python property

来源：http://www.cnblogs.com/coder2012/p/4433703.html

在2.6版本中，添加了一种新的类成员函数的访问方式--property。

原型

class property([fget[, fset[, fdel[, doc]]]])

fget：获取属性

fset：设置属性

fdel：删除属性

doc：属性含义

用法

1.让成员函数通过属性方式调用

class C(object):
    def __init__(self):
        self._x = None
    def getx(self):
        return self._x
    def setx(self, value):
        self._x = value
    def delx(self):
        del self._x
    x = property(getx, setx, delx, "I'm the 'x' property.")

a = C()
print C.x.__doc__ #打印doc
print a.x #调用a.getx()

a.x = 100 #调用a.setx()
print a.x

try:
    del a.x #调用a.delx()
    print a.x #已被删除，报错
except Exception, e:
    print e

输出结果：

I'm the 'x' property.
None
100
'C' object has no attribute '_x'

2.利用property装饰器，让成员函数称为只读的

class Parrot(object):
    def __init__(self):
        self._voltage = 100000

    @property
    def voltage(self):
        """Get the current voltage."""
        return self._voltage

a = Parrot()
print a.voltage #通过属性调用voltage函数
try:
    print a.voltage() #不允许调用函数，为只读的
except Exception as e:
    print e

输出结果：

100000
'int' object is not callable

3.利用property装饰器实现property函数的功能

class C(object):
    def __init__(self):
        self._x = None

    @property
    def x(self):
        """I'm the 'x' property."""
        return self._x

    @x.setter
    def x(self, value):
        self._x = value

    @x.deleter
    def x(self):
        del self._x

其他应用

1.bottle源码中的应用

class Request(threading.local):
    """ Represents a single request using thread-local namespace. """
    ...

    @property
    def method(self):
        ''' Returns the request method (GET,POST,PUT,DELETE,...) '''
        return self._environ.get('REQUEST_METHOD', 'GET').upper()

    @property
    def query_string(self):
        ''' Content of QUERY_STRING '''
        return self._environ.get('QUERY_STRING', '')

    @property
    def input_length(self):
        ''' Content of CONTENT_LENGTH '''
        try:
            return int(self._environ.get('CONTENT_LENGTH', '0'))
        except ValueError:
            return 0

    @property
    def COOKIES(self):
        """Returns a dict with COOKIES."""
        if self._COOKIES is None:
            raw_dict = Cookie.SimpleCookie(self._environ.get('HTTP_COOKIE',''))
            self._COOKIES = {}
            for cookie in raw_dict.values():
                self._COOKIES[cookie.key] = cookie.value
        return self._COOKIES

2.在django model中的应用，实现连表查询

from django.db import models

class Person(models.Model):
     name = models.CharField(max_length=30)
     tel = models.CharField(max_length=30)

class Score(models.Model):
      pid = models.IntegerField()
      score = models.IntegerField()
      
      def get_person_name():
            return Person.objects.get(id=pid)

       name = property(get_person_name) #name称为Score表的属性，通过与Person表联合查询获取name