怎么可能在 Python 中计算 +5？

Question

如何评估 + 5（剧透警告：结果为 5）？

+ 不是通过调用某些东西的 __add__ 方法来工作吗？ 5 将是“other”在：

>>> other = 5
>>> x = 1
>>> x.__add__(other)
6

那么允许加5的"void"是什么？

void.__add__(5)

另一个线索是：

/ 5

抛出错误：

TypeError: 'int' object is not callable

Answer 1

根据 language reference on numeric literals:

Note that numeric literals do not include a sign; a phrase like -1 is actually an expression composed of the unary operator - and the literal 1.

和 section on unary operators:

The unary - (minus) operator yields the negation of its numeric argument.

The unary + (plus) operator yields its numeric argument unchanged.

没有一元 /（除法）运算符，因此出现错误。

相关的"magic methods"(__pos__,__neg__)在the data model documentation.

Answer 2

本例中的+调用了一元魔法方法__pos__ rather than __add__:

>>> class A(int):
    def __pos__(self):
        print '__pos__ called'
        return self
...
>>> a = A(5)
>>> +a
__pos__ called
5
>>> +++a
__pos__ called
__pos__ called
__pos__ called
5

Python只支持其中的4个（一元算术运算）__neg__、__pos__、__abs__和__invert__，因此SyntaxError 与 /。请注意，__abs__ 与称为 abs() 的内置函数一起使用，即此一元运算没有运算符。

请注意 /5（/ 后跟一些东西）仅在 IPython shell 中有不同的解释，对于正常的 shell 是语法错误正如预期的那样：

Ashwinis-MacBook-Pro:py ashwini$ ipy
Python 2.7.6 (default, Sep  9 2014, 15:04:36)
Type "copyright", "credits" or "license" for more information.

IPython 3.0.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.
>>> /5
Traceback (most recent call last):
  File "<ipython-input-1-2b14d13c234b>", line 1, in <module>
    5()
TypeError: 'int' object is not callable

>>> /float 1
1.0
>>> /sum (1 2 3 4 5)
15

Ashwinis-MacBook-Pro:~ ashwini$ python
Python 2.7.6 (default, Sep  9 2014, 15:04:36)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> /5
  File "<stdin>", line 1
    /5
    ^
SyntaxError: invalid syntax
>>> /float 1
  File "<stdin>", line 1
    /float 1
    ^
SyntaxError: invalid syntax

Answer 3

看起来你找到了三个之一 unary operators:

一元加运算+x调用__pos__()方法。
一元否定运算-x调用__neg__()方法。
一元非（或反转）操作 ~x 调用 __invert__() 方法。

怎么可能在 Python 中计算 +5？

How is it possible to evaluate +5 in Python?

python

operators

addition