In Python there is a wide-spread idiom of using specific object instances as markers: MARKER = object() def f(arg) -> if arg is MARKER: do_something_special() else: process_arg_as_string(arg) When I add type hints my initial thought was decorating MARKER as constant literal: from typing import Final, Literal, Union MARKER: Final[object] = object() def f(arg: Union[str, Literal[MARKER]]) -> None: pass MARKER is declared as a final value, it cannot be changed from the typing perspective and IMHO satisfies requirements for Literal. Sure, it doesn't work with the current mypy but maybe the proposal makes sense? -- Thanks, Andrew Svetlov
Am 24.11.20 um 07:17 schrieb Andrew Svetlov:
In Python there is a wide-spread idiom of using specific object instances as markers:
MARKER = object()
def f(arg) -> if arg is MARKER: do_something_special() else: process_arg_as_string(arg)
When I add type hints my initial thought was decorating MARKER as constant literal:
from typing import Final, Literal, Union
MARKER: Final[object] = object()
def f(arg: Union[str, Literal[MARKER]]) -> None: pass
MARKER is declared as a final value, it cannot be changed from the typing perspective and IMHO satisfies requirements for Literal.
Sure, it doesn't work with the current mypy but maybe the proposal makes sense?
There is some prior discussion about such a feature in this issue: https://github.com/python/typing/issues/689. - Sebastian
Thanks for the link! On Tue, Nov 24, 2020 at 2:15 PM Sebastian Rittau <srittau@rittau.biz> wrote:
Am 24.11.20 um 07:17 schrieb Andrew Svetlov:
In Python there is a wide-spread idiom of using specific object instances as markers:
MARKER = object()
def f(arg) -> if arg is MARKER: do_something_special() else: process_arg_as_string(arg)
When I add type hints my initial thought was decorating MARKER as constant literal:
from typing import Final, Literal, Union
MARKER: Final[object] = object()
def f(arg: Union[str, Literal[MARKER]]) -> None: pass
MARKER is declared as a final value, it cannot be changed from the typing perspective and IMHO satisfies requirements for Literal.
Sure, it doesn't work with the current mypy but maybe the proposal makes sense?
There is some prior discussion about such a feature in this issue: https://github.com/python/typing/issues/689.
- Sebastian _______________________________________________ Typing-sig mailing list -- typing-sig@python.org To unsubscribe send an email to typing-sig-leave@python.org https://mail.python.org/mailman3/lists/typing-sig.python.org/ Member address: andrew.svetlov@gmail.com
-- Thanks, Andrew Svetlov
I recently ran across some Django code that has an interesting solution for typing a sentinel object: It uses a *type* object as a sentinel rather than just an `object()`: # django/db/models/fields/__init__.py class NOT_PROVIDED: pass class Field(RegisterLookupMixin): def __init__(self, ..., default=NOT_PROVIDED, ...): ... Presumably the type of the `default` parameter could then be written as: Union[Type[NOT_PROVIDED], Any] Not exactly direct, but may work. -- David Foster | Seattle, WA, USA Contributor to TypedDict support for mypy
That's a very expensive sentinel though. Classes are notoriously heavy objects. On Tue, Nov 24, 2020 at 7:17 PM David Foster <davidfstr@gmail.com> wrote:
I recently ran across some Django code that has an interesting solution for typing a sentinel object: It uses a *type* object as a sentinel rather than just an `object()`:
# django/db/models/fields/__init__.py
class NOT_PROVIDED: pass
class Field(RegisterLookupMixin): def __init__(self, ..., default=NOT_PROVIDED, ...): ...
Presumably the type of the `default` parameter could then be written as:
Union[Type[NOT_PROVIDED], Any]
Not exactly direct, but may work.
-- David Foster | Seattle, WA, USA Contributor to TypedDict support for mypy _______________________________________________ Typing-sig mailing list -- typing-sig@python.org To unsubscribe send an email to typing-sig-leave@python.org https://mail.python.org/mailman3/lists/typing-sig.python.org/ Member address: guido@python.org
-- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
participants (4)
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Andrew Svetlov -
David Foster -
Guido van Rossum -
Sebastian Rittau