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MAINT: Give the _<X>Like and _ArrayLike<X> type aliases a more descriptive name #18185

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Merged
merged 4 commits into from
Jan 19, 2021
Merged

MAINT: Give the _<X>Like and _ArrayLike<X> type aliases a more descriptive name #18185

merged 4 commits into from
Jan 19, 2021

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BvB93
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@BvB93 BvB93 commented Jan 18, 2021

Closes #18096.

This PR renames the _<X>Like and _ArrayLike<X> aliases to, respectively, _<X>Like_co and _ArrayLike<X>_co.

The goal of this change is to emphasize that aforementioned type aliases are not invariant;
they represent all objects that can be cast into <X> (using casting="same_kind").
The aliases are now thus marked (by convention) as covariant, despite the objects contained
therein not necessarily being formal sub-types of <X>.

Examples

from typing import Union
import numpy as np

# `_BoolLike_co` and `_BoolLike` would actually be equivalent here
_BoolLike_co = Union[bool, np.bool_]
_IntLike_co = Union[_BoolLike_co, int, np.integer]

Bas van Beek added 4 commits January 18, 2021 16:12
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BvB93 commented Jan 18, 2021

As a side note: this PR has some minor (name-related) merge conflicts with #18128.

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charris commented Jan 18, 2021

Looks good to me. Maybe cov instead of co? The latter has no obvious meaning.

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BvB93 commented Jan 19, 2021

Looks good to me. Maybe cov instead of co? The latter has no obvious meaning.

It is used consistently as suffix in both typeshed and typing when referring to covariant typevariables,
so I'd argue there is very much a precedent for using co.

@charris charris merged commit da54e54 into numpy:master Jan 19, 2021
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charris commented Jan 19, 2021

Thanks Bas.

@BvB93 BvB93 deleted the x-like branch January 19, 2021 16:00
@pfheatwole
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Looks good to me. Maybe cov instead of co? The latter has no obvious meaning.

It is used consistently as suffix in both typeshed and typing when referring to covariant typevariables,
so I'd argue there is very much a precedent for using co.

For anyone else trying to educate themselves on this issue, another reference is PEP 484, in the section Covariance and Contravariance:

By convention, it is recommended to use names ending in _co for type variables defined with covariant=True and names ending in _contra for that defined with contravariant=True.

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typing aliases for integer-like
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@BvB93 @charris @pfheatwole