Python中递增和递减运算符的行为

#1 楼

++count

+(+count)

count

count += 1

#2 楼

b++

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

b = b + 1

b += 1

#3 楼

class ValueKeeper(object):
    def __init__(self, value): self.value = value
    def __str__(self): return str(self.value)

class A(ValueKeeper):
    def __pos__(self):
        print 'called A.__pos__'
        return B(self.value - 3)

class B(ValueKeeper):
    def __pos__(self):
        print 'called B.__pos__'
        return A(self.value + 19)

x = A(430)
print x, type(x)
print +x, type(+x)
print ++x, type(++x)
print +++x, type(+++x)

#4 楼

def PreIncrement(name, local={}):
    #Equivalent to ++name
    if name in local:
        local[name]+=1
        return local[name]
    globals()[name]+=1
    return globals()[name]

def PostIncrement(name, local={}):
    #Equivalent to name++
    if name in local:
        local[name]+=1
        return local[name]-1
    globals()[name]+=1
    return globals()[name]-1

x = 1
y = PreIncrement('x') #y and x are both 2
a = 1
b = PostIncrement('a') #b is 1 and a is 2

x = 1
def test():
    x = 10
    y = PreIncrement('x') #y will be 2, local x will be still 10 and global x will be changed to 2
    z = PreIncrement('x', locals()) #z will be 11, local x will be 11 and global x will be unaltered
test()

x = 1
print(PreIncrement('x'))   #print(x+=1) is illegal!

x = 1
x+=1
print(x)

def PreDecrement(name, local={}):
    #Equivalent to --name
    if name in local:
        local[name]-=1
        return local[name]
    globals()[name]-=1
    return globals()[name]

def PostDecrement(name, local={}):
    #Equivalent to name--
    if name in local:
        local[name]-=1
        return local[name]+1
    globals()[name]-=1
    return globals()[name]+1

#5 楼

a += 1

a = 1

a += 1

a = 1
print(hex(id(a)))
a += 1
print(hex(id(a)))

#6 楼

if x = 0:
  y = 1

#7 楼

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

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

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

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

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

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

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

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

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

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

#8 楼

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

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

    
1
2
3
4

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

    
1
2
3
4

#9 楼

---count

-(-(-count)))

-(+count)

-count