数字文字的调用方法 [英] Call methods of a number literal

问题描述

我可以在字符串文字上调用 str 方法.但是我不能在整数文字上调用 int 方法:

I can call str methods on string literal. But I cannot call int methods on a integer number literal:

Python 2.7.5+ (default, Sep 19 2013, 13:48:49) 
>>> 1.bit_length()
  File "<stdin>", line 1
    1.bit_length()
               ^
SyntaxError: invalid syntax
>>> a = 1
>>> a.bit_length()
1
>>> ', '.join(['1', '2'])
'1, 2'
>>> 

这是为什么?

推荐答案

Python 将 1. 视为浮点数的开头，但无法解析该行的其余部分.改成

Python treats 1. as a beginning of a float number, but fails to parse the rest of the line. Change it to

(1).bit_length()

通过用括号括起数字文字，我们确保 python 计算括号内的表达式，即 1 并调用该数字的方法.

by enclosing the number literal with parenthesis, we make sure that python evaluates the expression within the parens, which is 1 and call the method on that number.

Python 像这样定义了浮点文字

Python defines floating point literal like this

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

根据该定义，词法分析器认为 1.bit_length() 将是一个浮点文字，因为 1. 匹配 [intpart] 分数 的开头.但该行的其余部分不匹配.这就是它失败的原因.

As per that definition, the lexical analyzer thinks that 1.bit_length() would be a float literal, since 1. matches the [intpart] fraction's beginning. But the rest of the line doesn't match. That is why it fails.

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