Splitting a list of strings

[Alex Martelli]

Sean Ross wrote:
        ...
> filter, where you drop handfuls of change into the top of the device and
> they come out the other side sorted into pennies, nickels, dimes,
> quarters. In this case I had comments, attributes, and data. The comments
        ...
> You can probably just chalk this all up to an infatuation with list
> comprehensions.

Yep.  A list comprehension is a device that Python got from functional
programming (Haskell, specifically): in functional programming, data
structures don't mutate.  Thus, a list comprehension doesn't mutate
existing structures -- it just produces a new one.

For the "change sorter" effect, use a normal loop and a dictionary
of lists.  Say you have a function "classifier" that accepts any
item in the list and returns how the item should be classified (as
a number, say, or a string -- any hashables that distinguish item
classifications will do).  "Sieving" a list that way is then trivial:

def sieve(alist, classifier):
    result = {}
    for item in alist:
        result.setdefault(classifier(item),[]).append(result)
    return result

If you don't want the result as a dictionary of lists, but e.g. a
list of lists in ascending order of classifier-returned keys,
change the last statement to:
    keys = result.keys()
    keys.sort()
    return [result[key] for key in keys]

See, you get to use a list comprehension again in this case:-).


Alex