[Python-Dev] os.path.walk() lacks 'depth first' option

[Guido van Rossum]

> I like ValueError best when an input is of the right type but
> outside the defined domain of a function, like math.sqrt(-1.0) or
> chr(500).  Failing to find something feels more like a (possibly
> proper subclass of) LookupError to me.

Yeah, [].remove(42) raising ValueError is a bit weird.  It was put in
before we had the concept of LookupError, and the rationale for using
ValueError was that the *value* is not found -- can't use IndexError
because the value is chosen from a different set than the index, can't
use KeyError because lists don't have a concept of key.  In
retrospect, it would have been better to define a SearchError,
subclassing LookupError.

OTOH there's something to say for fewer errors, not more;
e.g. sometimes I wish AttributeError and TypeError were unified,
because AttributeError usually means that an object isn't of the
expected type.

--Guido van Rossum (home page: http://www.python.org/~guido/)