"/a" is not "/a" ?
steve at pearwood.info
Fri Mar 6 22:22:23 CET 2009
Gary Herron wrote:
>> Huh? How am I supposed to compare immutable types for identity then? Your
>> bizarre instruction would prohibit:
>> if something is None
> Just use:
> if something == None
> It does *exactly* the same thing.
"something is None" is a pointer comparison. It's blindingly fast, and it
will only return True if something is the same object as None. Any other
object *must* return False.
"something == None" calls something.__eq__(None), which is a method of
arbitrary complexity, which may cause arbitrary side-effects. It can have
false positives, where objects with unexpected __eq__ methods may return
True, which is almost certainly not the intention of the function author
and therefore a bug.
> If they use a couple "something==None" instead of "something is None"
> in their code while learning Python, it won't hurt,
Apart from the subtle bugs they introduce into their code.
> and they can change
> their style when they understand the difference. And meanwhile they
> will skip traps newbies fall into when they don't understand these
> things yet.
How about teaching them the right reasons for using "is" instead of giving
them false information by telling them they should never use it?
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