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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