python定制类(进阶6)
1. python中什么是特殊方法
![Python定制类(进阶6)]()
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] 10.下一步学习内容 IO:文件和Socket 多线程：进程和线程 数据库 Web开发