Consider the following script:

importastdeftest(node):matchnode:caseast.FunctionDef("foo",ast.arguments(args=[ast.arg("bar")])):print('matched! :)')case _:print("Didn't match :(")source=ast.parse("def foo(bar): pass")node=source.body[0]assertisinstance(node,ast.FunctionDef)test(node)

Running this script on 3.11 gets you this output:

>python repro.pymatched! :)

Running this script on CPythonmain, however, gets you this output:

>python repro.pyDidn't match :(

The reason for this is that the implementation ofPEP-695 (new in Python 3.12) added a number of new AST nodes to Python, and as a result, the__match_args__ attributes ofast.FunctionDef,ast.AsyncFunctionDef andast.ClassDef are all different on 3.12 compared to what they were on 3.11.

>>>import ast>>>for nodein ast.ClassDef, ast.FunctionDef, ast.AsyncFunctionDef:...print(node.__match_args__)...('name', 'bases', 'keywords', 'body', 'decorator_list')('name', 'args', 'body', 'decorator_list', 'returns', 'type_comment')('name', 'args', 'body', 'decorator_list', 'returns', 'type_comment')

>>>import ast>>>for nodein ast.ClassDef, ast.FunctionDef, ast.AsyncFunctionDef:...print(node.__match_args__)...('name', 'type_params', 'bases', 'keywords', 'body', 'decorator_list')('name', 'type_params', 'args', 'body', 'decorator_list', 'returns', 'type_comment')('name', 'type_params', 'args', 'body', 'decorator_list', 'returns', 'type_comment')

This 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 iftype_params had been added as the final item in the__match_args__ tuples, but at the moment it comes in second place.

Cc.@JelleZijlstra forPEP-695. Also curious if@brandtbucher has any thoughts (for pattern-matching expertise) or@isidentical (forast-module expertise).

Linked PRs

• gh-104799: Move location of type_params AST fields #104828
• gh-104799: Default missing lists in AST to the empty list #104834
• [3.12] gh-104799: Move location of type_params AST fields (GH-104828) #104974
• [3.12] gh-104799: Default missing lists in AST to the empty list (GH-104834) #105213
• gh-104799: PEP 695 backward compatibility for ast.unparse #105846
• [3.12] gh-104799: PEP 695 backward compatibility for ast.unparse (GH-105846) #105862