alternative to zip(*map(f, lst))?

Alex Martelli aleax at
Tue Jan 22 21:20:00 CET 2002

Huaiyu Zhu wrote:
>>>> def f(x): return x, x*3
> ...
>>>> a, b = zip(*map(f, range(2))); print a, b
> (0, 1) (0, 3)
>>>> a, b = zip(*map(f, range(1))); print a, b
> (0,) (0,)
>>>> a, b = zip(*map(f, range(0))); print a, b
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> TypeError: zip() requires at least one sequence
> Is is possible to get () () in the last case?  Thanks.

Possible, and even quite easy, although not necessarily
good programming practice:

a, b = n and zip(*map(f, range(n))) or 2*[()]

more generally, for an arbitrary sequence X and f
returning an N-tuple for an arbitrary given N,

        X and zip(*map(f, X)) or N*[()]

However, as I mentioned, I don't think that building such
"deep" (as in "complicated") inline expressions is best
programming practice.  You don't get a prize for squishing
everything into one expression.  I would code, NOT

def hz(f, X, N): return X and zip(*map(f, X)) or N*[()]

but rather:

def am(f, X, N):
    if X: return zip(*map(f, X))
    else: return N*[()]

(assuming for the sake of argument that I did want to
use the zip(*map trick at all, of course:-).


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