python - Python中递增和递减运算符的行为(Behaviour of increment and decrement operators in Python)

Question

I notice that a pre-increment/decrement operator can be applied on a variable (like ++count ).

(我注意到可以将预增/减运算符应用于变量（例如++count ）。)

It compiles, but it does not actually change the value of the variable!

(它可以编译，但实际上并不会改变变量的值！)

What is the behavior of the pre-increment/decrement operators (++/--) in Python?

(Python中预增/减运算符（++ /-）的行为是什么？)

Why does Python deviate from the behavior of these operators seen in C/C++?

(为什么Python会偏离C / C ++中看到的这些运算符的行为？)

  ask by Ashwin Nanjappa translate from so

Answer 1

++ is not an operator.

(++不是运算符。)

It is two + operators.

(它是两个+运算符。)

The + operator is the identity operator, which does nothing.

(+运算符是身份运算符，不执行任何操作。)

(Clarification: the + and - unary operators only work on numbers, but I presume that you wouldn't expect a hypothetical ++ operator to work on strings.)

(（澄清： +和-一元运算符仅对数字有效，但我想您不会期望假设的++运算符对字符串有效。）)

++count

Parses as

(解析为)

+(+count)

Which translates to

(转化为)

count

You have to use the slightly longer += operator to do what you want to do:

(您必须使用稍长的+=运算符来完成您想做的事情：)

count += 1

I suspect the ++ and -- operators were left out for consistency and simplicity.

(我怀疑++和--运算符由于一致性和简单性而被遗漏了。)

I don't know the exact argument Guido van Rossum gave for the decision, but I can imagine a few arguments:

(我不知道Guido van Rossum做出决定的确切论据，但我可以想象一些论点：)

• Simpler parsing.

  (更简单的解析。)

  Technically, parsing ++count is ambiguous, as it could be + , + , count (two unary + operators) just as easily as it could be ++ , count (one unary ++ operator).

  (从技术上讲，解析++count是模棱两可的，因为它可能是+ ， + ， count （两个一元+运算符），就像它可能是++ ， count （一个一元++运算符）一样容易。)

  It's not a significant syntactic ambiguity, but it does exist.

  (它不是语法上的明显歧义，但确实存在。)

• Simpler language.

  (语言更简单。)

  ++ is nothing more than a synonym for += 1 .

  (++只是+= 1的同义词。)

  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.

  (这是一种速记的发明，因为C编译器很愚蠢，并且不知道如何将a += 1优化a += 1大多数计算机拥有的inc指令。)

  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.

  (在优化编译器和字节码解释语言的这一天，通常不赞成在一种语言中添加运算符以允许程序员优化其代码，尤其是在像Python这样设计成一致且易读的语言中。)

• Confusing side-effects.

  (令人困惑的副作用。)

  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.

  (使用++运算符的语言中一个常见的新手错误是混合使用递增/递减运算符之间的差异（优先级和返回值），Python喜欢消除语言“陷阱”。)

  The precedence issues of pre-/post-increment in C are pretty hairy, and incredibly easy to mess up.

  (C中的预增/后增的优先级问题非常棘手，难以置信。)