为什么000在Python 3中评估为0？ [英] Why does 000 evaluate to 0 in Python 3?

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 Lexical Analysis （整数）文字部分）页面：

  integer :: = decinteger | bininteger | octinteger | hexinteger 
 decinteger :: = nonzerodigit（[_] digit）* | 0+（[_]0）*  
 ...  

这意味着 decinteger 以非零数字开头（后跟所有可能的数字和可选的下划线），或者是一个零序列，带有可选的下划线（映射为零）。

该文档还声明：

请注意，非零十进制数中的前导零不允许。

所以这意味着他们为零做了一个例外（在 python-3.3 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 python-2.x 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 python-3.6 的问题：在 3.5的规范它们在语法中没有提到。

在 python-2.7 文档指定零后跟其他数字（也是其他零作为 octinteger ：

  integer :: = decimalinteger | octinteger | hexinteger | bininteger 
 decimalinteger :: = nonzerodigit digit * | 0
 octinteger :: =0（o|O）octdigit + | 0octdigit + 
  

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:

integer      ::=  decinteger | bininteger | octinteger | hexinteger
decinteger   ::=  nonzerodigit (["_"] digit)* | "0"+(["_"] "0")*
...

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:

Note that leading zeros in a non-zero decimal number are not allowed.

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:

integer        ::=  decimalinteger | octinteger | hexinteger | bininteger
decimalinteger ::=  nonzerodigit digit* | "0"
octinteger     ::=  "0" ("o" | "O") octdigit+ | "0" octdigit+

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