Trivial performance questions

Alex Martelli aleax at
Sat Oct 18 20:02:54 CEST 2003

Bryan wrote:

>   > If you *get into the habit* of always checking with "if x is None:"
>> rather than "if x == None:" -- two equally readable constructs -- it
>> will cost you no increase in effort whatsoever to always use the
>> idiom you've gotten used to.  So, whether the (tiny) extra speed and
>> readability are important or not, it's still a good habit to pick up.
>   >
>> Alex
> can you explain in more detail why "if x is None:" is better/faster than
> "if x == None:"?  i guess i don't fully
> understand "is".  my fingers always seem to want to type "if not x:", but
> that is probably worse still since it includes (), [], {}, '', False, None

If you WANT to include other false values, then of course "if not x:" is
just perfect.  But with either 'is None' or '== None' you're checking
specifically for None -- quite different semantics.

"a is None" a bit more readable than "a == None" because it uses readable
words rather than punctuation.  It's a bit faster, because 'is' gets the
id (machine addresses) of its operands and just compares them -- it looks
for IDENTITY, the SAME objects, as opposed to two separate objects which
happen to have the same value; '==' necessarily must do a bit more work,
because many objects can be equal without being the same object.


More information about the Python-list mailing list