How to deal __getattr__
adalke at mindspring.com
Sat Oct 16 10:21:01 CEST 2004
> I want to compare the objects themself not the attributes of them. But
> it seems python invoke __getattr__ method, it's strange. How can I
> compare the object directly without calling __getattr__?
You need to know a bit more about Python's object model.
In Python, attributes and methods are fetched with the
same notation, using the dot, as "a.z"
If it's a function then doing a.z returns a "bound method".
That means it knows it's a method of the specific instance
a. That's how a.z() gets passed 'a' as the "self" parameter.
When you ask for a == b Python does the following steps.
(It's slightly more complicated and a bit different,
if you had used new-style classes derived from "object".)
First see if a rich comparison is defined
if that fails it tries the three-way comparison
if that fails it uses the result of
id(a) == id(b)
See the first step? That gets the '__eq__' property
of the instance a. Because Python doesn't distinguish
at this level between attributes and methods it first
looks for the instance in a.__dict__. That fails so
it uses the backup plan of calling __getattr__. In
you case it succeeds and returns None.
So "a.__eq__" returns None for you. The next step is
to call None(b) . But that doesn't work because None
is not a callable object.
An exception is raised and that's why you see that
dalke at dalkescientic.com
More information about the Python-list