return an object of a different class
steve+comp.lang.python at pearwood.info
Tue Feb 22 00:33:03 CET 2011
On Mon, 21 Feb 2011 14:23:10 +0100, Jean-Michel Pichavant wrote:
> What is not legit, is to return different objects for which the caller
> has to test the type to know what attributes he can use.
Well, I don't know... I'm of two minds.
On the one hand, I find it *really annoying* when this happens:
>>> re.search(pattern, text).group()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'group'
The problem is that re.search and re.match return None instead of a match-
object when they don't find anything. Perhaps they should return an empty
But on the other hand, there is a well known failure mode caused by doing
# Strip everything after a comment symbol.
offset = text.find("#")
text = text[:offset]
See the bug? If there's no # in the string, it drops the last character.
The most insidious part of the bug is that you might not notice, if your
strings end with whitespace. If str.find() returned None, at least you
would get a nice TypeError as soon as you tried to use it as a integer
So, it's not clear to me whether under *these* circumstances it's better
to return a completely different type, which cannot possibly be mistaken
for success, or a magic value of the same type.
(The third alternative is to raise an exception, and the unspeakably
awful alternative is to have a single global error flag for the operation
which must be checked.)
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