New submission from Dutcho <dut...@ziggo.nl>:
While issue #34498 signalled a break in Python 3.7 of legal Python 3.6 code that registers a single-dispatch function with a 'pseudo-type' like typing.Sequence, both Python 3.6 and Python 3.7 don't work with a 'parametrized pseudo-type' like typing.Sequence[int]. However, to fully benefit from Python 3.7's use of annotations for registering single-dispatch functions, these 'parametrized pseudo-types' should also be acceptable. E.g. in Python 3.6: $ py -3.6 Python 3.6.6 (v3.6.6:4cf1f54eb7, Jun 27 2018, 03:37:03) [MSC v.1900 64 bit (AMD64)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> from functools import singledispatch >>> from typing import Iterable, Sequence, T >>> >>> @singledispatch ... def last(iterable: Iterable[T]) -> T: ... for last in iterable: ... pass ... return 'slow', last ... >>> @last.register(Sequence) ... def _(sequence: Sequence[T]) -> T: ... return 'fast', sequence[-1] ... >>> print(*last(iter([1, 2, 3]))) slow 3 >>> print(*last([1, 2, 3])) fast 3 Note in Python 3.6 no match results if the 'parametrized pseudo-type' Sequence[T] is specified as argument to last.register() as isinstance(..., Sequence[T]) fails. To fully benefit from Python 3.7's use of annotations for single-dispatch functions, it should (1) accept >>> @last.register ... def _(sequence: Sequence[T]) -> T: ... return 'fast', sequence[-1] and (2) remove the T parameter of Sequence for matching when dispatching. I recognize this could potentially create a clash between e.g. Sequence[int] and Sequence[str] ---------- components: Library (Lib) messages: 324063 nosy: Dutcho priority: normal severity: normal status: open title: Extend registering of single-dispatch functions to parametrized generic pseudo-types type: enhancement versions: Python 3.7 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue34499> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
