［Python进阶－5］类的特殊方法

（1）str和repr

——一般我们用print打印一个变量时，一般是调用了它的str方法。这里打印实例也是如此，调用了str，所以，我们可通过重写该方法获得我们想到的输入。

——但是如上的重写str只能在print x时有用，如果直接用x，输出的仍然是内存地址之类的，所以有另一个方法，就是repr这个其实是面向开发者的输出。也可以重新定义。

class Person(object):
def __init__(self, name, gender):
self.name = name
self.gender = gender

class Student(Person):
def __init__(self, name, gender, score):
super(Student, self).__init__(name, gender)
self.score = score

def __str__(self):
return '(Student:%s,%s,%s)'%(self.name,self.gender,self.score)
__repr__=__str__

s = Student('Bob', 'male', 88)
print s

（2）排序实现复习，按分数排序，分数相同则按名字

class Student(object):

def __init__(self, name, score):
self.name = name
self.score = score

def __str__(self):
return '(%s: %s)' % (self.name, self.score)

__repr__ = __str__

def __cmp__(self, s):
if self.score==s.score:
return cmp(self.name,s.name)
return -cmp(self.score,s.score)

L = [Student('Tim', 99), Student('Bob', 88), Student('Alice', 99)]
print sorted(L)

＃[(Alice: 99), (Tim: 99), (Bob: 88)]

（3）联系str和len特殊方法

class Fib(object):

def __init__(self, num):
self.num=num
a,b,L=0,1,[]
for n in range(num):
L.append(a)
a,b=b,a+b
self.nums=L

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

def __len__(self):
return self.num

f = Fib(10)
print f
print len(f)

（4）实现有理数的加减乘除

class Rational(object):
def __init__(self, p, q):
self.p = p
self.q = q

def __add__(self, r):
return Rational(self.p * r.q + self.q * r.p, self.q * r.q)

def __sub__(self, r):
return Rational(self.p * r.q - self.q * r.p, self.q * r.q)

def __mul__(self, r):
return Rational(self.p * r.p, self.q * r.q)

def __div__(self, r):
return Rational(self.p * r.q, self.q * r.p)

def __str__(self):
return '%s/%s'%(self.p,self.q)

__repr__ = __str__

r1 = Rational(1, 2)
r2 = Rational(1, 4)
print r1 + r2
print r1 - r2
print r1 * r2
print r1 / r2

（5）int和float实现类型转换

class Rational(object):
def __init__(self, p, q):
self.p = p
self.q = q

def __int__(self):
return self.p // self.q

def __float__(self):
return float(self.p)/self.q

print float(Rational(7, 2))
print float(Rational(1, 3))

（6）利用@property可添加一个属性。也可以配套@属性.setter来设置属性值。

class Student(object):

def __init__(self, name, score):
self.name = name
self.__score = score

@property
def score(self):
return self.__score

@score.setter
def score(self, score):
if score < 0 or score > 100:
raise ValueError('invalid score')
self.__score = score

@property
def grade(self):
if self.score>=80:
return 'A'
elif self.score>=60:
return 'B'
else:
return 'C'

s = Student('Bob', 59)
print s.grade

s.score = 60
print s.grade

s.score = 99
print s.grade

（7）slot固定了实例有的属性个数，不能随意添加

class Person(object):
__slots__ = ('name', 'gender')
def __init__(self, name, gender):
self.name = name
self.gender = gender

class Student(Person):
__slots__ = ('name','gender','score')
def __init__(self,name,gender,score):
super(Student,self).__init__(name,gender)
self.score=score

s = Student('Bob', 'male', 59)
s.name = 'Tim'
s.score = 99
print s.score

（8）call，重写输出feb数列

class Fib(object):
def __call__(self,n):
a=0
b=1
L=[]
for x in range(n):
L.append(a)
a,b=b,a+b
return L
f = Fib()
print f(10)