[英]how to use @ in python.. and the @property and the @classmethod
this is my code: 这是我的代码:
def a():
print 'sss'
@a()
def b():
print 'aaa'
b()
and the Traceback is: 跟踪是:
sss
Traceback (most recent call last):
File "D:\zjm_code\a.py", line 8, in <module>
@a()
TypeError: 'NoneType' object is not callable
so how to use the '@' 那么如何使用'@'
thanks 谢谢
updated 更新
class a:
@property
def b(x):
print 'sss'
aa=a()
print aa.b
it print : 它打印:
sss
None
how to use @property 如何使用@property
thanks 谢谢
updated2 updated2
and the classmethod: 和类方法:
class a:
@classmethod
def b(x):
print 'sss'
aa=a()
print aa.b
the it print : 它打印:
<bound method classobj.b of <class __main__.a at 0x00B2DC00>>
A decorator needs to be a callable object (either a function or an object implementing __call__), where the parameter is the function that has been decorated, and the result is a function that will replace the function that has been decorated, so, to use your example of printing 'sss' instead of printing 'aaa': 装饰器需要是一个可调用的对象(一个函数或一个实现__call__的对象),其中参数是已经装饰的函数,结果是一个函数,它将替换已经装饰的函数,因此,使用打印'sss'而不是打印'aaa'的示例:
>>> def a(f): ... def replacementfunc(): ... print 'sss' ... return replacementfunc; ... >>> @a ... def b(): ... print 'aaa' ... >>> b() sss
Or, a more elaborate example: 或者,更详细的例子:
>>> class print_decorator(object): ... def __init__(self,text): ... self.text = text; ... def __call__(self,f): ... def replacement(): ... print self.text; ... return replacement; ... >>> @print_decorator("Hello world!") ... def b(): ... print 'aaa'; ... >>> b() Hello world!
Edit
编辑
As for your updated question, you need to look at the documentation for @property . 至于您更新的问题,您需要查看@property的文档。 It's not clear exactly what you are trying to accomplish, although my guess is that you want:
目前尚不清楚你想要完成什么,虽然我的猜测是你想要的:
class a: @property def b(self): return 'sss' aa=a() print aa.b # prints 'sss', whereas without @property, prints <function ... >
The line @a()
means " a
is a callable, returning some other callable, which can be called with a function as its only argument and will return a callable". 行
@a()
表示“ a
是可调用的,返回一些其他可调用的,可以使用函数作为其唯一参数调用,并返回可调用的”。 If you're not familiar with the term callable
, it's just a generalization of function
: it can be a function, a class, or an instance of a class that has a __call__
method. 如果您不熟悉术语
callable
,它只是function
的泛化:它可以是函数,类或具有__call__
方法的类的实例。
Your def a
is returning None
, so you're clearly violating the "decorator contract" that you're requesting with the @a()
syntax. 你的
def a
返回None
,所以你明显违反了你用@a()
语法请求的“装饰者合同”。
If you just used @a
, without the ()
, then a
would have to accept the function as its argument and return a callable. 如果你只是使用
@a
,没有()
那么a
必须接受功能作为它的参数和返回调用。
I'm not sure what you're trying to accomplish, but if it's just "print something in a decorator for a function, then this would work: 我不确定你要完成什么,但如果它只是“在装饰器中为函数打印一些东西,那么这将起作用:
def a():
print 'sss'
return lambda f: f
Now you can use @a()
and live happily ever after, since this version does respect the "decorator contract" I explained in the first paragraph. 现在你可以使用
@a()
并从此过上幸福的生活,因为这个版本确实尊重我在第一段中解释的“装饰者合同”。
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