python实例属性和类属性精讲

42ew222

一：访问与改变

# -*-coding: UTF-8 -*-
class ShuXing(object):
    class_shuxing = "class_shuxing"
    class_shuxing_list = [1, 2, 3]
    def __init__(self):
        self.instance_shuxing = "instance_shuxing"
        self.instance_shuxing_list = [5, 6, 7]

print ShuXing.class_shuxing
print ShuXing.class_shuxing_list
#print ShuXing.instance_shuxing   类名不能访问instance不可变变量
#print ShuXing.instance_shuxing_list  类名不能访问instance可变变量
a = ShuXing()
#instance可以访问任何变量
print a.class_shuxing
print a.class_shuxing_list
print a.instance_shuxing
print a.instance_shuxing_list
print "+++++++++++++++++++"
print "instance gaipian class"
a.class_shuxing = "gaipian class_shuxing"
a.class_shuxing_list.append("gaipian class_shuxing_list")

print ShuXing.class_shuxing  #类不可变变量没改变
print ShuXing.class_shuxing_list  #类可变变量有改变
#print ShuXing.instance_shuxing
#print ShuXing.instance_shuxing_list
a = ShuXing()
print a.class_shuxing
print a.class_shuxing_list   #类可变变量有改变
print a.instance_shuxing
print a.instance_shuxing_list

print "+++++++++++++++++++"
print "class gaipian class"
ShuXing.class_shuxing = "gaipian class_shuxing"
ShuXing.class_shuxing_list.append("gaipian class_shuxing_list")

print ShuXing.class_shuxing   #类不可变变量有改变
print ShuXing.class_shuxing_list  #类可变变量有改变
#print ShuXing.instance_shuxing
#print ShuXing.instance_shuxing_list
a = ShuXing()
print a.class_shuxing   #类不可变变量有改变
print a.class_shuxing_list  #类可变变量有改变
print a.instance_shuxing
print a.instance_shuxing_list

1.Python是一门动态语言，任何实体都可以动态地添加或删除属性。
2.一个类定义了一个作用域。
3.类实例也引入了一个作用域，这与相应类定义的作用域不同。
4.在类实例中查找属性的时候，首先在实例自己的作用域中查找，如果没有找到，则再在类定义的作用域中查找。
5.在对类实例属性进行赋值的时候，实际上会在类实例定义的作用域中添加一个属性（如果还不存在的话），并不会影响到相应类中定义的同名属性。

class ShuXing2(object):
    a = 0
    def __init__(self):
        self.a = 10
        self.b = 20

print ShuXing2.a  # 0
#print ShuXing2.b  #类名不能访问instance变量
a = ShuXing2()  
print a.a  # 10
print a.b  # 20

二：用方法访问变量


1 hasattr()：根据输入参数判断某个容器中是否有已该参数为名称的内容，如果有则返回true
2 getattr()：取出容器中以参数为名称的内容
3 setattr()：对容器中以参数为名称的内容进行修改
4 delattr()：删除容器中以参数为名称的内容


class ShuXing2(object):
    a = 0
    def __init__(self):
        self.a = 10
        self.b = 20

a = ShuXing2()
print hasattr(a, 'a')  #True
print getattr(a, 'a')  #10
print setattr(a, 'a', 100)  #None
print getattr(a, 'a') #100
print delattr(a, 'a') #None
print getattr(a, 'a') #0

三：使用特性来管理类实例的属性访问控制

# -*- coding:UTF-8 -*-
class Student(object):

    def __init__(self, score):
        self.__score = score

    @property
    def score(self):
        return 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

s = Student(80)
#print s.__score
s.score = 60
print s.score

s.score = 9999
'''
60
Traceback (most recent call last):
  File "lizi2.py", line 24, in <module>
    s.score = 9999
  File "lizi2.py", line 16, in score
    raise ValueError('score must between 0 ~ 100!')
ValueError: score must between 0 ~ 100!
'''

四：描述符管理“类属性”（不是类实例）访问



class Descriptor(object):

    def __init__(self):
        self._name = ''

    def __get__(self, instance, owner):
        print "Getting: %s" % self._name
        return self._name

    def __set__(self, instance, name):
        print "Setting: %s" % name
        self._name = name.title()

    def __delete__(self, instance):
        print "Deleting: %s" % self._name
        del self._name

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

>>> user = Person()
>>> user.name = 'john smith'
Setting: john smith
>>> user.name
Getting: John Smith
'John Smith'
>>> del user.name
Deleting: John Smith