Python定制类(进阶6)

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http://www.cnblogs.com/why168888/p/6411919.html

本文出自:【Edwin博客园】

Python定制类(进阶6)

1. python中什么是特殊方法

任何数据类型的实例都有一个特殊方法：__str__()

• 用于print的__str__
• 用于len的__len__
• 用于cmp的__cmp__
• 特殊方法定义在class中
• 不需要直接调用
• Python的某些函数或操作符会调用对应的特殊方法

正确实现特殊方法

• 只需要编写用到的特殊方法
• 有关联性的特殊方法都必须实现
• __getattr__,__setattr__,__delattr__

2. python中 __str__和__repr__

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

3. python中 __cmp__

对 int、str 等内置数据类型排序时，Python的 sorted() 按照默认的比较函数 cmp 排序，但是，如果对一组 Student 类的实例排序时，就必须提供我们自己的特殊方法 __cmp__()

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.name < s.name:
            return -1
        elif self.name > s.name:
            return 1
        else:
            return 0

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)

4. python中 __len__

如果一个类表现得像一个list，要获取有多少个元素，就得用 len() 函数.

要让 len() 函数工作正常，类必须提供一个特殊方法__len__()，它返回元素的个数。

class Students(object):
    def __init__(self, *args):
        self.names = args
    def __len__(self):
        return len(self.names)
ss = Students('Bob', 'Alice', 'Tim')
print len(ss) # 3

class Fib(object):

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

    def __len__(self):
        return len(self.num)

f = Fib(10)
print f.num # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
print len(f) # 10

5. python中数学运算

Python 提供的基本数据类型 int、float 可以做整数和浮点的四则运算以及乘方等运算。

def gcd(a, b):
    if b == 0:
        return a
    return gcd(b, a % b)

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):
        g = gcd(self.p, self.q)
        return '%s/%s' % (self.p / g, self.q / g)

    __repr__ = __str__

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

6. python中类型转换

print int(12.34) # 12
print float(12) # 12.0

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)) # 3.5
print float(Rational(1, 3)) # 0.333333333333

7. python中 @property

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 < 60:
            return 'C'
        if self.score < 80:
            return 'B'
        return 'A'

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

s.score = 60
print s.grade

s.score = 99
print s.grade

8. python中 __slots__

slots 的目的是限制当前类所能拥有的属性，如果不需要添加任意动态的属性，使用__slots__也能节省内存。

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

s = Student('Bob', 'male', 59)
s.name = 'Tim' # OK
s.score = 99 # OK
s.grade = 'A' # Error

class Person(object):

    __slots__ = ('name', 'gender')

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

class Student(Person):

    __slots__ = {'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

9. python中__call__

一个类实例也可以变成一个可调用对象，只需要实现一个特殊方法__call__()

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

    def __call__(self, friend):
        print 'My name is %s...' % self.name
        print 'My friend is %s...' % friend

p = Person('Bob', 'male')
p('Tim') # My name is Bob... My friend is Tim...

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

f = Fib()
print f(10) # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

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