Python内置函数(51)——property

英文文档：

class property(fget=None, fset=None, fdel=None, doc=None)

Return a property attribute.

fget is a function for getting an attribute value. fset is a function for setting an attribute value. fdel is a function for deleting an attribute value. And doc creates a docstring for the attribute.

If given, doc will be the docstring of the property attribute. Otherwise, the property will copy fget‘s docstring (if it exists). This makes it possible to create read-only properties easily using property() as a decorator。

The @property decorator turns the voltage() method into a “getter” for a read-only attribute with the same name, and it sets the docstring for voltage to “Get the current voltage.”

A property object has getter, setter, and deleter methods usable as decorators that create a copy of the property with the corresponding accessor function set to the decorated function. 

说明：

　　1. property是一个类，其作用是用来包装类的属性，这个属性可以根据实际需要，控制是否可读(设置fget参数)、可写(设置fset参数)、可删除(设置fdel参数)。

class C:
    def __init__(self):
        self._x = '_x in C'

    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.")

>>> c = C()
>>> c.x # 调用 getx
'_x in C'

>>> c.x = 'x had changed' # 调用 setx
>>> c.x # 调用 getx
'x had changed'

>>> del c.x  # 调用 delx
>>> c.x # 调用 getx
Traceback (most recent call last):
  File "<pyshell#34>", line 1, in <module>
    c.x
  File "<pyshell#28>", line 6, in getx
    return self._x
AttributeError: 'C' object has no attribute '_x'

　　2. 参数doc表示的是属性的说明，如果没有指定，将从fget参数指定的方法中读取。

>>> help(c)
Help on C in module __main__ object:

class C(builtins.object)
 |  Methods defined here:
 |  
 |  __init__(self)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |  
 |  delx(self)
 |  
 |  getx(self)
 |  
 |  setx(self, value)
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  __dict__
 |      dictionary for instance variables (if defined)
 |  
 |  __weakref__
 |      list of weak references to the object (if defined)
 |  
 |  x
 |      I'm the 'x' property.

# 最后一行中 文档字符串 为I'm the 'x' property.

>>> class C:
    def __init__(self):
        self._x = '_x in C'

    def getx(self):
        """I'm the 'x' property. provide by getx"""
        return self._x

    def setx(self, value):
        self._x = value

    def delx(self):
        del self._x

    x = property(getx, setx, delx)

    
>>> help(C)
Help on class C in module __main__:

class C(builtins.object)
 |  Methods defined here:
 |  
 |  __init__(self)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |  
 |  delx(self)
 |  
 |  getx(self)
 |      I'm the 'x' property. provide by getx
 |  
 |  setx(self, value)
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  __dict__
 |      dictionary for instance variables (if defined)
 |  
 |  __weakref__
 |      list of weak references to the object (if defined)
 |  
 |  x
 |      I'm the 'x' property. provide by getx

# 最后一行中 文档字符串 为 I'm the 'x' property. provide by getx

　　3. property更优雅的做法是用作装饰器，装饰过的方法就可以以属性方式调用。同时将生成.setter和.deleter装饰器，用于指定可入方法，删除方法。

>>> class C:
    def __init__(self):
        self._x = '_x in C'

    @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

>>> c = C()
>>> c.x # 调用 getx
'_x in C'

>>> c.x = 'x had changed' # 调用 setx
>>> c.x # 调用 getx
'x had changed'

>>> del c.x  # 调用 delx
>>> c.x # 调用 getx
Traceback (most recent call last):
  File "<pyshell#34>", line 1, in <module>
    c.x
  File "<pyshell#28>", line 6, in getx
    return self._x
AttributeError: 'C' object has no attribute '_x'

　　4. 不直接使用字段，而使用属性，主要因为可以控制外部对类字段的恶意修改和删除，而且可以再设置属性值的时候进行适当的验证。

>>> class C:
    def __init__(self):
        self._name = ''
    @property
    def name(self):
        """i'm the 'name' property."""
        return self._name
    @name.setter
    def name(self,value):
        if value is None:
            raise RuntimeError('name can not be None')
        else:
            self._name = value

            
>>> c = C()

>>> c.name # 访问属性
''
>>> c.name = None # 设置属性时进行验证
Traceback (most recent call last):
  File "<pyshell#84>", line 1, in <module>
    c.name = None
  File "<pyshell#81>", line 11, in name
    raise RuntimeError('name can not be None')
RuntimeError: name can not be None

>>> c.name = 'Kim' # 设置属性
>>> c.name # 访问属性
'Kim'

>>> del c.name # 删除属性，不提供deleter则不能删除
Traceback (most recent call last):
  File "<pyshell#87>", line 1, in <module>
    del c.name
AttributeError: can't delete attribute
>>> c.name
'Kim'