Can a simple a==b 'hang' in and endless loop?

Steve Holden steve at
Wed Jan 18 09:47:42 EST 2006

Claudio Grondi wrote:
> Yes, I know about 'is',
> but I mean, that it is not possible to use 'is' as replacement for '==' 
> operator to achieve in Python same behaviour as it is the case in C and 
> Javascript when comparing values with '=='.
> 'is' does the C, Javascript job when comparing lists, but I mean it 
> fails to give fully predictable results when applied to elements of 
> lists in case there exist duplicate objects with same 'value' i.e. e.g. 
> there are two different objects storing the integer value 1, what I mean 
> can happen when there is enough other code between the Python code lines 
> assigning the integer value 1 to a list element or any other identifier.

Perhaps you could try again in English? :-) Sorry, that's a very complex 
sentence and it isn't clear what yo mean.

In C, of course, a == b requires that a and b be of compatible types and 
that they have the same value. This means that if they are pointers they 
must point to the same thing (which is exactly what "is" tests for).

> Or is there in Python a 100% reliable mechanism assuring, that there is 
> one and _only one_ object carrying a given 'value' (at least for the 
> built in types as integer, long integer, string, float) and if this 
> value is to be assigned to a list element or any other literal the 
> already existing object (if any) will be found and used/referenced?
No more than there is in C or, presumably, Java.

If you want to test for identity, use "is". If you want to test for 
equality, use "==". Of you want to test for something else, kindly 
explain what you want to test for.

Steve Holden       +44 150 684 7255  +1 800 494 3119
Holden Web LLC           
PyCon TX 2006        

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