我如何使用预递增/递减操作符(++,——),就像在c++ ?

为什么++计数运行,但不改变变量的值?


当前回答

一个直截了当的解决办法

c = 0
c = (lambda c_plusplus: plusplus+1)(c)
print(c)
1

不再打字

 c = c + 1

你也可以直接写 c++ 完成所有代码,然后搜索/替换“c++”,替换为“c=c+1”。只要确保正则表达式搜索是关闭的。

其他回答

++不是运算符。它是两个+运算符。+运算符是单位运算符,它什么也不做。(澄清:+和-一元操作符仅适用于数字,但我假定您不会期望假设的++操作符适用于字符串。)

++count

解析为

+(+count)

翻译过来就是

count

你必须使用稍微长一点的+=运算符来做你想做的事情:

count += 1

我怀疑省略++和——操作符是为了保持一致性和简单性。我不知道Guido van Rossum为这个决定给出的确切理由,但我可以想象出一些理由:

Simpler parsing. Technically, parsing ++count is ambiguous, as it could be +, +, count (two unary + operators) just as easily as it could be ++, count (one unary ++ operator). It's not a significant syntactic ambiguity, but it does exist. Simpler language. ++ is nothing more than a synonym for += 1. It was a shorthand invented because C compilers were stupid and didn't know how to optimize a += 1 into the inc instruction most computers have. In this day of optimizing compilers and bytecode interpreted languages, adding operators to a language to allow programmers to optimize their code is usually frowned upon, especially in a language like Python that is designed to be consistent and readable. Confusing side-effects. One common newbie error in languages with ++ operators is mixing up the differences (both in precedence and in return value) between the pre- and post-increment/decrement operators, and Python likes to eliminate language "gotcha"-s. The precedence issues of pre-/post-increment in C are pretty hairy, and incredibly easy to mess up.

python中不像C语言中那样有后/前自增/自减操作符。

我们可以看到++或-作为多个符号相乘,就像我们在数学中做的(-1)*(-1)=(+1)。

E.g.

---count

解析为

-(-(-count)))

翻译过来就是

-(+count)

因为-号和-号的乘法是+

最后,

-count

在Python中,表达式和语句之间有严格的区别 与Common Lisp、Scheme或 Ruby。

维基百科

因此,通过引入这样的操作符,可以打破表达式/语句的分割。

和你不能写作的原因一样

if x = 0:
  y = 1

就像在其他一些不保留这种区别的语言中一样。

一个直截了当的解决办法

c = 0
c = (lambda c_plusplus: plusplus+1)(c)
print(c)
1

不再打字

 c = c + 1

你也可以直接写 c++ 完成所有代码,然后搜索/替换“c++”,替换为“c=c+1”。只要确保正则表达式搜索是关闭的。

扩展Henry的回答,我实验性地实现了一个语法糖库,实现了++:hdytto。

使用方法简单。从PyPI安装后,放置sitecustomize.py:

from hdytto import register_hdytto
register_hdytto()

在项目目录中。然后,使main.py:

# coding: hdytto

a = 5
print(a++)
print(++a)
b = 10 - --a
print(b--)

并通过PYTHONPATH=运行它。python main.py。输出将是

5
7
4

在解码脚本文件时,Hdytto将++替换为((a:=a+1)-1),因此它可以工作。