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 = genderclass 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) # 3class 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 + r2print r1 - r2print r1 * r2print r1 / r2

6. python中类型转换

print int(12.34) # 12print float(12) # 12.0class 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.qprint float(Rational(7, 2)) # 3.5print 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.grades.score = 60print s.grades.score = 99print 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 = scores = Student('Bob', 'male', 59)s.name = 'Tim' # OKs.score = 99 # OKs.grade = 'A' # Errorclass Person(object):    __slots__ = ('name', 'gender')    def __init__(self, name, gender):        self.name = name        self.gender = genderclass Student(Person):    __slots__ = {'score'}    def __init__(self, name, gender, score):        super(Student, self).__init__(name, gender)        self.score = scores = Student('Bob', 'male', 59)s.name = 'Tim's.score = 99print 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...' % friendp = 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 Lf = Fib()print f(10) # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]