python / python/mypy

Float multiplication of numpy.ndarray in lambda is incorrectly analysed

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#8,001 3 comments 0 reactions 0 assignees View on GitHub

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bug false-positive priority-1-normal
Dominant language
Python
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Description

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

  • Are you reporting a bug, or opening a feature request?

Bug.

  • Please insert below the code you are checking with mypy,
    or a mock-up repro if the source is private. We would appreciate
    if you try to simplify your case to a minimal repro.
from typing import NewType, Tuple, Deque

import numpy as np


class Point(np.ndarray):
    def __new__(cls, x: float, y: float) -> np.ndarray:
        return np.array((x, y), dtype=np.float32)


Nose = NewType("Nose", Tuple[Point, ...])


def average_position(positions: Deque) -> Nose:
    factor = 1.0 / len(positions)
    point_sum = [Point(0.0, 0.0) for _ in range(10)]
    for nose in positions:
        for i, point in enumerate(nose):
            point_sum[i] += point
    avg_pos = Nose(tuple(map(lambda p: factor * p, point_sum)))
    return avg_pos


def average_position_with_multiply(positions: Deque) -> Nose:
    factor = 1.0 / len(positions)
    point_sum = [Point(0.0, 0.0) for _ in range(10)]
    for nose in positions:
        for i, point in enumerate(nose):
            point_sum[i] += point
    avg_pos = Nose(tuple(map(lambda p: np.multiply(factor, p), point_sum)))
    return avg_pos

  • What is the actual behavior/output?

Mypy gives the errors below:

bug.py:20: error: Argument 1 to "map" has incompatible type "Callable[[Point], float]"; expected "Callable[[Point], Point]"
bug.py:20: error: Incompatible return value type (got "float", expected "Point")
Found 2 errors in 1 file (checked 1 source file)

In the lambda in average_position(), mypy incorrectly (?) determines the type as Callable[[Point], float], while using the NumPy function np.multiply() (which does the same thing) gives no error. Both functions output the same correct value.

  • What is the behavior/output you expect?
    There should be no error.

  • What are the versions of mypy and Python you are using?
    Do you see the same issue after installing mypy from Git master?

mypy==0.740
Python 3.8.0
  • What are the mypy flags you are using? (For example --strict-optional)
[mypy]
python_version = 3.8

[mypy-cv2,dlib,numpy]
ignore_missing_imports = True

I am unsure if this is actually an error, or if I've done something wrong somewhere, but both functions run and output correctly.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running mypy on the minimal Python example shown in the issue with the provided Python 3.8 configuration, comparing the lambda expression with np.multiply(). Trace the type inference for the lambda and confirm the fix with a regression case where the example reports no errors and preserves the expected Nose return type.

Written by the indexing model from the issue text.

Assessment

Tech stack
numpy, python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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