TYP: Suboptimally annotated arithmetic operator types #27965

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1. adamjstewart

   added

   41 - Static typing

   on Dec 10, 2024
2. jorenham commented on Dec 19, 2024

   

   jorenham

   on Dec 19, 2024

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   this example shows why type-checkers reject the x = x / 1 from your example"

   # issue_27965.pyi
   
   import numpy as np
   
   reveal_type(x := np.zeros(1))
   reveal_type(x / 1)

   relevant mypy output:

   [...]/issue_27965.pyi:3: note: Revealed type is "numpy.ndarray[tuple[builtins.int], numpy.dtype[numpy.float64]]"
   [...]/issue_27965.pyi:4: note: Revealed type is "numpy.ndarray[builtins.tuple[builtins.int, ...], numpy.dtype[numpy.floating[Any]]]"
   

   relevant pyright output:

   [...]/issue_27965.pyi:3:13 - information: Type of "x := np.zeros(1)" is "ndarray[tuple[int], dtype[float64]]"
   [...]/issue_27965.pyi:4:13 - information: Type of "x / 1" is "ndarray[tuple[int, ...], dtype[floating[Any]]]"
   

   The np.zeros(1) is inferred by both mypy and pyright as a ndarray with 1-d shape and a dtype of float64. This is correct in the most accurate way that's (reasonably) possible.

   But np.zeros(1) / 1 isn't as precisely typed, and is inferred as ndarray with ?-d shape and a dtype of floating.

   Now the problem emerges when we do x = x / 1, which means we're assigning ndarray[tuple[int, ...], floating] to ndarray[(int), float64]. But this is invalid, because

   1. tuple[int, ...] isn't assignable to tuple[int]
   2. floating isn't assignable to float64 ()

   This is because we can't assign something less specific to something more specific, just like you can't assign int to a bool.

   But if we flip the LHS and RHS, then it actually is allowed. So in the same way that bool is assignable to int, tuple[int] is assignable to tuple[int, ...] and float64 is assignable to floating.

   We can use this to create a very clean workaround to this issue:

   import numpy as np
   import numpy.typing as npt
   
   x: npt.NDArray[np.floating]  # ndarray[tuple[int, ...], dtype[floating]]
   x = np.zeros(1)  # accepted
   x = x / 1  # accepted

   ---

   Note that this workaround is only required for mypy users: Pyright infers the type of x as ndarray[tuple[int, ...], dtype[floating]], because it looks at more than one line of code at a time, and isn't "greedy" (algorithmically speaking) like mypy is.

   👍React with 👍1Reacted by Adam J. Stewart
3. charris

   closed this as completedin #28108on Jan 6, 2025