为什么Python中的@ foo.setter对我不起作用?

问题:为什么Python中的@ foo.setter对我不起作用?

因此,我正在使用Python 2.6中的装饰器,并且在使它们工作时遇到了一些麻烦。这是我的类文件:

class testDec:

    @property
    def x(self): 
        print 'called getter'
        return self._x

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

我认为这意味着将其视为x属性,但是在get和set上调用这些函数。因此,我启动了IDLE并检查了它:

>>> from testDec import testDec
from testDec import testDec
>>> t = testDec()
t = testDec()
>>> t.x
t.x
called getter
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "testDec.py", line 18, in x
    return self._x
AttributeError: testDec instance has no attribute '_x'
>>> t.x = 5
t.x = 5
>>> t.x
t.x
5

显然,第一次调用按预期方式工作,因为我调用了getter,并且没有默认值,并且失败。好的,我了解。但是,对assign的调用t.x = 5似乎会创建一个新属性x,而现在getter不起作用!

我想念什么?

So, I’m playing with decorators in Python 2.6, and I’m having some trouble getting them to work. Here is my class file:

class testDec:

    @property
    def x(self): 
        print 'called getter'
        return self._x

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

What I thought this meant is to treat x like a property, but call these functions on get and set. So, I fired up IDLE and checked it:

>>> from testDec import testDec
from testDec import testDec
>>> t = testDec()
t = testDec()
>>> t.x
t.x
called getter
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "testDec.py", line 18, in x
    return self._x
AttributeError: testDec instance has no attribute '_x'
>>> t.x = 5
t.x = 5
>>> t.x
t.x
5

Clearly the first call works as expected, since I call the getter, and there is no default value, and it fails. OK, good, I understand. However, the call to assign t.x = 5 seems to create a new property x, and now the getter doesn’t work!

What am I missing?


回答 0

您似乎在python 2中使用了经典的老式类。为了使属性正常工作,您需要使用新型类(在python 2中,您必须继承自object)。只需将您的类声明为MyClass(object)

class testDec(object):

    @property
    def x(self): 
        print 'called getter'
        return self._x

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

有用:

>>> k = testDec()
>>> k.x
called getter
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/devel/class_test.py", line 6, in x
    return self._x
AttributeError: 'testDec' object has no attribute '_x'
>>> k.x = 5
called setter
>>> k.x
called getter
5
>>> 

可能导致问题的另一个细节是,这两种方法都需要相同的名称才能使该属性起作用。如果您使用类似这样的其他名称来定义设置器,它将无法正常工作

@x.setter
def x_setter(self, value):
    ...

首先,还不完全容易发现的另一件事是顺序:必须先定义吸气剂。如果首先定义设置器,则会 name 'x' is not defined出错。

You seem to be using classic old-style classes in python 2. In order for properties to work correctly you need to use new-style classes instead (in python 2 you must inherit from object). Just declare your class as MyClass(object):

class testDec(object):

    @property
    def x(self): 
        print 'called getter'
        return self._x

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

It works:

>>> k = testDec()
>>> k.x
called getter
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/devel/class_test.py", line 6, in x
    return self._x
AttributeError: 'testDec' object has no attribute '_x'
>>> k.x = 5
called setter
>>> k.x
called getter
5
>>> 

Another detail that might cause problems is that both methods need the same name for the property to work. If you define the setter with a different name like this it won’t work:

@x.setter
def x_setter(self, value):
    ...

And one more thing that is not completely easy to spot at first, is the order: The getter must be defined first. If you define the setter first, you get name 'x' is not defined error.


回答 1

只是为那些偶然发现此异常的其他人提供的注释:两个函数必须具有相同的名称。按以下方式命名方法将导致异常:

@property
def x(self): pass

@x.setter
def x_setter(self, value): pass

而是给这两种方法取相同的名称

@property
def x(self): pass

@x.setter
def x(self, value): pass

同样重要的是要注意声明的顺序很重要。必须先在文件中的setter之前定义getter,否则您将获得NameError: name 'x' is not defined

Just a note for other people who stumble here looking for this exception: both functions need to have the same name. Naming the methods as follows will result in an exception:

@property
def x(self): pass

@x.setter
def x_setter(self, value): pass

Instead give both methods the same name

@property
def x(self): pass

@x.setter
def x(self, value): pass

It is also important to note that the order of the declaration matters. The getter must be defined before the setter in the file or else you will get a NameError: name 'x' is not defined


回答 2

您需要通过从对象派生类来使用新型类:

class testDec(object):
   ....

然后它应该工作。

You need to use new-style classes which you do by deriving your class from object:

class testDec(object):
   ....

Then it should work.


回答 3

如果有人来自谷歌,除了上面的答案,我想补充一点__init__,在基于此答案从您的类的方法调用设置器时,需要特别注意 :

class testDec(object):                                                                                                                                            

    def __init__(self, value):
        print 'We are in __init__'
        self.x = value # Will call the setter. Note just x here
        #self._x = value # Will not call the setter

    @property
    def x(self):
        print 'called getter'
        return self._x # Note the _x here

    @x.setter
    def x(self, value): 
        print 'called setter'
        self._x = value # Note the _x here

t = testDec(17)
print t.x 

Output:
We are in __init__
called setter
called getter
17

In case anybody comes here from google, in addition to the above answers I would like to add that this needs careful attention when invoking the setter from the __init__ method of your class based on this answer Specifically:

class testDec(object):                                                                                                                                            

    def __init__(self, value):
        print 'We are in __init__'
        self.x = value # Will call the setter. Note just x here
        #self._x = value # Will not call the setter

    @property
    def x(self):
        print 'called getter'
        return self._x # Note the _x here

    @x.setter
    def x(self, value): 
        print 'called setter'
        self._x = value # Note the _x here

t = testDec(17)
print t.x 

Output:
We are in __init__
called setter
called getter
17