英文:
What is the purpose of "if False:" here?
问题
Here is the translated content from the provided URL:
if False: # pragma: no cover (mypy)
from typing import Type
if sys.version_info >= (3,):
AsyncFunctionDef = ast.AsyncFunctionDef
else:
AsyncFunctionDef = ast.stmt
False 可以在 Python 2 中为真（可以被重新定义），但在 Python 3 中不能为真。这可能是一个笑话，或者是一个正在进行中的工作，或者是注释掉代码的方式，但这是一个相当成熟的工具，我是否漏掉了什么？
英文:
https://github.com/asottile/pyupgrade/blob/fecacc91e57c224a0bd2564579ef01238650126c/pyupgrade.py#L53
if False: # pragma: no cover (mypy)
from typing import Type
if sys.version_info >= (3,):
AsyncFunctionDef = ast.AsyncFunctionDef
else:
AsyncFunctionDef = ast.stmt
The commit is not revealing:
https://github.com/asottile/pyupgrade/commit/fecacc91e57c224a0bd2564579ef01238650126c#diff-8213eba6a28bcc759225cd8cf49b2fd1
False can be truthy in Python 2 (where it can be re-defined) but not in Python 3. It might be a joke, or work in progress, or way of commenting out the code, but this is quite a mature tool- am I missing something?
答案1
得分: 5
AsyncFunctionDef 的值在运行时永远不需要，只在两个与 Python 2 兼容的类型提示（位于第 1298 和 1318 行）中被 mypy 使用。if False 阻止了在运行时发生赋值，但让 mypy 在类型检查期间看到正确的底层类型。（它还防止在 Python 2 中尝试导入 typing 模块时引发 ImportError。）
在这里使用 typing.TYPE_CHECKING 会更清晰（其值在运行时为 False，但在 mypy 运行时为 True），但是要注意 typing.TYPE_CHECKING 在 Python 2 中同样不可用。
英文:
The value of AsyncFunctionDef is never needed at runtime, only by mypy in two Python-2-compatible type hints (at lines 1298 and 1318). The if False prevents the assignments from occurring at run-time, but lets mypy see the correct underlying type to use during type-checking. (It also prevents an ImportError from being raised at the attempt to import the typing module under Python 2.)
It would be clearer to use typing.TYPE_CHECKING (whose value is False at run-time, but True when mypy runs) here, except typing.TYPE_CHECKING is also unavailable in Python 2.
答案2
得分: 5
if False被用于if typing.TYPE_CHECKING之上，因为python3.5.0-3.5.2受pyupgrade支持，不具有typing.TYPE_CHECKING -- 你可以在flake8-typing-imports中找到更多信息（我也是这个flake8插件的作者）
在3.5.3+和typing的后移版（在任何<3.5的版本中都可用）中，TYPE_CHECKING可用
免责声明：我是pyupgrade的作者。
英文:
if False is used over if typing.TYPE_CHECKING because python3.5.0-3.5.2 is supported by pyupgrade and does not have typing.TYPE_CHECKING -- you can find more information in flake8-typing-imports (I'm also the author of this flake8 plugin)
in 3.5.3+ and the typing backport (available in anything <3.5) TYPE_CHECKING is available
disclaimer: I'm the author of pyupgrade
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