GeezerVison is showing github.com — Add a constant that's False at runtime but True when type checking · Issue #230 · python/typing
Add a constant that's False at runtime but True when type checking #230DescriptionThis would provide a standard idiom for including imports that are only necessary while type checking, see e.g. python/mypy#1646 (which currently recommends if False: import ... because that's the only way to do it). Activitygvanrossum commented on Jun 6, 2016 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 commented on Jun 6, 2016 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 commented on Jun 6, 2016 /cc @markshannon , since this came up in discussion at PyCon last week JukkaL commented on Jun 7, 2016 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 commented on Jun 7, 2016 IMO it really should be some kind of present tense verb to emphasize "if currently type checking".
Maybe typing.TYPE_CHECKING? JukkaL commented on Jun 7, 2016 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 commented on Jun 7, 2016 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 commented on Jun 7, 2016 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 commented on Jun 7, 2016 matthiaskramm commented on Jun 7, 2016 I'm happy with typing.TYPE_CHECKING. gvanrossum commented on Jun 7, 2016 OK, typing.TYPE_CHECKING it is. I'll send the changes soon. added a commit that references this issue on Jun 7, 2016This issue will close once commit 2496ab4 is merged into the 'main' branch.2496ab4 added a commit that references this issue on Jun 8, 2016This issue will close once commit 600c07f is merged into the 'main' branch.600c07f
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