为什么000在Python 3中评估为0？

问题描述

由于Python 3中的八进制前缀现在是 0 ，因此再写 0777 是不合法的。好的。

那么为什么写 00 合理地评估 0 而其他数字会触发语法错误？

 >>> 01 
 ... 
文件<互动输入>，第1行
 01 
 ^ 
语法错误：无效令牌
>>> ; 
>>> 00 
 0 
  

解决方案

如果有人看看在（整数）文字部分）页面：

所以这意味着他们为零做了一个例外（在可以找到那里）：你可以将零写为零序列。我的猜测当然是他们必须包含0（你怎么指定零作为 decinteger ？ ），那么为什么在这种情况下不允许更多的零，无论数字系统如何， 000 都保持为零。他们可能不希望允许 01 作为 decinteger 来防止意外运行代码，从而获得完全不同的结果。

最后请注意，下划线只是该规范的一部分，因为的问题：在它们在语法中没有提到。

在 指定零后跟其他数字（也是其他零作为 octinteger ：

Since the octal prefix is now 0o in Python 3 it's not legal to write 0777 any more. Okay.

So why is it legal to write 00 which evaluates properly to 0 whereas other digits trigger a syntax error?

>>> 01
  ...
  File "<interactive input>", line 1
    01
     ^
SyntaxError: invalid token
>>>
>>> 00
0

解决方案

If one takes a look at the Lexical Analysis (Integer Literal Section) page:

So that means that a decinteger either begins with a nonzero digit (followed by all possible digits and optionally underscores), or is a sequence of zeros with optionally underscores (which maps to zero).

The documentation furthermore states that:

So it means they make an exception for zero (in all documentation for python-3.3 one can find there): you can write zero as a sequence of zeros. My guess is that of course they have to include "0" (how else would you specify zero as a decinteger?), so why not allow more zeros in that case, regardless of the number system, 000 is and stays zero. They probably do not want to allow 01 as a decinteger to prevent accidentally running python-2.x code and thus obtaining totally different results.

Finally note that the underscores are only part of that specification since python-3.6: in the specifications for 3.5 they are not mentioned in the grammar.

In python-2.7 the documentation specifies a zero followed by other digits (also other zeros as an octinteger:

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