[Chicago] Resolving lists within lists within lists within .....

Brad Martsberger bradley.marts at gmail.com
Tue Feb 16 14:13:49 EST 2016

> you can get almost there with itertools.chain.from_iterable

It's tempting, but it fails in a lot of ways. It successfully flattens a
list of lists, but doesn't go any deeper than that and fails completely on
a list composed of some lists and some non-list objects. You can also get
the same behavior out of a list comprehension.

Douglas, you have written a recursive function, but I think you've missed
on what the base case is. The base case is not whether or not you've been
passed an empty list, but rather whether an element is a list or not (if
it's not, you don't need any further flattening. Also, all those indices
don't look very pythonic.

Here is a recursive flatten function that will handle any depth and mixed
depths at different elements (no indexing required)

def flatten(lst):
    new_list = []
    for element in lst:
        if isinstance(element, list):
            # This is the base case where the recursion ends

    return new_list

>>> flatten([[1, 1.1], 2, 3, [4, 5, [6, 7, 8]]])
[1, 1.1, 2, 3, 4, 5, 6, 7, 8]

On Mon, Feb 15, 2016 at 9:09 PM, Joshua Herman <zitterbewegung at gmail.com>

> The idea of flattening a object or datatype is a functional programming
> technique and not just a part of Ruby and Mathematica According to this
> answer on the programming stack exchange there is no method / function that
> implements flatten for build in Python functions but you can get almost
> there with itertools.chain.from_iterable . See
> http://programmers.stackexchange.com/questions/254279/why-doesnt-python-have-a-flatten-function-for-lists
> .
> On Feb 15, 2016, at 4:12 PM, Lewit, Douglas <d-lewit at neiu.edu> wrote:
> Hi everyone,
> Well it's President's Day and I've got the day off!  Hooray!!!  Finally
> some time to just relax and mess around.  So I'm at my computer playing
> around with Python and wondering how to resolve the issue of multiple lists
> embedded within other lists.  I came up with two functions that I think
> solve the problem.  But I was wondering if Guido or someone else added a
> builtin function or method that does this automatically for the
> programmer.  Or is there an easier way?  Okay.... thanks.  ( In case you're
> wondering why I called the function "flatten" it's because I know from
> experience that Wolfram Mathematica and Ocaml have these "flatten"
> functions.  I think Ruby has something similar, but I haven't played with
> Ruby in a while so I'm not really sure. )  The try: except block is
> important because you can't subscript non-list data structures in Python.
> The IndexError is what you get when you try to index an empty list.  So I
> ****think**** my try: except block covers most commonly encountered
> exceptions when working with lists embedded within other lists.
> Best,
> Douglas.
> def flatten(lst):
> if lst == [ ]:
> return lst
> else:
> try:
> return [lst[0][0]] + flatten(lst[0][1:] + lst[1:])
> except TypeError:
> return [lst[0]] + flatten(lst[1:])
> except IndexError:
> return flatten(lst[1:])
> def flattenAgain(lst):
> newList = lst[:]
> while newList != flatten(newList):
> newList = flatten(newList)
> return newList
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