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Docs: Replace "if False" trick with "if TYPE_CHECKING". #2181

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Sep 25, 2016
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Docs: Replace "if False" trick with "if TYPE_CHECKING". #2181

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13 changes: 5 additions & 8 deletions docs/source/common_issues.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -260,17 +260,16 @@ imports to a module and those imports cause cycles that didn't exist
before. If those cycles become a problem when running your program,
there's a trick: if the import is only needed for type annotations in
forward references (string literals) or comments, you can write the
imports inside ``if False:`` so that they are not executed at runtime.
The reason this works is that mypy (currently) does not analyze
unreachable code like this. Example:
imports inside ``if TYPE_CHECKING:`` so that they are not executed at runtime.
Example:

File ``foo.py``:

.. code-block:: python

from typing import List
from typing import List, TYPE_CHECKING

if False:
if TYPE_CHECKING:
import bar

def listify(arg: 'bar.BarClass') -> 'List[bar.BarClass]':
Expand All @@ -289,7 +288,5 @@ File ``bar.py``:

.. note::

It is possible that in the future, mypy will change its dead code
analysis and this trick will stop working. We will then offer an
alternative, e.g. a constant defined by the ``typing`` module that
The ``TYPE_CHECKING`` constant defined by the ``typing`` module
is ``False`` at runtime but ``True`` while type checking.