Python getattr 行为？在 ECLIPSE/PyDev 控制台中？

发布于 2024-10-13 07:00:54 字数 580 浏览 2 评论 0原文

以下：

class A(object):
    def __getattr__(self, attr):
        try:
            return self.__dict__[attr]
        except KeyError:
            self.__dict__[attr] = 'Attribute set to string'
            print 'Assigned attribute'
            return self.__dict__[attr]

obj = A()
obj.foo
Assigned attribute
Assigned attribute
Assigned attribute
'Attribute set to string'

魔法发生在哪里？

（我使用的是 2.6.6）

编辑：感谢您的反馈。事实上，这个问题无法从 Python 命令行本身重现。似乎只有在 Eclipse/PyDev 中使用控制台时才会出现这种情况。

原文

The following:

class A(object):
    def __getattr__(self, attr):
        try:
            return self.__dict__[attr]
        except KeyError:
            self.__dict__[attr] = 'Attribute set to string'
            print 'Assigned attribute'
            return self.__dict__[attr]

returns:

obj = A()
obj.foo
Assigned attribute
Assigned attribute
Assigned attribute
'Attribute set to string'

Where is the magic happening?

(I'm on 2.6.6)

Edit: Thanks for your feedback. Indeed, this problem can't be reproduced from the Python command line itself. It seems that it only occurs when using the console in Eclipse/PyDev.

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黯然 2024-10-20 07:00:54

我的代码版本略有不同，可能会有所帮助。

class A(object):
    def __getattr__(self, attr):
        try:
            return self.__dict__[attr]
        except KeyError:
            self.__dict__[attr] = 'Attribute set to string'
            print 'Assigned attribute', attr
            return self.__dict__[attr]

>>> o = A()
>>> o.foo
Assigned attribute foo
'Attribute set to string'

我不知道你如何多次看到“分配的属性”。这是 Python 2.6.6 的情况。

值得指出的是，如果调用 __getattr__，则 try 总是会失败。

I have a slightly different version of your code that might help.

class A(object):
    def __getattr__(self, attr):
        try:
            return self.__dict__[attr]
        except KeyError:
            self.__dict__[attr] = 'Attribute set to string'
            print 'Assigned attribute', attr
            return self.__dict__[attr]

>>> o = A()
>>> o.foo
Assigned attribute foo
'Attribute set to string'

I don't know how you see "Assigned attribute" more than once though. This is with Python 2.6.6.

It's worth pointing out that the try always fails if __getattr__ is called.

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叹倦 2024-10-20 07:00:54

这不会发生：

class A(object):
    def __getattr__(self, attr):
        try:
            return self.__dict__[attr]
        except KeyError:
            self.__dict__[attr] = 'Attribute set to string'
            print 'Assigned attribute'
            return self.__dict__[attr]

obj = A()
print obj.foo

给出：

Assigned attribute
Attribute set to string

仅当属性不存在时才调用 __getattr__ ！所以 try .. except 每次都会进入 except ...

它相当于：

class A(object):
    def __getattr__(self, attr):
        val = 'Attribute set to string'
        setattr(self, attr, val)
        print 'Assigned attribute'
        return val

That doesn't happen:

class A(object):
    def __getattr__(self, attr):
        try:
            return self.__dict__[attr]
        except KeyError:
            self.__dict__[attr] = 'Attribute set to string'
            print 'Assigned attribute'
            return self.__dict__[attr]

obj = A()
print obj.foo

gives:

Assigned attribute
Attribute set to string

__getattr__ is only called when the attribute does not exist! So the try .. except will go into the except every time ...

It's equivalent to:

class A(object):
    def __getattr__(self, attr):
        val = 'Attribute set to string'
        setattr(self, attr, val)
        print 'Assigned attribute'
        return val

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