why __repr__ affected after __getattr__ overloaded?

[Peter Otten]

Roc Zhou wrote:

> I'm sorry but I still have a question, look at this example:
>>>> class test:
> ...     def __init__(self):
> ...         self.x = 1
> ...     def __getattr__(self, attr_name):
> ...         print attr_name
> ...         if attr_name == 'y':
> ...             return 2
> ...         else:
> ...             raise AttributeError, attr_name
> ...
>>>> t = test()
>>>> t.x
> 1
>>>> t.y
> y
> 2
>>>> print t.x
> 1
>>>> print t
> __str__
> __repr__
> <__main__.test instance at 0xb7f6d6cc>
> 
> Since __str__ and __repr__ does not exist because their names was
> printed, why not the "AttributeError" be raised?

Because classic classes invoke

t.__getattr__(self, "__repr__")

and expect that to return a proper __repr__() method -- unless __getattr__()
raises an AttributeError: 

>>> class Test:
...     def __getattr__(self, name):
...             if name == "__repr__":
...                     raise AttributeError
...             return "<inexistent %r>" % name
...
>>> t = Test()
>>> t
<__main__.Test instance at 0x401d42ac>

If you use newstyle classes you won't run into that particular problem:

>>> class Test(object):
...     def __getattr__(self, name):
...             return "<inexistent %r>" % name
...
>>> t = Test()
>>> t
<__main__.Test object at 0x401d426c>
>>> t.yadda
"<inexistent 'yadda'>"