Float multiplication of numpy.ndarray in lambda is incorrectly analysed
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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
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- 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