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

是的，我也错过了++和-功能。几百万行c代码让这种想法在我的老头脑中根深蒂固，而不是与它斗争……这里是一个我拼凑起来实现的类:

pre- and post-increment, pre- and post-decrement, addition,
subtraction, multiplication, division, results assignable
as integer, printable, settable.

这里的是:

class counter(object):
    def __init__(self,v=0):
        self.set(v)

    def preinc(self):
        self.v += 1
        return self.v
    def predec(self):
        self.v -= 1
        return self.v

    def postinc(self):
        self.v += 1
        return self.v - 1
    def postdec(self):
        self.v -= 1
        return self.v + 1

    def __add__(self,addend):
        return self.v + addend
    def __sub__(self,subtrahend):
        return self.v - subtrahend
    def __mul__(self,multiplier):
        return self.v * multiplier
    def __div__(self,divisor):
        return self.v / divisor

    def __getitem__(self):
        return self.v

    def __str__(self):
        return str(self.v)

    def set(self,v):
        if type(v) != int:
            v = 0
        self.v = v

你可以这样使用它:

c = counter()                          # defaults to zero
for listItem in myList:                # imaginary task
     doSomething(c.postinc(),listItem) # passes c, but becomes c+1

.．.已经有了c，你可以这样做。

c.set(11)
while c.predec() > 0:
    print c

．...或者只是……

d = counter(11)
while d.predec() > 0:
    print d

.．.和(重)赋值为整数…

c = counter(100)
d = c + 223 # assignment as integer
c = c + 223 # re-assignment as integer
print type(c),c # <type 'int'> 323

.．.而这将保持c作为类型计数器:

c = counter(100)
c.set(c + 223)
print type(c),c # <class '__main__.counter'> 323

编辑:

还有一点出乎意料(完全不受欢迎)的行为，

c = counter(42)
s = '%s: %d' % ('Expecting 42',c) # but getting non-numeric exception
print s

.．.因为在该元组中，没有使用getitem()，而是将对象的引用传递给格式化函数。叹息。所以:

c = counter(42)
s = '%s: %d' % ('Expecting 42',c.v) # and getting 42.
print s

.．.或者，更详细，更明确地说，我们实际上想要发生什么，尽管在实际形式中与冗长相反(使用c.v代替)……

c = counter(42)
s = '%s: %d' % ('Expecting 42',c.__getitem__()) # and getting 42.
print s

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 3.8+中，你可以做:

(a:=a+1) #same as ++a (increment, then return new value)
(a:=a+1)-1 #same as a++ (return the incremented value -1) (useless)

你可以用这个做很多思考。

>>> a = 0
>>> while (a:=a+1) < 5:
    print(a)

    
1
2
3
4

或者如果你想写一些更复杂的语法(目标不是优化):

>>> del a
>>> while (a := (a if 'a' in locals() else 0) + 1) < 5:
    print(a)

    
1
2
3
4

即使'a'不存在也会返回0，然后将其设置为1

++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没有前增量和后增量操作符。

在Python中，整数是不可变的。那就是你不能改变他们。这是因为整数对象可以在多个名称下使用。试试这个:

>>> b = 5
>>> a = 5
>>> id(a)
162334512
>>> id(b)
162334512
>>> a is b
True

上面的A和b实际上是同一个物体。如果你增加a，你也会增加b，这不是你想要的。所以你必须重新分配。是这样的:

b = b + 1

许多使用python的C程序员都想要一个自增操作符，但是这个操作符看起来像是对对象进行了自增操作，而实际上是对对象进行了重赋。因此-=和+=运算符加在一起，比b = b+ 1更短，同时比b++更清晰和灵活，所以大多数人会用:

b += 1

将b重新赋值给b+1。这不是一个自增运算符，因为它不加b，而是重新赋值。

简而言之:Python在这里表现不同，因为它不是C语言，也不是机器代码的低级包装器，而是一种高级动态语言，其中增量没有意义，也不像C语言那样必要，例如，在C语言中，每次有循环时都要使用增量。