String Identity Test

Gabriel Genellina gagsl-py2 at yahoo.com.ar
Wed Mar 4 15:30:44 CET 2009


En Wed, 04 Mar 2009 07:07:44 -0200, Avetis KAZARIAN <avetis.k at gmail.com>  
escribió:

> Gary Herron wrote:
>> The question now is:  Why do you care?   The properties of strings do
>> not depend on the implementation's choice, so you shouldn't care because
>> of programming considerations.  Perhaps it's just a matter of curiosity
>> on your part.
>>
>> Gary Herron
>
> Well, it's not about curiosity, it's more about performance.
>
> I will make a PHP example (a really quite simple )
>
> PHP :
>
> Stat 1 : $aVeryLongString == $anOtherVeryLongString
> Stat 2 : $aVeryLongString === $anOtherVeryLongString
>
> Stat 2 is really faster than Stat 1 (due to the binary comparison)
>
> As I said, I'm coming from PHP, so I was wondering if there was such a
> difference in Python.
>
> Because I was trying to use "is" as for "===".

PHP '==' has no direct correspondence in Python. '===' in PHP is more like  
'==' in Python (but not exactly the same).
In PHP, $x === $y is true if both variables are of the same type *and*  
both have the same value. $x == $y checks only the values, doing type  
conversions as needed, even string -> number; there is no equivalent  
operator in Python. PHP === is called "identity" but isn't related to the  
"is" operator in Python; there is no identity test in PHP with the Python  
semantics.

PHP:
1 == 1
TRUE

1 == 1.0
TRUE

1 == "1"
TRUE

1 == "1.0"
TRUE

1 === 1
TRUE

1 === 1.0
FALSE

1 === "1"
FALSE

1 === "1.0"
FALSE

array(1,2,3) == array(1,2,3)
TRUE

array(1,2,3) === array(1,2,3)
TRUE


Python:
1 == 1
True

1 == 1.0
True

1 == "1"
False

1 == "1.0"
False

[1,2,3] == [1,2,3]
True

[1,2,3] is [1,2,3]
False


So, don't try to translate concepts from one language to another. (Ok,  
it's natural to try to do that if you know PHP, but doesn't work. You have  
to know the differences).
-- 
Gabriel Genellina




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