[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Change by Ivan Levkivskyi :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions: +Python 3.8 -Python 3.6

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:


New changeset 4c23aff065fb28aba789a211937a2af974842110 by Ivan Levkivskyi in 
branch 'master':
bpo-29262: Add get_origin() and get_args() introspection helpers to typing 
(GH-13685)
https://github.com/python/cpython/commit/4c23aff065fb28aba789a211937a2af974842110


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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Change by Ivan Levkivskyi :


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keywords: +patch
pull_requests: +13573
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/13685

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

The discussed functionality is published as a separate package:
https://pypi.python.org/pypi/typing-inspect
https://github.com/ilevkivskyi/typing_inspect

After the API is settled, some introspection functions may be added directly to 
typing.

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

Cross-posting the link to upstream work on this: 
https://github.com/python/typing/pull/377

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[issue29262] Provide a way to check for *real* typing.Union instances

[flying sheep]

flying sheep added the comment:

Cool! This set of basic initial check will consist of all the is_* functions 
that were mentioned right?

FWIW I also think that this is the way to go, as it’s not obvious if the 
semantics should be “conforms to this type annotation” or “is a type annotation 
of that kind” or other variants.

In case this isn’t already too much future think: What should be the way 
forward from there? E.g. when working with Union[A, B], you will probably want 
to get “(A, B)”.

So will that mean more introspection functions (like 
union_types(Union[str,int]),
or public APIs for typings (e.g. a_union.__iter__() or a_union.types)?

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

OK, I will make a PR to typing adding a folder (as it is done for 
mypy_extensions) with a basic set of runtime type inspection functions.

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[issue29262] Provide a way to check for *real* typing.Union instances

[Guido van Rossum]

Guido van Rossum added the comment:

Up to you, but the latter might make it clearer that the two are to be kept
in sync.

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

OK, I agree.

How then it should be done logistically? Should I just make a separate repo on 
GitHub for this? Or will it be inside typing (like mypy_extesions is inside 
mypy) but published on PyPI separately?

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[issue29262] Provide a way to check for *real* typing.Union instances

[Guido van Rossum]

Guido van Rossum added the comment:

I think it should be separate from mypy_extensions, since it's not even
related to mypy.

Regarding typing.re and typing.io, typing.inspect would be a typed version
of the inspect module, so that's not quite the same. (Though I consider
typing.re/io mistakes now -- people seem to mostly ignore it and use the
toplevel names.)

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

I have a similar idea. We already have mypy_extensions for runtime counterparts 
of experimental features. However, the runtime inspections are not related to 
mypy, so that I am not sure. I am just a bit worried there will be to many 
things to keep in mind/in sync. What do you think?

By the way maybe later we could add ``typing.inspect`` similar to ``typing.re`` 
and ``typing.io``?

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[issue29262] Provide a way to check for *real* typing.Union instances

[Guido van Rossum]

Guido van Rossum added the comment:

Posting to python-dev would probably cause more friction than a PR for the
PEPs repo. But maybe the best way to do this is to use a third party module
with a proposed API? E.g. typing_inspect. We could then iterate quickly on
the design and implementation there, and we could commit to keeping it in
sync with changes to typing.py, so flying sheep's package could depend on
it.

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

> Maybe a proposal should be discussed as an addendum to PEP 484? Or would
Mark Shannon reject that?

On one hand, I would like to involve a wider audience to this discussion. On 
the other hand, an addition to the PEP could slow things down. Maybe a 
discussion on python-dev followed by implementation plus extensive docs would 
work?

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[issue29262] Provide a way to check for *real* typing.Union instances

[Guido van Rossum]

Guido van Rossum added the comment:

Maybe a proposal should be discussed as an addendum to PEP 484? Or would
Mark Shannon reject that?

On Sun, Jan 15, 2017 at 2:48 PM, Ivan Levkivskyi 
wrote:

>
> Ivan Levkivskyi added the comment:
>
> > Maybe we need to take a step back and look at the needs for code that
> wants to implement runtime type checking more in general?
>
> I would say that the most convenient way for me would be a set of
> "inspect-style" simple helpers like ``is_union``, ``is_generic``,
> ``get_parameters`` (similar to inspect.ismethod, inspect.getargspec etc).
>
> > ISTM we have ways to access the parameters of a parameterized type
> (typically t.__parameters__) but we don't have a reasonable way yet to
> determine what kind of thing t is.
>
> There is one way that I see right now: using _gorg. For example,
> ``_gorg(Tuple[int, str]) is Tuple``, ``_gorg(Callable[..., str]) is
> Callable``, etc. This will also work for ``Union`` if we relax the assert
> that requires type to be instance of GenericMeta (now there is a common
> internal API used by almost everything in typing).
>
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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

> Maybe we need to take a step back and look at the needs for code that wants 
> to implement runtime type checking more in general?

I would say that the most convenient way for me would be a set of 
"inspect-style" simple helpers like ``is_union``, ``is_generic``, 
``get_parameters`` (similar to inspect.ismethod, inspect.getargspec etc).

> ISTM we have ways to access the parameters of a parameterized type (typically 
> t.__parameters__) but we don't have a reasonable way yet to determine what 
> kind of thing t is.

There is one way that I see right now: using _gorg. For example, 
``_gorg(Tuple[int, str]) is Tuple``, ``_gorg(Callable[..., str]) is Callable``, 
etc. This will also work for ``Union`` if we relax the assert that requires 
type to be instance of GenericMeta (now there is a common internal API used by 
almost everything in typing).

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[issue29262] Provide a way to check for *real* typing.Union instances

[Guido van Rossum]

Guido van Rossum added the comment:

Hm, let me back-pedal a bit. The situation with Callable is murky, as e.g. 
isinstance(typing.Tuple[int, int], typing.Callable) returns True.

Maybe we need to take a step back and look at the needs for code that wants to 
implement runtime type checking more in general? ISTM we have ways to access 
the parameters of a parameterized type (typically t.__parameters__) but we 
don't have a reasonable way yet to determine what kind of thing t is.

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

This will be a bit unusual since ``isinstance`` is typically called for 
instances (i.e. not types) as in ``isinstance(func, Callable)``. But on the 
other hand this is probably what one would expect when one sees 
``isinstance(tp, Union)``. Thus I am fine with doing it this way.

If there are no objections/other ideas, then I will make a PR upstream in next 
few days.

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[issue29262] Provide a way to check for *real* typing.Union instances

[Guido van Rossum]

Guido van Rossum added the comment:

Hm, maybe isinstance(t, Union) should actually be allowed? (Though 
isinstance(x, Union[int, str]) should not be!)  After all we can write 
isinstance(t, Callable) as well, even though isinstance(x, Callable[[int], 
int]) is disallowed.

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[issue29262] Provide a way to check for *real* typing.Union instances

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

In principle, such a function could be added if it is not called 
``isinstance``. For example, we could add a helper ``is_union(tp)`` (or maybe 
also ``is_generic(tp)`` etc). Such function(s) will be simple one-liners 
wrapping private API in a right way (e.g. using _gorg instead of __origin__ 
where needed etc).

Guido, what do you think?

--
nosy: +gvanrossum, levkivskyi
type:  -> enhancement

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[issue29262] Provide a way to check for *real* typing.Union instances

[Evan]

Evan added the comment:

I'm also interested in this. I've been using 'thing.__origin__ is 
typing.Union', but this doesn't work in some of the older versions of typing.

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nosy: +evan_

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[issue29262] Provide a way to check for *real* typing.Union instances

[flying sheep]

New submission from flying sheep:

typing.Union prevents the use of `isinstance` and `issubclass` via a hook.

This is presumably to prevent errors that would arise if someone tried to do 
issubclass(A, Union[A, B]) or isinstance(A(), Union[A, B]), because Union isn’t 
specified in the PEP to allow a check like this, and doesn’t implement it. 
(Instead it throws said error)

However, as far as I can see there is no blessed way to check if an object was 
returned by Union.__getitem__(). A simple way that works is:

sig = inspect.signature(f)
ann = sig.parameters['arg1'].annotation
is_an_union = isinstance(ann, typing._Union)

but _Union is a private class, and an implementation detail.

is there a blessed way to do this? If not, one should be added.

--
components: Library (Lib)
messages: 285410
nosy: flying sheep
priority: normal
severity: normal
status: open
title: Provide a way to check for *real* typing.Union instances
versions: Python 3.6

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