How to annotate a numpy array of strings with mypy?

Question

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 ?

Asked By: Gabriel Cia

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Source

Answer 1

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

How to annotate a numpy array of strings with mypy?

Question:

Answers:

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