Claudio Grondi wrote: > 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. > 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?
I think you fundamentally can't get what you want here. It would be quite possible to implement an optimization on the == operator in Python which checked whether two items were identical (i.e. "is", the same as comparing their addresses). This would do just what C is doing in the case of comparing two lists which are the same, but then the following code could not be written: >>> class A: ... def __eq__(self, other): ... return False ... >>> a = A() >>> a == a False If you eliminate the possibility of writing the above code, you probably don't have Python any more (or possibly many other very dynamic languages either). -Peter -- http://mail.python.org/mailman/listinfo/python-list
