Scott David Daniels wrote:
> Ron Adam wrote:
>
>> George Sakkis wrote:
>>
>>> I get:
>>>
>>> None: 0.549999952316
>>> String: 0.498000144958
>>> is None: 0.450000047684
>>
>>
>>
>> What do yo get for "name is 'string'" expressions?
>
>
> >>> 'abc' is 'abcd'[:3]
> False
Well of course it will be false... your testing two different strings!
And the resulting slice creates a third.
Try:
ABC = 'abc'
value = ABC
if value is ABC: # Test if it is the same object
pass
In the previous discussion I was comparing using a string as an
alternative to using None as a "flag" object. Not as a value to be
calculated.
And just to be clear, I'm not disagreeing with you.
Yes, you can have problems with string comparisons if you create a copy
instead of pointing a name to the object like you did above. Something
to be aware of.
To avoid that you either need to define the flag string as a global name
or use it strictly in the local scope it's defined in. Python will also
sometimes reuse strings as an optimization instead of creating a second
string if they are equal. Something else to be aware of.
So I'm not suggesting it is a good idea to use strings in place of None.
But I still wonder why bool and other object comparisons are slightly
slower than string comparisons. (?)
Cheers,
Ron
> You need to test for equality (==), not identity (is) when
> equal things may be distinct. This is true for floats, strings,
> and most things which are not identity-based (None, basic classes).
> This is also true for longs and most ints (an optimization that makes
> "small" ints use a single identity can lead you to a mistaken belief
> that equal integers are always identical.
>
> >>> (12345 + 45678) is (12345 + 45678)
> False
>
> 'is' tests for identity match. "a is b" is roughly equivalent to
> "id(a) == id(b)". In fact an optimization inside string comparisons
> is the C equivalent of "return (id(a) == id(b) of len(a) == len(b)
> and <elements match>)
>
> --Scott David Daniels
> [EMAIL PROTECTED]
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