Peter Hansen wrote: > 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: > [snip..]
What you could do is define a subclass of list that compared for identity rather than comparing the contents of the list. Alternatively you could test the contents by identity. All the best, Fuzzyman http://www.voidspace.org.uk/python/idnex.shtml -- http://mail.python.org/mailman/listinfo/python-list
