为什么Python 3允许“00”。作为0的文字但不允许“01”作为1的字面值？ [英] Why does Python 3 allow "00" as a literal for 0 but not allow "01" as a literal for 1?

问题描述

为什么Python 3允许00作为0的文字而不允许01作为1的文字？有充分的理由吗？这种不一致让我感到困惑。 （我们正在谈论Python 3，它故意破坏向后兼容性以实现一致性等目标。）

Why does Python 3 allow "00" as a literal for 0 but not allow "01" as a literal for 1? Is there a good reason? This inconsistency baffles me. (And we're talking about Python 3, which purposely broke backward compatibility in order to achieve goals like consistency.)

例如：

>>> from datetime import time
>>> time(16, 00)
datetime.time(16, 0)
>>> time(16, 01)
  File "<stdin>", line 1
    time(16, 01)
              ^
SyntaxError: invalid token
>>>

推荐答案

每 https://docs.python.org/3/reference/lexical_analysis.html#integer-literals ：

整数文字由以下词汇定义描述：

Integer literals are described by the following lexical definitions:

integer        ::=  decimalinteger | octinteger | hexinteger | bininteger
decimalinteger ::=  nonzerodigit digit* | "0"+
nonzerodigit   ::=  "1"..."9"
digit          ::=  "0"..."9"
octinteger     ::=  "0" ("o" | "O") octdigit+
hexinteger     ::=  "0" ("x" | "X") hexdigit+
bininteger     ::=  "0" ("b" | "B") bindigit+
octdigit       ::=  "0"..."7"
hexdigit       ::=  digit | "a"..."f" | "A"..."F"
bindigit       ::=  "0" | "1"

除了
之外，整数文字的长度没有限制存储在可用内存中。

There is no limit for the length of integer literals apart from what can be stored in available memory.

请注意，不允许使用非零十进制数的前导零。
这是用于消除C风格八进制文字的消歧，在3.0版之前使用Python
。

Note that leading zeros in a non-zero decimal number are not allowed. This is for disambiguation with C-style octal literals, which Python used before version 3.0.

As这里注意到，不允许在非零十进制数字中前导零。 0+ 作为一个非常特殊的案例是合法的，在Python 2中不存在：

As noted here, leading zeros in a non-zero decimal number are not allowed. "0"+ is legal as a very special case, which wasn't present in Python 2:

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

---

SVN commit r55866 在tokenizer中实现了PEP 3127，禁止旧版本 0< octal> 数字。然而，奇怪的是，它还添加了这个注释：

---

SVN commit r55866 implemented PEP 3127 in the tokenizer, which forbids the old 0<octal> numbers. However, curiously, it also adds this note:

/* in any case, allow '0' as a literal */

带有特殊的非零标志，只会抛出一个 SyntaxError 如果以下数字序列包含非零数字。

with a special nonzero flag that only throws a SyntaxError if the following sequence of digits contains a nonzero digit.

这是奇怪的，因为 PEP 3127 不允许这种情况：

This is odd because PEP 3127 does not allow this case:

这个PEP建议通过使用前导零来指定八进制数的能力将从Python 3.0中的语言（以及2.6的Python 3.0预览模式）中删除，并且 a只要前导0紧跟另一个数字，就会引发SyntaxError。

This PEP proposes that the ability to specify an octal number by using a leading zero will be removed from the language in Python 3.0 (and the Python 3.0 preview mode of 2.6), and that a SyntaxError will be raised whenever a leading "0" is immediately followed by another digit.

（强调我的）

所以，多个零的事实是允许技术上违反PEP，并且基本上由Georg Brandl作为特例执行。他做了相应的文档更改，注意0+ 是 decimalinteger 的有效案例（之前已经过涵盖在 octinteger ）。

So, the fact that multiple zeros are allowed is technically violating the PEP, and was basically implemented as a special case by Georg Brandl. He made the corresponding documentation change to note that "0"+ was a valid case for decimalinteger (previously that had been covered under octinteger).

我们可能永远不会知道完全为什么Georg选择了使0+ 有效 - 在Python中它可能永远是一个奇怪的角落。

We'll probably never know exactly why Georg chose to make "0"+ valid - it may forever remain an odd corner case in Python.

更新 [2015年7月28日]：这个问题导致生动的讨论主题，其中 Georg chimed in ：

UPDATE [28 Jul 2015]: This question led to a lively discussion thread on python-ideas in which Georg chimed in:

Steven D'Aprano写道：

Steven D'Aprano wrote:

为什么这样定义？ [...]为什么我们写0000才能获得零？

Why was it defined that way? [...] Why would we write 0000 to get zero?

我可以告诉你，但是我必须杀了你。

I could tell you, but then I'd have to kill you.

Georg

稍后，该线程产生了此错误报告旨在摆脱这种特殊情况。在这里，乔治说：

Later on, the thread spawned this bug report aiming to get rid of this special case. Here, Georg says:

我不记得这种故意改变的原因（从文档更改中可以看出）。

I don't recall the reason for this deliberate change (as seen from the docs change).

我现在无法想出这个改变的好理由[...]

I'm unable to come up with a good reason for this change now [...]

因此我们拥有它：这种不一致背后的确切原因是时间丢失。

and thus we have it: the precise reason behind this inconsistency is lost to time.

最后，请注意错误报告被拒绝：对于Python 3.x的其余部分，前导零将继续只接受零整数。

Finally, note that the bug report was rejected: leading zeros will continue to be accepted only on zero integers for the rest of Python 3.x.

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