Add a constant that's False at runtime but True when type checking

[gvanrossum]

This would provide a standard idiom for including imports that are only necessary while type checking, see e.g. https://github.com/python/mypy/pull/1646 (which currently recommends if False: import ... because that's the only way to do it).

[gvanrossum]

There's one case where I think the if False: idiom may still be needed: to import the typing module itself. Some code has the constraint that it shouldn't depend on any 3rd party packages. For Python 2.7 and for 3.2--3.4 the only reasonable ways to do that are either

if False:
    from typing import ...

or

try:
    from typing import ...
except ImportError:
    ...

To survive the non-existence of the typing module, some idioms can't be used (especially type aliases, type variables, and generic classes) but many others are still available (especially # type comments for function signatures and variables, and function annotations in string quotes).

We probably should add some language to the PEP to direct type checkers to support at least one of these (which would then become the preferred way). Currently I've mostly been recommending if False: for this, because it's easiest to write.

[gnprice]

This would be nice to get into the 3.5.2 release if we can.

Some name ideas:

typing.in_typechecker
typing.in_type_system
not typing.at_runtime
typing.typechecker is not None  # else 'mypy', 'pytype', 'semmle', etc.

Out of those I think I'd go for in_typechecker (or in_type_checker), but I don't have a strong view.

[gnprice]

/cc @markshannon , since this came up in discussion at PyCon last week

[JukkaL]

Here are some more ideas (with an usage example to better illustrate what it would look like):

if typing.type_checker:   # could be similar to Greg's typechecker, but with _
    from model import Person

if typing.type_check:
    from model import Person

if typing.TYPE_CHECK:   # this is probably my favorite
    from model import Person

if typing.CHECKER:
    from model import Person

if typing.STATIC:
    from model import Person

if typing.ANALYZE:
    from model import Person

if not typing.RUNNING:
    from model import Person

if not typing.RUNTIME:
    from model import Person

Also, should this be okay if I don't want to use string literals in annotations:

if TYPE_CHECK:
    from model import Person
else:
    Person = 'Person'

def show(p: Person) -> None: ...

[refi64]

IMO it really should be some kind of present tense verb to emphasize "if currently type checking".

Maybe typing.TYPE_CHECKING?

[JukkaL]

TYPE_CHECKING sounds pretty good to me. This (along with many of the other suggestions) has the minor drawback that it implies a particular sort of tool, even though other tools such as pure type inference tools, refactoring tools or IDEs that don't do type checking can also use these imports.

[gvanrossum]

Maybe ANALYZING? Or would the Brits and Aussies get mad at us?

I do like TYPE_CHECKING, and at least for IDEs that seems a fine usage. It also feels intuitive so we won't have to explain it to most people who see it for the first time.

[markshannon]

We could generali(s|z)e TYPE_CHECKING to STATIC_CHECKING, but TYPE_CHECKING is fine (I don't want to bike shed too much).

[gvanrossum]

@matthiaskramm any objection to TYPE_CHECKING?

--Guido (mobile) On Jun 7, 2016 11:24 AM, "Mark Shannon" notifications@github.com wrote:

We could generali(s|z)e TYPE_CHECKING to STATIC_CHECKING, but TYPE_CHECKING is fine (I don't want to bike shed too much).

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[matthiaskramm]

I'm happy with typing.TYPE_CHECKING.

[gvanrossum]

OK, typing.TYPE_CHECKING it is. I'll send the changes soon.