为什么在Python 3中将True和False更改为关键字

问题描述

在Python 2中，我们可以重新分配True和False(但不能重新分配None)，但是三个(True，False和None)都被视为内置变量.但是，在Py3k中，根据文档将这三个变量都更改为关键字.. >

根据我自己的推测，我只能猜测这是为了防止像，它源自旧的True, False = False, True恶作剧.但是，在Python 2.7.5以及之前的版本中，重新分配了None的None = 3这样的语句会引发SyntaxError: cannot assign to None.

从语义上讲，我不相信True，False和None是关键字，因为它们最终在语义上是文字，这是Java所做的.我检查了PEP 0(索引)，却找不到说明为什么更改它们的PEP.

将关键字设置为文字或像python2中的None那样特殊设置是否有性能优势或其他原因?

解决方案

可能是因为Python 2.6不仅允许True = False，而且还允许您说一些有趣的事情，例如:

__builtin__.True = False

，整个过程会将True重置为False.它可以导致发生真正有趣的事情:

>>> import __builtin__
>>> __builtin__.True = False
>>> True
False
>>> False
False
>>> __builtin__.False = True
>>> True
False
>>> False
False

编辑:如 Mike 所述， Python Wiki 在核心语言更改下也声明了以下内容:

• 输入True和False关键字.
  • 原因:无法分配给他们.

In Python 2, we could reassign True and False (but not None), but all three (True, False, and None) were considered builtin variables. However, in Py3k all three were changed into keywords as per the docs.

From my own speculation, I could only guess that it was to prevent shenanigans like this which derive from the old True, False = False, True prank. However, in Python 2.7.5, and perhaps before, statements such as None = 3 which reassigned None raised SyntaxError: cannot assign to None.

Semantically, I don't believe True, False, and None are keywords, since they are at last semantically literals, which is what Java has done. I checked PEP 0 (the index) and I couldn't find a PEP explaining why they were changed.

Are there performance benefits or other reasons for making them keywords as opposed to literals or special-casing them like None in python2?

解决方案

Possibly because Python 2.6 not only allowed True = False but also allowed you to say funny things like:

__builtin__.True = False

which would reset True to False for the entire process. It can lead to really funny things happening:

>>> import __builtin__
>>> __builtin__.True = False
>>> True
False
>>> False
False
>>> __builtin__.False = True
>>> True
False
>>> False
False

EDIT: As pointed out by Mike, the Python wiki also states the following under Core Language Changes:

• Make True and False keywords.
  • Reason: make assignment to them impossible.

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