How do I implement __getattribute__ without an infinite recursion error?

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 

Answer 1

You get a recursion error because your attempt to access the self.__dict__ attribute inside __getattribute__ invokes your __getattribute__ again. If you use object's __getattribute__ instead, it works:

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

This works because object (in this example) is the base class. By calling the base version of __getattribute__ you avoid the recursive hell you were in before.

Ipython output with code in foo.py:

In [1]: from foo import *  In [2]: d = D()  In [3]: d.test Out[3]: 0.0  In [4]: d.test2 Out[4]: 21 

Update:

There's something in the section titled More attribute access for new-style classes in the current documentation, where they recommend doing exactly this to avoid the infinite recursion.

Answer 2

Actually, I believe you want to use the __getattr__ special method instead.

Quote from the Python docs:

__getattr__( self, name)

Called when an attribute lookup has not found the attribute in the usual places (i.e. it is not an instance attribute nor is it found in the class tree for self). name is the attribute name. This method should return the (computed) attribute value or raise an AttributeError exception.
Note that if the attribute is found through the normal mechanism, __getattr__() is not called. (This is an intentional asymmetry between __getattr__() and __setattr__().) This is done both for efficiency reasons and because otherwise __setattr__() would have no way to access other attributes of the instance. Note that at least for instance variables, you can fake total control by not inserting any values in the instance attribute dictionary (but instead inserting them in another object). See the __getattribute__() method below for a way to actually get total control in new-style classes.

Note: for this to work, the instance should not have a test attribute, so the line self.test=20 should be removed.