Functions with global inside should not have their function bodies be stripped #22240

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1. A5rocks

   added

   bugmypy got something wrongmypy got something wrong

   on Oct 21, 2026
2. Madin-H23 commented on Oct 21, 2026

   

   Madin-H23

   on Oct 21, 2026

   More actions

   I looked into this and found the root cause. The fix for the default (native) parser is in mypyc/ast_serialize#98.

   Root cause. When a module is only analyzed for its types (e.g. it is imported with --follow-imports=silent), function bodies are skipped if they have no "externally visible effects". There are two implementations of that check, and both miss global/nonlocal declarations:

   • native parser (the default): mypyc/ast_serialize, src/func_effect_visitor.rs, has_externally_visible_effect() — it keeps bodies that assign to an attribute of the first parameter (self.x = 1) or that contain a yield (the inferred return type depends on it), but a global/nonlocal declaration is not considered;
   • old Python parser (--no-native-parser): mypy/fastparse.py, strip_function_bodies — same gap.

   A function that declares a global can assign to it, so skipping the body changes the inferred type of a variable outside the function:

   # b.py
   x = None
   
   def f() -> None:
       global x
       x = 1

   # a.py
   import b
   
   reveal_type(b.x)

   With --follow-imports=silent this reveals None; without the flag it reveals int | None. The same happens with nonlocal and with a global declaration nested in an if block or in a nested function.

   Fix. Treat a global/nonlocal declaration as an externally visible effect. Nested functions and classes have to be searched as well, since a declaration in a nested scope also refers to a variable outside the current function — and the nested body would be dropped together with the enclosing one.

   The linked PR adds this to the native parser, together with unit tests. The old Python parser needs the same change in mypy/fastparse.py; I have a verified patch for that as well (roughly 10 lines, plus a regression test in test-data/unit/check-flags.test) and can send it as a separate PR if that is useful.

   I can also add the end-to-end test to test-data/unit/native-parser.test once a new ast_serialize release is available.

   🤖 This analysis and the patches were drafted with AI assistance.
3. ilevkivskyi

   added

   topic-parser

   on Oct 21, 2026