Python - Using __getattribute__ method

By : Greg
Source: Stackoverflow.com
Question!

I want to override access to one variable in a class, but return all others normally. How do I accomplish this with __getattribute__?

I tried the following (which should also illustrate what I'm trying to do) but I get a recursion error:

class D(object):
    def __init__(self):
        self.test=20
        self.test2=21
    def __getattribute__(self,name):
        if name=='test':
            return 0.
        else:
            return self.__dict__[name]

>>> print D().test
0.0
>>> print D().test2
...
RuntimeError: maximum recursion depth exceeded in cmp
By : Greg


Answers

Python language reference:

In order to avoid infinite recursion in this method, its implementation should always call the base class method with the same name to access any attributes it needs, for example, object.__getattribute__(self, name).

Meaning:

def __getattribute__(self,name):
    ...
        return self.__dict__[name]

You're calling for an attribute called __dict__. Because it's an attribute, __getattribute__ gets called in search for __dict__ which calls __getattribute__ which calls ... yada yada yada

return  object.__getattribute__(self, name)

Using the base classes __getattribute__ helps finding the real attribute.

By : ttepasse


Best Practice

The proper way to programmatically control access to a single attribute is with property. Class D should be written as follows (with the setter and deleter optionally to replicate apparent intended behavior):

class D(object):
    def __init__(self):
        self.test2=21

    @property
    def test(self):
        return 0.

    @test.setter
    def test(self, value):
        '''dummy function to avoid AttributeError on setting property'''

    @test.deleter
    def test(self):
        '''dummy function to avoid AttributeError on deleting property'''

And usage:

>>> o = D()
>>> o.test
0.0
>>> o.test = 'foo'
>>> o.test
0.0
>>> del o.test
>>> o.test
0.0

Options for __getattribute__

If you insist on using the __getattribute__ (really only best when you need to control access to all attributes) you can either call __getattr__ (which, if __getattribute__ is defined, isn't called unless __getattribute__ calls it) or call the parent class's __getattribute__ with super, i.e.:

class D(object):
    def __init__(self):
        self.test=20
        self.test2=21
    def __getattribute__(self,name):
        if name=='test':
            return 0.
        else:
            return self.__getattr__(name)

or

class D(object):
    def __init__(self):
        self.test=20
        self.test2=21
    def __getattribute__(self,name):
        if name=='test':
            return 0.
        else:
            return super(D, self).__getattribute__(name)

And both will behave like this (with an error on a repeated deletion):

>>> o = D()
>>> o.test = 'foo'
>>> o.test
0.0
>>> del o.test
>>> o.test
0.0
>>> del o.test

Traceback (most recent call last):
  File "<pyshell#216>", line 1, in <module>
    del o.test
AttributeError: test


Are you sure you want to use __getattribute__? What are you actually trying to achieve?

The easiest way to do what you ask is:

class D(object):
    def __init__(self):
        self.test = 20
        self.test2 = 21

    test = 0

or:

class D(object):
    def __init__(self):
        self.test = 20
        self.test2 = 21

    @property
    def test(self):
        return 0

Edit: Note that an instance of D would have different values of test in each case. In the first case d.test would be 20, in the second it would be 0. I'll leave it to you to work out why.

Edit2: Greg pointed out that example 2 will fail because the property is read only and the __init__ method tried to set it to 20. A more complete example for that would be:

class D(object):
    def __init__(self):
        self.test = 20
        self.test2 = 21

    _test = 0

    def get_test(self):
        return self._test

    def set_test(self, value):
        self._test = value

    test = property(get_test, set_test)

Obviously, as a class this is almost entirely useless, but it gives you an idea to move on from.



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By: admin