python基础---->python的使用(六)，,　　这里记录一下py

　　这里记录一下python中关于class类的一些知识。不解释就弄不懂的事，就意味着怎样解释也弄不懂。

python中的类知识

一、class的属性引用与实例

class MyClass():    ‘‘‘A simple exampel class‘‘‘    i = 12345 # class variable shared by all instances     def __init__(self, realpart, imagpart):        self.real = realpart # instance variable unique to each instance        self.imag = imagpart    def f(self):        return self.real + ‘hello‘x = MyClass(‘huhx‘, ‘linux‘)print(x.real, x.imag, x.i, MyClass.i)  # MyClass.real会报错print(MyClass.__doc__, x.__doc__)  # A simple exampel classprint(MyClass.f(x), x.f())  # huhxhello huhxhello

When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance.所以python类的__init__()方法类似于java中构造方法。MyClass类的属性i在所有MyClass的实例中共享，而real和imag就是实例私有，每个MyClass的实例这两个属性值可能是不一样的。关于这个，请看下面的这个例子

 1 class Dog(): 2     tricks = [] 3  4     # def __init__(self): 5     #     self.tricks = [] 6  7     def add_tricks(self, trick): 8         self.tricks.append(trick) 9 d = Dog()10 d.add_tricks(‘roll over‘)11 e = Dog()12 e.add_tricks(‘play dead‘)13 print(d.tricks, e.tricks) # [‘roll over‘, ‘play dead‘] [‘roll over‘, ‘play dead‘]

如果注释掉第二行，打开4、5行。运行的结果：[‘roll over‘] [‘play dead‘]。类的方法还可以定义在类的外面，测试用例如下：

def f1(self, x, y):    return min(x, y)class C():    f = f1    def g(self):        return ‘hello world‘    h = gclassC = C()print(C.f(classC, 2, 45), classC.f(2, 45)) # 2 2print(classC.h()) # hello worldclassC.h = ‘hello abc‘print(classC.g(), classC.h) # hello world hello abc

上述的例子可以看到f1定义在类C的外面，可以正常使用。而且在类中赋值h = g，修改h的值。不会影响到g，说明类中的方法赋值是值传递。

二、python类的继承与访问权限

python的继承语法如下，可以支持多层继承。

class DerivedClassName(Base1, Base2, Base3):    <statement-1>    .    .    .    <statement-N>

关于python的私有变量，提供下述的代码：

class Student(object):    def __init__(self, name, score):        self.__name = name        self.__score = score        self._name = name    def print_score(self):        print(‘%s: %s‘ % (self.__name, self.__score))bart = Student(‘Bart Simpson‘, 59)# print(bart.__name) # AttributeError: ‘Student‘ object has no attribute ‘_name‘print(bart._Student__name)print(bart._name) # 约定是外部不能访问，但是实际上外部可以访问。print(type(bart)) # <class ‘__main__.Student‘>print(isinstance(bart, Student), issubclass(Student, object)) # True True

python中可以定义一个空的类，属性和方法可以自行添加。

class Employee:    passjohn = Employee()  # Create an empty employee record# Fill the fields of the recordEmployee.name = ‘John Doe‘john.dept = ‘computer lab‘john.salary = 1000print(john.name) # John Doe

三、python类中的Generators与Iterators

# one wayfor ele in [1, 2, 3]:    print(ele, end=‘ ‘)print()# iters = ‘abc‘it = iter(s)print(next(it), next(it), next(it), end=‘ ‘)print()# Generatorsdef reverse(data):    for index in range(len(data)-1, -1, -1):        yield data[index]for char in reverse(‘huhx‘):    print(char, end=‘ ‘)print()# class next and iterclass Reverse:    """Iterator for looping over a sequence backwards."""    def __init__(self, data):        self.data = data        self.index = len(data)    def __iter__(self):        return self    def __next__(self):        if self.index == 0:            raise StopIteration        self.index = self.index - 1        return self.data[self.index]rev = Reverse(‘linux‘)for char in rev:    print(char, end=‘ ‘)# 1 2 3# a b c# x h u h# x u n i l

四、python类的一些特殊方法

　　Python允许在定义class的时候，定义一个特殊的__slots__变量，来限制该class实例能添加的属性：

class Student(object):    __slots__ = (‘name‘, ‘age‘)s = Student()s.name = ‘huhx‘s.age = 45s.score = 45

运行会报错：

Traceback (most recent call last):  File "G:/Java/Go/program/2017-05-18/LearnPython1/test10/huhx5.py", line 8, in <module>    s.score = 45AttributeError: ‘Student‘ object has no attribute ‘score‘

　　Python内置的@property装饰器就是负责把一个方法变成属性调用的。当调用不存在的属性时，比如score，Python解释器会试图调用__getattr__(self, ‘score‘)来尝试获得属性。__str__()方法类似于java类中的toString方法。如下案例

class Student(object):    @property    def score(self):        return ‘score = ‘ + str(self._score)    @score.setter    def score(self, value):        if not isinstance(value, int):            raise ValueError(‘score must be an integer!‘)        if value < 0 or value > 100:            raise ValueError(‘score must between 0 ~ 100!‘)        self._score = value    def __str__(self):        return ‘student info: ‘ + self.score    def __getattr__(self, item):        if item == ‘address‘:            return ‘china‘        elif item == ‘attr_fun‘:            return lambda x: x * xs = Student()s.score = 60print(s.score) # score = 60print(s.address) # chinaprint(s.attr_fun(4)) # 16print(s) # student info: score = 60s.score = 999 # 抛出异常

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python基础---->python的使用(六)