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

Question

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.

Answer 1

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.

Answer 2

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