Hello,

and thanks for your comments! I am attaching a new patch.

On Sat, Sep 07, 2013 at 02:49:07PM +0200, Ludovic Courtès wrote:
> Still it’s better to keep it, as a generic version.  Then, we can have:
>   (define package-with-python-2
>     (cut package-with-explicit-python <> python-2))
> and then use that as needed.

I modified with two additional parameters, OLD-PREFIX and NEW-PREFIX:
When going to python-2, one rewrites the prefix "python-" to "python2-".
The current patch would allow to go back to Python 3 by calling
  (package-with-explicit-python p python "python2-" "python-").

> Sylistic note: never use ‘car’, ‘cdr’, and co; use ‘match’ instead: it
> leads to code that is both more readable and more robust (info "(guile)
> Pattern Matching").  In that case, you can even use ‘match-lambda’,
> which is equivalent to:
>   (lambda (x)
>     (match x
>       ...))

This is rather surprising for a language based on lists...
But admittedly, the code becomes more readable.

> Second remark: inputs are actually tuples of one of two forms:
>   (name package)
>   (name package output)

Or a third one, (name-of-patch store-path-of-patch), which was already
handled. The current patch should handle all cases.

> (map rewrite (package-inputs p)) is enough; if (package-inputs p) is
> null?, then that’ll return the empty list too.

This is something that I had tested, and strangely, it did not work at the
time I made the test. But the error must have lain somewhere else then.

Andreas