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

Aaron Elmquist elmq0022 at umn.edu
Thu Feb 18 14:43:36 EST 2016

```Douglas,

Here's one more version for you and the rest of the list. It's based on
Brad's code.  I will let you think about why this version might be better
or worse.  Also, recursion is great.  It's just too bad it's not one of
python's strong points.

def flatten(lst):
for item1 in lst:
if hasattr(item1, '__iter__'):
for item2 in flatten(item1):
yield item2
else:
yield item1

print([x for x in flatten([1, [2,3,[4,5,6,[7,8,9]]]]) if x%2 == 1])

y = flatten([1, [2,3,[4,5,6,[7,8,9]]]])

print(next(y))
print(next(y))
print(next(y))
.
.
.

On Wed, Feb 17, 2016 at 11:21 PM, Lewit, Douglas <d-lewit at neiu.edu> wrote:

> Hey Massimo,
>
> That one-liner is so cool!  Thanks!  Simple and gets the job done.  Thanks
> for sharing.
>
> On Wed, Feb 17, 2016 at 9:48 PM, DiPierro, Massimo <
> MDiPierro at cs.depaul.edu> wrote:
>
>> here is a one liner:
>>
>> def flatten(x):
>>     return [z for y in x for z in flatten(y)] if isinstance(x,list) else
>> [x]
>>
>>
>>
>> On Feb 17, 2016, at 9:30 PM, Lewit, Douglas <d-lewit at neiu.edu<mailto:
>> d-lewit at neiu.edu>> wrote:
>>
>> Hi Mark,
>>
>> Thanks for those links.  Yes, Linus Torvalds is quite a legend in his own
>> time.  I'll be content to become 1% of the programmer that he is!   :-)
>>
>> My original question was simple, "Does Python have a builtin function for
>> flattening lists?"  It was a very simple question that provoked a very
>> strange and hostile thread!  I'm not sure why that is.  Anyhow, someone
>> mentioned itertools.chain( ).  Can someone provide a concrete example of
>> how that function works?  Or is that question inappropriate?  And please,
>> type of reply, okay?  Thank you.
>>
>>
>>
>> On Wed, Feb 17, 2016 at 2:52 PM, Mark Graves <mgraves87 at gmail.com<mailto:
>> mgraves87 at gmail.com>> wrote:
>> I don't mean to be argumentative or add to this discussion in a negative
>> way.
>>
>> Could we have a little direction from a higher up around the code of
>> conduct here?  For reference, this is the only one I found:
>>
>> http://www.chipy.org/pages/conduct/
>>
>> I am in support of Doug asking his questions and agree with Adam on this.
>> I've met Doug, and sometimes his humor is lost on people through the
>> mailing list.  If you are bothered, you can always create an email filter.
>>
>> FWIW, imagine if the developer at the top of this list had been
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> On Wed, Feb 17, 2016 at 11:50 AM, Mike Tamillow <
>> mikaeltamillow96 at gmail.com<mailto:mikaeltamillow96 at gmail.com>> wrote:
>> Yeah, but I generally agree that this list shouldn't be used for help
>> with personal programming problems. There is a website called stack
>> overflow as well as much documentation that can be consulted for this.
>>
>> What I like best is when messages come out exposing me to some open
>> source tool I have yet to hear about that may be useful.
>>
>> I'm sure there's other great discussions but I don't think code by email
>> is quite a good thing to send out to hundreds of people.
>>
>> Sent from my iPhone
>>
>>
>> Hey everyone,
>>
>> Please remember that intentions can be hard to judge on the mailing list.
>> I've met Douglas in person and he's a nice guy. Please don't assign motives
>> just because there are issues communicating.
>>
>>
>>
>> On Wed, Feb 17, 2016 at 10:02 AM, Chris Foresman <foresmac at gmail.com
>> <mailto:foresmac at gmail.com>> wrote:
>> Honestly, Douglas, you come to the list all the time asking for help or
>> opinions and then you precede to generally be a jerk to people that respond
>> to you. The fact is your solution is sloppy, confusing, and doesn’t work at
>> least as far as you originally explained it was supposed to work. Brad
>> pointed all this out and suggested a vastly better alternative, and did so
>> in an extremely polite way. Your response was just acerbic and doltish.
>> Please consider either accepting constructive criticism with humility or
>> just stop asking for help.
>>
>>
>> Regards,
>> Chris Foresman
>> chris at chrisforesman.com<mailto:chris at chrisforesman.com>
>>
>>
>>
>> On Feb 16, 2016, at 10:44 PM, Lewit, Douglas <d-lewit at neiu.edu<mailto:
>> d-lewit at neiu.edu>> wrote:
>>
>> Use flattenAgain.... which calls flatten repeatedly until there's no
>> change in the list.  You have to use BOTH functions!
>>
>> Sarcasm?  What's that?
>>
>>
>> On Tue, Feb 16, 2016 at 10:37 PM, Brad Martsberger <
>> Douglas, I don't know if that was supposed to be sarcastic or what.
>>
>> In fact, your code does not work.
>>
>> >>> flatten([[[1, 2], 3], 4])
>> [[1, 2], 3, 4]
>>
>> Looks like it fails to fully flatten the list.
>>
>> I assumed from your original email you were interested in other
>> approaches, so I gave one that looks to me like it's much less complex (no
>> need for try/except, no need for indexing, 1 recursive call instead of 3).
>> Less complex code is usually easier to reason about and less prone to bugs.
>>
>> In purely functional languages there is no for loop, so if you want to
>> iterate over a list, you have to do it with recursive function calls.
>> Recursion stops when there's nothing left in the list, so the base case is
>> the empty list. Since iterating over a list is so common in programming, it
>> can start to feel like this is the way recursion and lists go together.
>>
>> But a good rule of thumb is only good if it doesn't trip you up when you
>> com across an exception to the rule. In the problem of flattening a list,
>> the recursion is down the depth of nesting, not across the list. In this
>> case, you can stop flattening when you hit a non-list object, so that's
>>
>>
>> On Tue, Feb 16, 2016 at 2:50 PM, Lewit, Douglas <d-lewit at neiu.edu<mailto:
>> d-lewit at neiu.edu>> wrote:
>> Whether it looks pythonic or not Joshua, it works!  Try it before you
>> criticize it!!!   ;-)   In implementing recursive functions on lists, one
>> of the base cases is almost always whether the list is empty or not.  A
>> little lesson I learned from studying lists in Haskell and Ocaml.  Hard
>> languages for sure, but they made me a stronger programmer when it comes to
>> the dreaded "R" ( Recursion ).   ;-)
>>
>>
>> On Tue, Feb 16, 2016 at 1:13 PM, Brad Martsberger <
>> > 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):
>>             new_list.extend(flatten(element))
>>         else:
>>             # This is the base case where the recursion ends
>>             new_list.append(element)
>>
>>     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
>> <mailto:zitterbewegung at gmail.com>> wrote:
>> 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<mailto:
>> 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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