interleaving dictionary values

j1o1h1n at j1o1h1n at
Tue Nov 21 15:34:32 CET 2006


I was trying to create a flattened list of dictionary values where each
value is a list, and I was hoping to do this in some neat functionally
style, in some brief, throwaway line so that it would assume the
insignificance that it deserves in the grand scheme of my program.

I had in mind something like this:

>>> interleave([1, 2, 3], [4,5], [7, 8, 9])
[1, 4, 7, 2, 5, 8, 3, 9]

I played for a while with zip(), [some newfangled python keyword, that
I was truly shocked to find has been hiding at the bottom of the list
built in functions since version 2.0], before giving up and going back
to trusty old map(), long celebrated for making code hard to read:

>>> map(None, [1, 2, 3], [4,5], [7, 8, 9])
[(1, 4, 7), (2, 5, 8), (3, None, 9)]

This is basically it.  It then becomes:

>>> filter(None, flatten(map(None, [1, 2, 3], [4,5], [7, 8, 9])))
[1, 4, 7, 2, 5, 8, 3, 9]

filter(None, - my brain parses that automatically now.  This is not so
bad.  Flatten is snitched from ASPN/Cookbook/Python/Recipe/363051,
thank you Jordan Callicoat, Mike C. Fletcher:

def flatten(l, ltypes=(list, tuple)):
    i = 0
    while (i < len(l)):
       while (isinstance(l[i], ltypes)):
           l[i:i+1] = list(l[i])
       i += 1
    return l

Trouble is then getting map() to play with the result of dict.values().
 I only worked this out while writing this post, of course.

Given a dictionary like d = { "a" : [1, 2, 3], "b" : [4, 5], "c" : [7,
8, 9]} - I was hoping to do this:

  map(None, d.values())

But instead I (finally worked out I could) do this:

  apply(map, tuple([None] + d.values()))

So... my bit of code becomes:

  filter(None, flatten(map(None, apply(map, tuple([None] +

It fits on one line, but it feels far more evil than I set out to be.
The brackets at the end are bad for my epilepsy.

Surely there is there some nice builtin function I have missed?

| John J. Lehmann, j1o1h1n(@)
+ [lost-in-translation] "People using public transport look stern, and
+ snatchers increase the ill feeling."  A  Japanese woman, Junko, told
the paper:
+ "For us, Paris is the dream city. The  French are all beautiful and
+ And then, when we arrive..."

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