[Python-Dev] Int FutureWarnings and other 2.4 TODOs

Guido van Rossum guido at python.org
Wed Dec 3 15:42:29 EST 2003

> > So the question is, does long have operations that int doesn't
> > have?  And if so, why can't those operations be added to int?  And
> > if there's a reason, is it good enough?
> If every operation that might conceivably yield an int quietly
> yields a long when the result doesn't fit in an int, then it is
> possible for int and long to have the same operations.
> In which case, why do they have different types?  Just as an
> implementation detail?

Right.  The implementation uses the type as the vtable of a C++ class,
and the functions in the vtable "know" the representation, which is
very different for short and long ints.  This is all nice and
efficient, except that the identity of the vtable is returned as the
type of the object, so two different vtables means two different
types.  The alternative implementation technique is to have a single
type/vtable, embed a flag in each instance that tells the
implementation which representation is used, and have an "if" at the
top of each implementation function switching between the two.  The
downslide should be obvious: not only an extra test+branch for each
operation, but also extra space in the object (the short
representation has no spare bits).

--Guido van Rossum (home page: http://www.python.org/~guido/)

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