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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"PEP-695: Potentially breaking changes made to `__match_args__` attributes of AST nodes","articleBody":"Consider the following script:\r\n\r\n```py\r\nimport ast\r\n\r\ndef test(node):\r\n    match node:\r\n        case ast.FunctionDef(\"foo\", ast.arguments(args=[ast.arg(\"bar\")])):\r\n            print('matched! :)')\r\n        case _:\r\n            print(\"Didn't match :(\")\r\n\r\n\r\nsource = ast.parse(\"def foo(bar): pass\")\r\nnode = source.body[0]\r\nassert isinstance(node, ast.FunctionDef)\r\ntest(node)\r\n```\r\n\r\nRunning this script on 3.11 gets you this output:\r\n\r\n```\r\n\u003epython repro.py\r\nmatched! :)\r\n```\r\n\r\nRunning this script on CPython `main`, however, gets you this output:\r\n\r\n```\r\n\u003epython repro.py\r\nDidn't match :(\r\n```\r\n\r\nThe reason for this is that the implementation of PEP-695 (new in Python 3.12) added a number of new AST nodes to Python, and as a result, the `__match_args__` attributes of `ast.FunctionDef`, `ast.AsyncFunctionDef` and `ast.ClassDef` are all different on 3.12 compared to what they were on 3.11.\r\n\r\n\u003cdetails\u003e\r\n\u003csummary\u003e`__match_args__` attributes on 3.11:\u003c/summary\u003e\r\n\r\n```pycon\r\n\u003e\u003e\u003e import ast\r\n\u003e\u003e\u003e for node in ast.ClassDef, ast.FunctionDef, ast.AsyncFunctionDef:\r\n...     print(node.__match_args__)\r\n...\r\n('name', 'bases', 'keywords', 'body', 'decorator_list')\r\n('name', 'args', 'body', 'decorator_list', 'returns', 'type_comment')\r\n('name', 'args', 'body', 'decorator_list', 'returns', 'type_comment')\r\n```\r\n\r\n\u003c/details\u003e\r\n\r\n\u003cdetails\u003e\r\n\u003csummary\u003e`__match_args__` attributes on 3.12:\u003c/summary\u003e\r\n\r\n```pycon\r\n\u003e\u003e\u003e import ast\r\n\u003e\u003e\u003e for node in ast.ClassDef, ast.FunctionDef, ast.AsyncFunctionDef:\r\n...     print(node.__match_args__)\r\n...\r\n('name', 'type_params', 'bases', 'keywords', 'body', 'decorator_list')\r\n('name', 'type_params', 'args', 'body', 'decorator_list', 'returns', 'type_comment')\r\n('name', 'type_params', 'args', 'body', 'decorator_list', 'returns', 'type_comment')\r\n```\r\n\r\n\u003c/details\u003e\r\n\r\nThis feels like it has the potential to be quite a breaking change for people using pattern-matching to parse ASTs. It would probably be okay if `type_params` had been added as the final item in the `__match_args__` tuples, but at the moment it comes in second place.\r\n\r\nCc. @JelleZijlstra for PEP-695. Also curious if @brandtbucher has any thoughts (for pattern-matching expertise) or @isidentical (for `ast`-module expertise).\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-104828\n* gh-104834\n* gh-104974\n* gh-105213\n* gh-105846\n* gh-105862\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/AlexWaygood","@type":"Person","name":"AlexWaygood"},"datePublished":"2023-05-23T14:36:24.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":14},"url":"https://github.com/104799/cpython/issues/104799"}