The 'is' operator and check-code

[Kevin Bullock]

> I'm throwing this out in case any Python experts have opinions.
> 
> The 'is' operator is used in a bunch of places as a synonym for ==. This
> happens to work with short strings and small integers (0 to 256) in
> CPython, but that's by no means guaranteed by the language, so there are
> places where we're just getting lucky. So I propose we disallow using it
> for anything that's not one of the True/False/None singletons. Here's my
> regex hack:
> 
> diff -r 340c46028ebd contrib/check-code.py
> --- a/contrib/check-code.py	Tue Nov 16 12:55:07 2010 -0600
> +++ b/contrib/check-code.py	Tue Nov 16 13:35:04 2010 -0600
> @@ -130,6 +130,7 @@
>     (r'[\x80-\xff]', "non-ASCII character literal"),
>     (r'("\')\.format\(', "str.format() not available in Python 2.4"),
>     (r'^\s*with\s+', "with not available in Python 2.4"),
> +    (r' is\s+(not\s+)?[^TFNn]', "object comparison with non-singleton"),
>     (r'(?<!def)\s+(any|all|format)\(',
>      "any/all/format not available in Python 2.4"),
>     (r'(?<!def)\s+(callable)\(',

I'm no python expert, but the first thing I wondered was the performance difference:

❧  time python -c $'for i in xrange(10000000):\n  if "a" is "a": pass'

real	0m0.809s
user	0m0.795s
sys	0m0.013s

❧  time python -c $'for i in xrange(10000000):\n  if "a" == "a": pass'

real	0m0.890s
user	0m0.876s
sys	0m0.012s

(Python 2.6.1, OS X 10.6.5)

Not much difference in a simple string case. Now what if the two operations return a different result? (a == b but not a is b):

>>> 'ab' is ''.join(['a','b'])
False
>>> 'ab' is 'a' + 'b'
True

❧  time python -c $'a="".join(["a","b"])\nb="ab"\nfor i in xrange(10000000):\n  if a is b: pass'

real	0m1.264s
user	0m1.250s
sys	0m0.012s

❧  time python -c $'a="".join(["a","b"])\nb="ab"\nfor i in xrange(10000000):\n  if a == b: pass'

real	0m1.345s
user	0m1.330s
sys	0m0.012s

Still not much difference.

pacem in terris / mir / shanti / salaam / heiwa
Kevin R. Bullock