Python 3.8+typingterm

typing.SupportsIndex

Supports* protocols

A runtime-checkable protocol for objects usable as an index (has __index__).

Syntax

isinstance(obj, typing.SupportsIndex)

__index__ marks a 'genuine integer' as opposed to a float: it is what slices, range(), hex()/bin()/oct() and operator.index() demand. So float lacks it — isinstance(2.0, SupportsIndex) is False, while int and bool pass. The protocol itself arrived in Python 3.8.

Examples

from typing import SupportsIndex
print(isinstance(5, SupportsIndex))   # → True
print(isinstance(2.5, SupportsIndex))  # → False
print(isinstance(True, SupportsIndex))  # → True

from typing import SupportsIndex
class Idx: __index__ = lambda self: 1
print(isinstance(Idx(), SupportsIndex))  # → True
print('abc'[Idx()])  # → b

See also

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