Python学习之路（13）——类与对象的内置函数

这里介绍几个类与对象的内置函数。

1、判断对象关系：issubclass和isinstance

issubclass(class, classinfo)：如果class是classinfo的子类，则返回True。

注意：

1）一个类可以被认为是自身的子类

2）classinfo可以使类对象组成的元组，只要class是其中任何一个候选类的子类，就返回True

>>> class A:
	pass

>>> class B(A):
	pass

>>> class C:
	pass

>>> issubclass(B, A)
True
>>> issubclass(B, B)
True
>>> issubclass(B, C)
False
>>> issubclass(B, object)
True
>>> 
>>> issubclass(B, (A, C))
True
>>> 

　　

isinstance(object, classinfo)：判断一个实例对象object是否属于类classinfo

注意：

1）如果参数一不是对象，则一定返回False

2）如果参数二不是类或者由类对象组成的元组，则抛出TypeError异常

>>> class A:
	pass

>>> class B(A):
	pass

>>> class C:
	pass

>>> b1 = B()
>>> a = 123
>>> isinstance(a, B)
False
>>> isinstance(123, B)
False
>>> isinstance(b1, B)
True
>>> isinstance(b1, A)
True
>>> isinstance(b1, C)
False
>>> isinstance(b1, (A, B, C))
True
>>> 

　　

2、属性相关：hasattr、getattr、setattr、delattr、property

hasattr(object, name)：测试对象objec中是否有指定的属性名name

注意：属性名name在该方法中以字符串的形式传入

>>> class C:
	def __init__(self, x = 0):
		self.name = x

		
>>> c1 = C()
>>> hasattr(c1, 'name')
True
>>> hasattr(c1, 'x')
False
>>> 

　　

getattr(object, name[, default])：返回对象指定的属性值。如果指定的属性值不存在，且设置了default，则返回default；否则返回一个AttributeError异常。

>>> class C:
	def __init__(self, x = 0):
		self.name = x

		
>>> c1 = C()
>>> getattr(c1, 'name')
0
>>> getattr(c1, 'age')
Traceback (most recent call last):
  File "<pyshell#141>", line 1, in <module>
    getattr(c1, 'age')
AttributeError: 'C' object has no attribute 'age'
>>> getattr(c1, 'age', '该属性不存在')
'该属性不存在'
>>> 

　　

setattr(object, name, value)：设置对象中指定属性的值，如果指定属性不存在，则创建并复制该指定属性。

>>> class C:
	def __init__(self, x = 0):
		self.name = x

		
>>> c1 = C()
>>> getattr(c1, 'name')
0
>>> getattr(c1, 'age')
Traceback (most recent call last):
  File "<pyshell#141>", line 1, in <module>
    getattr(c1, 'age')
AttributeError: 'C' object has no attribute 'age'
>>> setattr(c1, 'name', 123)
>>> getattr(c1, 'name')
123
>>> setattr(c1, 'age', 999)
>>> getattr(c1, 'age')
999
>>>

　　

delattr(object, name)：删除对象中指定的属性

>>> class C:
	def __init__(self, x = 0):
		self.name = x


>>> c1 = C()
>>> c2 = C()
>>> delattr(c1, 'name')
>>> getattr(c1, 'name')
Traceback (most recent call last):
  File "<pyshell#160>", line 1, in <module>
    getattr(c1, 'name')
AttributeError: 'C' object has no attribute 'name'
>>> getattr(c2, 'name')
0
>>> 

　　

property(fget = None, fset = None, fdel = None, doc = None)：通过属性来设置属性

>>> class C:
	def __init__(self, size = 10):
		self.size = size
	def getsize(self):
		return self.size
	def setsize(self, value):
		self.size = value
	def delsize(self):
		del self.size
	x = property(getsize, setsize, delsize)


>>> c1 = C()
>>> c1.getsize()
10
>>> c1.x
10
>>> c1.x = 18
>>> c1.x
18
>>> c1.size
18
>>> c1.getsize()
18
>>> del c1.x
>>> c1.x
Traceback (most recent call last):
  File "<pyshell#198>", line 1, in <module>
    c1.x
  File "<pyshell#189>", line 5, in getsize
    return self.size
AttributeError: 'C' object has no attribute 'size'
>>> c1.size
Traceback (most recent call last):
  File "<pyshell#199>", line 1, in <module>
    c1.size
AttributeError: 'C' object has no attribute 'size'
>>> c1.getsize()
Traceback (most recent call last):
  File "<pyshell#200>", line 1, in <module>
    c1.getsize()
  File "<pyshell#189>", line 5, in getsize
    return self.size
AttributeError: 'C' object has no attribute 'size'
>>>