[Python-Dev] A `cogen' module - an observation
Tue, 27 Aug 2002 02:27:40 -0400
On Tue, Aug 27, 2002 at 01:56:36AM -0400, Raymond Hettinger wrote:
> From: "Oren Tirosh" <email@example.com>
> > [f(x, y) for x in X for y in Y]
> > is equivalent to:
> > [f(x, y) for x, y in cartesian(X, Y)]
> Is the order guaranteed to be the same?
All generators below have the property of yielding successive results
in sorted order, given than input sequences were already sorted.
> Will each work the same for a non-restartable
> iterator, say a file object (equivalently put,
> does the second one read Y once or many times)?
They have exactly the same re-iterability wart as nested loops or list
comprehensions - an exhausted iterator is indistinguishable from an empty
> Would Descartes object to his name being used thusly?
The cartesian product is a set operation and therefore has no defined
order. When generating it you need some specific order and this one makes
the most sense. If you use it with a 'nested loop' mindset instead of a
'set theory' mindset Rene would have had some grounds for objection :-)