访问文字的属性适用于所有类型，但不适用于`int`；为什么? [英] Accessing attributes on literals work on all types, but not `int`; why?

问题描述

我已经阅读到python中的所有内容都是一个对象，因此我开始尝试不同的类型并对它们调用 __str__ —起初我感到非常兴奋，但后来我感到困惑.

I have read that everything in python is an object, and as such I started to experiment with different types and invoking __str__ on them — at first I was feeling really excited, but then I got confused.

>>> "hello world".__str__()
'hello world'
>>> [].__str__()
'[]'
>>> 3.14.__str__()
'3.14'
>>> 3..__str__()
'3.0'
>>> 123.__str__()
  File "<stdin>", line 1
    123.__str__()
              ^
SyntaxError: invalid syntax

• 为什么something.__str__()除了int之外还适用于一切"?
• 123不是类型为int的对象吗?
• Why does something.__str__() work for "everything" besides int?
• Is 123 not an object of type int?

推荐答案

所以您认为您可以  dance  浮点数?

123和3.14一样是一个对象，问题"位于语言的语法规则之内；解析器认为我们将要定义 float —不是带有跟踪方法调用的 int .

So you think you can  dance  floating-point?

123 is just as much of an object as 3.14, the "problem" lies within the grammar rules of the language; the parser thinks we are about to define a float — not an int with a trailing method call.

如果将数字用括号括起来，我们将获得预期的行为，如下所示.

We will get the expected behavior if we wrap the number in parenthesis, as in the below.

>>> (123).__str__()
'123'

或者如果我们只是在 123 之后添加一些空格:

Or if we simply add some whitespace after 123:

>>> 123 .__str__()
'123'




不适用于123.__str__()的原因是 123 之后的 dot 被解释为小数点一些部分声明的 floating-point .

The reason it does not work for 123.__str__() is that the dot following the 123 is interpreted as the decimal-point of some partially declared floating-point.

>>> 123.__str__()
  File "", line 1
    123.__str__()
              ^
SyntaxError: invalid syntax

解析器尝试将__str__()解释为数字序列，但显然失败—.并且我们得到一个 SyntaxError ，基本上是说解析器偶然发现了一些意外的东西.

The parser tries to interpret __str__() as a sequence of digits, but obviously fails — and we get a SyntaxError basically saying that the parser stumbled upon something that it did not expect.




查看123.__str__()时，python解析器可以使用 3 个字符，并将这些 3 个字符解释为 integer ，或，它可以使用 4 个字符并将它们解释为浮点的开始.

When looking at 123.__str__() the python parser could use either 3 characters and interpret these 3 characters as an integer, or it could use 4 characters and interpret these as the start of a floating-point.

123.__str__()
^^^ - int

123.__str__()
^^^^- start of floating-point

就像一个小孩想要在盘子上放尽可能多的蛋糕一样，解析器很贪婪，并且想一次吞下尽可能多的东西—即使这并不总是最好的主意，因此选择了后者(更好").

Just as a little child would like as much cake as possible on their plate, the parser is greedy and would like to swallow as much as it can all at once — even if this isn't always the best of ideas —as such the latter ("better") alternative is chosen.

当后来意识到__str__()绝不能解释为浮点数的小数位时，已经为时已晚. SyntaxError .

When it later realizes that __str__() can in no way be interpreted as the decimals of a floating-point it is already too late; SyntaxError.

^注意

^Note

 123 .__str__() # works fine

在上面的代码段中，由于没有 number 可以包含空格，因此必须将123 (请注意空格)解释为整数.这意味着它在语义上等同于(123).__str__().

In the above snippet, 123  (note the space) must be interpreted as an integer since no number can contain spaces. This means that it is semantically equivalent to (123).__str__().

^注意

^Note

 123..__str__() # works fine

上面的方法也有效，因为一个数字最多可以包含一个小数点，这意味着它等同于(123.).__str__().

The above also works because a number can contain at most one decimal-point, meaning that it is equivalent to (123.).__str__().




本节包含相关文字的词汇定义.

This section contains the lexical definition of the relevant literals.

_{词汇分析-2.4.5浮点文字}

floatnumber   ::=  pointfloat | exponentfloat
pointfloat    ::=  [intpart] fraction | intpart "."
exponentfloat ::=  (intpart | pointfloat) exponent
intpart       ::=  digit+
fraction      ::=  "." digit+
exponent      ::=  ("e" | "E") ["+" | "-"] digit+

词汇分析-2.4.4整数文字

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"

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