Re: [Python-ideas] Function to unnest for-statements

May 23, 2008


      On Thu, 22 May 2008, Carl Johnson wrote:
...
So, going back to the first example, wouldn't it be nicer to write:
for x, y, z in combine(xs, ys, zs):
    #Do something
If nothing else, this has the advantage that if you don't have to nest
the for-loops, things don't end up being so deeply indented on the
screen.
I think extra indentation helps, because it makes it clearer how the
loops nest, but I suppose you could do this if you really wanted;
...
...
...
xs = ys = zs = [True, False]
for a, b, c in [(x, y, z) for x in xs for y in ys for z in zs]:
...     print a, b, c
...
True True True
True True False
True False True
True False False
False True True
False True False
False False True
False False False
...
def combine(*sequences):
    #Test to guard against infinite recursion
    if sequences:
        def _inner(*seqs):
            if len(seqs) == 1:
                for i in seqs[0]: yield (i, )
            else:
                for rest in combine(*seqs[:-1]):
                    for i in seqs[-1]:
                        yield rest + (i, )
        return _inner(*sequences)
    else:
        #Empty generator that yields StopIteration
        def _inner():
            return
            yield
        return _inner()
However, I'm also convinced that there is a more efficient way to do
this.
I've worked this out (I'm sure I did this in intro CS once before...),
although there is probably some optimization trick I'm missing, and
I'm probably doing some silly double-allocation thing or something.
...
...
...
def combine(lists):
...     if len(lists) is 0:
...             return [[]]
...     first = combine(lists[0:-1])
...     total = []
...     for item in lists[-1]:
...             total += [[item] + subset for subset in first]
...     return total
...
for set in combine([[True, False]] * 4):
...     print set
...
[True, True, True, True]
[True, True, True, False]
[True, True, False, True]
[True, True, False, False]
[True, False, True, True]
[True, False, True, False]
[True, False, False, True]
[True, False, False, False]
[False, True, True, True]
[False, True, True, False]
[False, True, False, True]
[False, True, False, False]
[False, False, True, True]
[False, False, True, False]
[False, False, False, True]
[False, False, False, False]
...
So, what do you think? Is this a common enough need that it should be
built into itertools? The main namespace? Or should we leave it out,
since adding it would encourage people writing O(n^x) algorithms? If
nothing else, list members can enjoy rewriting this function for fun.
I really, really don't think this is worth putting in any distributed
package, as it's a pretty simple thing to code up if you're careful.
That said, it _was_ a lot of fun rewriting it.  Thanks.  :D

-- 
Cheers,
Leif

Re: [Python-ideas] Function to unnest for-statements

Leif Walsh