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bpo-29974: Improve typing.TYPE_CHECKING example (GH-982) 

* Fix PEP 8 (SomeType instead of some_type)
* Add a function parameter annotation
* Explain, using wording from PEP 484 and PEP 526,
  why one annotation is in quotes and another is not.

Suggested by Ivan Levkevskyi.
This commit is contained in:
Mathias Rav 2017-04-26 06:49:45 -04:00 committed by Berker Peksag
parent 0d637e236d
commit 87c07fe9d9
1 changed files with 7 additions and 2 deletions
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9
Doc/library/typing.rst
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@ -1041,5 +1041,10 @@ The module defines the following classes, functions and decorators:
if TYPE_CHECKING:
import expensive_mod
def fun():
local_var: expensive_mod.some_type = other_fun()
def fun(arg: 'expensive_mod.SomeType') -> None:
local_var: expensive_mod.AnotherType = other_fun()
Note that the first type annotation must be enclosed in quotes, making it a
"forward reference", to hide the ``expensive_mod`` reference from the
interpreter runtime. Type annotations for local variables are not
evaluated, so the second annotation does not need to be enclosed in quotes.