClass'\u003e, super()=\u003csuper: \u003cclass 'MyInstance'\u003e, \u003cMyClass object\u003e\u003e\r\nTraceback (most recent call last):\r\n  File \"/Users/rkm/beeware/rubicon/objc/repro.py\", line 24, in \u003cmodule\u003e\r\n    class2 = MyClass(\"Class2\")\r\n             ^^^^^^^^^^^^^^^^^\r\n  File \"/Users/rkm/beeware/rubicon/objc/repro.py\", line 16, in __new__\r\n    self = super().__new__(cls, id(name), name, (MyInstance,), {})\r\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n  File \"/Users/rkm/beeware/rubicon/objc/repro.py\", line 7, in __new__\r\n    super().__setattr__(self, \"ptr\", ptr)\r\nTypeError:  expected 2 arguments, got 3\r\n```\r\n\r\nThat is - the first `MyClass` instance can be instantiated; however, the second instance fails in mid-construction when invoking `__setattr__`. The state of the objects prior to the `__setattr__` invocation appear to be identical, but the `__setattr__` method behaves differently on the second invocation.\r\n\r\nGit bisect has narrowed the cause down to #104270 (CC @carljm, @markshannon).\r\n\r\nThe reproduction case also fails if the call to `super`:\r\n```\r\n        super(MyInstance, type(self)).__setattr__(self, \"ptr\", ptr)\r\n```\r\nis replaced with the simpler:\r\n```\r\n        super().__setattr__(self, \"ptr\", ptr)\r\n```\r\n## Background\r\n\r\nThis test case has been extracted from [rubicon-objc](https://github.com/beeware/rubicon-objc), causing beeware/rubicon-objc#313. Rubicon is a wrapper around the Objective C runtime used by macOS and iOS; `MyInstance` is an analog of `ObjCInstance`, and `MyClass` is a an analog of `ObjCClass`. `ObjCInstance` has an implementation of `__setattr__` to redirect attribute access to the underlying ObjC calls. However, during construction, `ObjCInstance` needs to store a `ptr` of the underlying ObjC instance. This isn't a valid ObjC attribute, so `super()` is used to access the underlying `__setattr__` implementation to set the attribute.\r\n\r\n# Your environment\r\n\r\n- CPython versions tested on: 3.7.9, 3.10.11, 3.12.0a7, 3.12.0b1\r\n- Operating system and architecture: macOS ARM64\r\n\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-105094\n* gh-105117\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/freakboy3742","@type":"Person","name":"freakboy3742"},"datePublished":"2023-05-28T04:34:48.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/105035/cpython/issues/105035"}