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Add TYPE_CHECKING constant. #234

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Merged
gvanrossum merged 1 commit into from
Jun 8, 2016
Merged

Add TYPE_CHECKING constant. #234

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29 changes: 28 additions & 1 deletion pep-0484.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -983,6 +983,31 @@ Don't expect a checker to understand obfuscations like
``"".join(reversed(sys.platform)) == "xunil"``.


Runtime or type checking?
-------------------------

Sometimes there's code that must be seen by a type checker (or other
static analysis tools) but should not be executed. For such
situations the ``typing`` module defines a constant,
``TYPE_CHECKING``, that is considered ``True`` during type checking
(or other static analysis) but ``False`` at runtime. Example::

import typing

if typing.TYPE_CHECKING:
import expensive_mod

def a_func(arg: 'expensive_mod.SomeClass') -> None:
a_var = arg # type: expensive_mod.SomeClass
...

(Note that the type annotation must be enclosed in quotes, making it a
"forward reference", to hide the ``expensive_mod`` reference from the
interpreter runtime. In the ``# type`` comment no quotes are needed.)

This approach may also be useful to handle import cycles.


Arbitrary argument lists and default argument values
----------------------------------------------------

Expand Down Expand Up @@ -1174,7 +1199,7 @@ in expressions, while type comments only apply to assignments.


NewType helper function
-----------------------
=======================

There are also situations where a programmer might want to avoid logical
errors by creating simple classes. For example::
Expand Down Expand Up @@ -1644,6 +1669,8 @@ Convenience definitions:
forward references (which are given as string literals) as expressions
in the context of the original function or method definition.

* TYPE_CHECKING, ``False`` at runtime but ``True`` to type checkers

Types available in the ``typing.io`` submodule:

* IO (generic over ``AnyStr``)
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5 changes: 5 additions & 0 deletions python2/typing.py
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Expand Up @@ -66,6 +66,7 @@
'no_type_check_decorator',
'overload',
'Text',
'TYPE_CHECKING',
]

# The pseudo-submodules 're' and 'io' are part of the public
Expand Down Expand Up @@ -1618,6 +1619,10 @@ def new_type(x):
Text = unicode


# Constant that's True when type checking, but False here.
TYPE_CHECKING = False


class IO(Generic[AnyStr]):
"""Generic base class for TextIO and BinaryIO.

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5 changes: 5 additions & 0 deletions src/typing.py
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Expand Up @@ -69,6 +69,7 @@
'no_type_check_decorator',
'overload',
'Text',
'TYPE_CHECKING',
]

# The pseudo-submodules 're' and 'io' are part of the public
Expand Down Expand Up @@ -1669,6 +1670,10 @@ def new_type(x):
Text = str


# Constant that's True when type checking, but False here.
TYPE_CHECKING = False


class IO(Generic[AnyStr]):
"""Generic base class for TextIO and BinaryIO.

Expand Down