Wrong comparison in type error for comparisons #22316

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1. A5rocks

   added

   bugmypy got something wrongmypy got something wrong

   on Oct 28, 2026
2. inchang-ing commented on Oct 29, 2026

   

   inchang-ing

   on Oct 29, 2026

   Contributor

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   I can reproduce this on master (2.5.0+dev @ f501cd0):

   def f(x: int | None) -> bool:
       return x >= 0

   main.py:2: error: Unsupported operand types for <= ("int" and "None")  [operator]
   main.py:2: note: Left operand is of type "int | None"
   

   Root cause: the message is rendered in mypy/messages.py:646 (incompatible_argument, the elif name.startswith('"__') branch) purely from the name of the method that failed:

   for method, op in op_methods_to_symbols.items():
       for variant in method, "__r" + method[2:]:
           if name.startswith(f'"{variant}" of'):

   NoneType has no __ge__, so check_op_reversible falls back to the reflected call (mypy/checkexpr.py:4215, rev_op_name = operators.reverse_op_methods[op_name]) and the callee that actually fails is "__le__" of "int". The symbol is therefore rendered as <= and the operands are printed in reflected order; nothing maps the reflected method back to the operator the user actually wrote. The result also disagrees with CPython's runtime message ('>=' not supported between instances of 'NoneType' and 'int').

   All four orderings are mirrored on master when the left operand lacks the forward method:

   written	reported
   x >= 0	<= ("int" and "None")
   x <= 0	>= ("int" and "None")
   x > 0	< ("int" and "None")
   x < 0	> ("int" and "None")

   (When the forward method exists the symbol is correct, e.g. x: int; s: str; x >= s -> >= ("int" and "str").)

   I'd like to work on this. One design question first, because the mirrored rendering is baked into existing assertions, e.g. test-data/unit/check-incremental.test:3101-3103 (1 <= A(1) -> >=, 1 >= A(1) -> <=) and test-data/unit/check-plugin-attrs.test:208-210 (roughly 8-12 assertions total):

   • Option A: always report the operator as written, with operands in source order (x >= 0 -> Unsupported operand types for >= ...), and update those assertions.
   • Option B: keep the reflected rendering, but add a note saying the error comes from the reflected call.

   My lean is A (it matches both the source text and CPython's message), but I'll follow your preference.
3. ilevkivskyi

   added

   topic-error-reportingHow we report errorsHow we report errors

   on Oct 29, 2026
4. xh123637

   added a commit that references this issue on Nov 6, 2026

   Fix flipped operator in comparison error message

   ...

   This issue will close once commit 825cf42 is merged into the 'master' branch.

   825cf42