Python attribute access error

Question

I am trying to customize attribute access in a class (python2.7). Say I define the following class:

class testAttrAccess(object):
    def __init__(self):
        self.data = {"attr1":"value1","attr2":"value2"}

    def __getattr__(self,name):
        try:
            return self.data[name]
        except KeyError:
            pass

        raise AttributeError

Then I get:

In [85]: a = testAttrAccess()

In [86]: a.attr2
Out[86]: 'value2'

In [87]: a.__getattr__('attr2')
Out[87]: 'value2'

In [88]: a.__getattribute__('attr2')
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)

/Users/laserson/temp/<ipython console> in <module>()

AttributeError: 'testAttrAccess' object has no attribute 'attr2'

However, according to the python documentation

If the class also defines __getattr__(), the latter will not be called unless __getattribute__() either calls it explicitly or raises an AttributeError.

So if __getattribute__() is raising an AttributeError, then why isn't __getattr__() being called and returning the proper value?

jon_darkstar · Accepted Answer · 2011-03-24 19:46:24Z

1

__getattribute__() is still default, so it is reaching for self.attr2 instead of self.data['attr2']

answered Mar 24, 2011 at 19:46

jon_darkstar

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3 Comments

jon_darkstar Over a year ago

oh, but then why a.attr2 works. maybes thats not it... good question

user395760 Over a year ago

a.attr2 works precisely because attr2 doesn't exist and thus __getattr__ is invoked. And the default __getattribute__ obviously doesn't invoke __getattr__.

jon_darkstar Over a year ago

right, i follow. thanks for making it clear, i was second guessing myself for a minute

Winston Ewert · Accepted Answer · 2011-03-24 20:47:12Z

1

So if __getattribute__() is raising an AttributeError, then why isn't __getattr__() being called and returning the proper value?

That only applies if python is calling __getattribute__ for you as a part of normal attribute access, not if you call __getattribute__ yourself. Basically, you should almost never call a python method of the form __foo__ Such methods are used to implement behaviors depended on by other operations.

answered Mar 24, 2011 at 20:47

Winston Ewert

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Comments

Lauritz V. Thaulow · Accepted Answer · 2011-03-24 20:41:23Z

0

a.__getattr__ gets called only if the attribute is not found in a.__dict__.

a.__getattribute__ is called in all cases, if it is defined.

As you know, a.__getattribute__(attr) is syntactic sugar for type(a).__getattribute__(a, attr), but since this method is not defined in testAttrAccess, the parent method object.__getattribute__(a, attr) is called. From your results we can then conclude that it is not object.__getattribute__ that calls __getattr__. Probably the interpreter takes care of that.

You can make your program work by changing it like this:

class testAttrAccess(object):
    def __init__(self):
        self.data = {"attr1": "value1", "attr2": "value2"}

    def __getattribute__(self, name):
        try:
            return object.__getattribute__(self, name)
        except AttributeError:
            if name == "__getattr__":
                return object.__getattribute__(self, "__getattribute__")
        try:
            return object.__getattribute__(self, "data")[name]
        except KeyError:
            raise AttributeError
a = testAttrAccess()

print a.attr2
print a.__getattr__('attr2')
print a.__getattribute__('attr2')
print a.qwerty

Output:

value2
value2
value2
Traceback (most recent call last):
  File "test.py", line 20, in 
    print a.qwerty
  File "test.py", line 13, in __getattribute__
    raise AttributeError
AttributeError

answered Mar 24, 2011 at 20:41

Lauritz V. Thaulow

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2 Comments

Uri Laserson Over a year ago

I was under the impression that whenever one wants to override attribute access to do something other than the default, they should only override __getattr__() and not __getattribute__(). Is this correct? Are there any hidden side-effects from the implementation you are proposing above?

Uri Laserson Over a year ago

Please see the answer I contributed as well. I would appreciate any comments.

Uri Laserson · Accepted Answer · 2011-03-24 23:11:34Z

0

Inspired by @lazyr, this solution seems to work too. It seems simpler to me...does it raise any red flags to anyone?

class testAttrAccess(object):
    def __init__(self):
        self.data = {"attr1":"value1","attr2":"value2"}

    def __getattribute__(self,name):
        try:
            return object.__getattribute__(self,name)
        except AttributeError:
            pass

        try:
            return self.data[name]
        except KeyError:
            pass

        raise AttributeError

The following session gives:

In [108]: a = testAttrAccess()

In [109]: a.attr2
Out[109]: 'value2'

In [110]: a.__getattribute__('attr2')
Out[110]: 'value2'

In [111]: a.__getattr__('attr2')
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)

/Users/laserson/temp/<ipython console> in <module>()

/Users/laserson/temp/<ipython console> in __getattribute__(self, name)

AttributeError: 

In [112]: a.qwerty
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)

/Users/laserson/temp/<ipython console> in <module>()

/Users/laserson/temp/<ipython console> in __getattribute__(self, name)

AttributeError:

answered Mar 24, 2011 at 23:11

Uri Laserson

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1 Comment

Ethan Furman Over a year ago

__getattribute__ is called for every non-magic method (__add__', '__repr__, etc.) so will have a big performance hit, as well as being completely unnecessary for what you want -- your original code using __getattr__ is appropriate for what you want to do.

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