Calling this a bug in Python is probably a misnomer.
Think of it as more of a "request for workaround".
Recent versions of SWIG create wrapper classes that
contain a Python class as a key in their __dict__'s.
Consequently, something like

    [k for k in dir(obj) if k.startswith("set_")]

because there is an element in dir(obj) that's not a
string or a unicode object.

I realize dir() is mostly a debugging aid, unlikely to be
used frequently in production code, but I'd like see the
docs updated so that it's guaranteed to only return
strings or unicode objects and I'd like its implementation
changed to enforce that (and maybe even warn if
something else is found).

If this is seen as an acceptable change I will work up a
patch.

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It seems that the documentation guarantees that it returns
"names" ("the list contains the object's attributes' names,
..."). Most people certainly expect names to be strings
(although they might be Unicode objects at some point in the
future :-).

So I think it is ok to fix dir in this respect.

Still present in trunk:69418:

class obj: pass

obj.__dict__[1] = 0
[k for k in dir(obj) if k.startswith("set_")]

Yes, the issue still exists in 2009. Attached patch contains a test + 
a patch to test the type of each value of the dir() result.

I'm not sure that it's a good idea to apply my patch. It's maybe not 
the job of Python to check the types of dir() result.

It's a feature request and it looks like except of Skip Montanaro in 
2004, nobody requires this feature. I don't like my patch because it 
slows down Python just for a very rare case (this issue).

I choose to close it. Reopen this issue if you think that this feature 
(check type of dir() result) is a must have ;-)