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

Claudio Grondi claudio.grondi at
Wed Jan 18 16:06:37 CET 2006

Steve Holden wrote:
> 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.
Here in English ;-) :
... many other statements here ...
a is b  # False
a == b  # True
a[0] is b[0] # unpredictable(?)
a[0] == b[0] # True

so a C, Javascript equivalent '==' operator (called here e.g. 'xx') 
should in case of code above _always_ give:
a    xx b    # False
a[0] xx b[0] # True
what becomes only possible if the object holding the integer value 1 is 
created only once when a=[1] and reused when executing b=[1].

> 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.
> regards
>  Steve

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