mypy error when reshaping an array with a list of tuples

December 9, 2021

I have a python module that looks as following:

import numpy as np

def test(order: str = 'C') -> np.ndarray:
    return np.array([1,2,3]).reshape((1,2), order = order)

evaluating this with mypy returns the error and notes

error: No overload variant of "reshape" of "_ArrayOrScalarCommon" matches argument types "Tuple[int, int]", "str"
note: Possible overload variants:
note:     def reshape(self, Sequence[int], *, order: Union[Literal['A'], Literal['C'], Literal['F'], None] = ...) -> ndarray
note:     def reshape(self, *shape: int, order: Union[Literal['A'], Literal['C'], Literal['F'], None] = ...) -> ndarray

Substituting the section order = order in the function call with order = ‘C’ stops this error from occuring. Why is it a problem for mypy if I choose to pass this parameter as a function argument?

>Solution :

test(order: str = 'C') only sets the default value of parameter C
but you can still call it with any str. For instance test('X') is
correctly typed. But this causes test to call reshape with
order='X' which is wrong and this is why mypy complains and this is
a good thing.

So the problem lies in the type definition of order. You can change
it match the reshape signature, and mypy is happy with it:

import typing as tp
import numpy as np

def test(order: tp.Optional[tp.Literal['A', 'C', 'F']] = 'C') -> np.ndarray:
    return np.array([1,2,3]).reshape((1,2), order = order)

test('C')  # ok
test('X')  # mypy error