[FIXED] How to annotate a numpy array of strings with mypy?

Issue

I'm using a numpy array to store pairs of strings in order to take advantage of numpy array's memory view capability when slicing,which normal python lists don't have.

import numpy as np
import numpy.typing as npt

pairs_of_identifiers = np.array([['A11', 'A19'], ['A2', 'A6'], ...]])

random_function(pairs_of_identifiers: npt.NDArray[str]) -> None:
    ...

So far I have tried to annotate it as npt.NDArray[str], but I get the following pylance/mypy error:

Could not specialize type "NDArray[ScalarType@NDArray]"
  Type "str" cannot be assigned to type "generic"
    "str" is incompatible with "generic"

What is the correct way of annotating a NDarray of strings with mypy ?

Solution

Refer to Built-in scalar types:

Array scalar type	Related Python type	Inherits?
int_	int	Python 2 only
float_	float	yes
complex_	complex	yes
bytes_	bytes	yes
str_	str	yes
bool_	bool	no
datetime64_	datetime.datetime	no
timedelta64	datetime.timedelta	no

So you can use npt.NDArray[np.str_].

Answered By - Mechanic Pig