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devel / comp.lang.python / flattening lists

SubjectAuthor
* flattening listsSquidBits _
+- Re: flattening listsLarry Martell
+* Re: flattening listsStefan Ram
|`* Re: flattening listsStefan Ram
| `- Re: flattening listsMRAB
+- Re: flattening listsDavid Lowry-Duda
+- Re: flattening listsThomas Passin
+- Re: flattening listsdn
+- Re: flattening listsDan Stromberg
`- Re: flattening listsAntoon Pardon

1
flattening lists

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Subject: flattening lists
From: gilads...@gmail.com (SquidBits _)
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 by: SquidBits _ - Tue, 11 Oct 2022 19:32 UTC

Does anyone else think there should be a flatten () function, which just turns a multi-dimensional list into a one-dimensional list in the order it's in. e.g.

[[1,2,3],[4,5,6,7],[8,9]] becomes [1,2,3,4,5,6,7,8,9].

I have had to flatten lists quite a few times and it's quite tedious to type out. It feels like this should be something built in to python, anyone else think this way?

Re: flattening lists

<mailman.653.1665518373.20444.python-list@python.org>

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From: larry.ma...@gmail.com (Larry Martell)
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Subject: Re: flattening lists
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 by: Larry Martell - Tue, 11 Oct 2022 19:58 UTC

On Tue, Oct 11, 2022 at 12:48 PM SquidBits _ <giladsmum@gmail.com> wrote:
>
> Does anyone else think there should be a flatten () function, which just turns a multi-dimensional list into a one-dimensional list in the order it's in. e.g.
>
> [[1,2,3],[4,5,6,7],[8,9]] becomes [1,2,3,4,5,6,7,8,9].
>
> I have had to flatten lists quite a few times and it's quite tedious to type out. It feels like this should be something built in to python, anyone else think this way?

x = [[1,2,3],[4,5,6,7],[8,9]]
[i for j in x for i in j]
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Re: flattening lists

<flatten-20221011205903@ram.dialup.fu-berlin.de>

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From: ram...@zedat.fu-berlin.de (Stefan Ram)
Newsgroups: comp.lang.python
Subject: Re: flattening lists
Date: 11 Oct 2022 20:01:12 GMT
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 by: Stefan Ram - Tue, 11 Oct 2022 20:01 UTC

SquidBits _ <giladsmum@gmail.com> writes:
>I have had to flatten lists quite a few times and it's quite
>tedious to type out. It feels like this should be something
>built in to python, anyone else think this way?

Isn't this a question of how you organize your code? Usually
you would paste the definition into your personal library
and use it from there the next time you need it after having
searched for it in the documentation of your personal library.

OT: When I was young, I was learning LISP and wrote a
flatten function in LISP. It looked something like

( SETQ FLATTEN
( LAMBDA( X )
( COND
( ( NULLP X ) '() )
( ( ATOMP X ) X )
( T ( APPEND
( COND
( ( ATOMP( CAR X ))( LIST( CAR X )))
( T ( FLATTEN( CAR X ))))
( FLATTEN( CDR X )))))))

. Here's a quick attempt at it in Python:

source = [[1,2,3],[4,5,6,7],[8,9]]

def flatten( source ):
if isinstance( source, list ):
for entry in source:
yield from flatten( entry )
else: yield source

print( [ *flatten( source )])

. I never understood "yield from" until just now, when I was
thinking, "Maybe this could be the piece that fits in here!"

Re: flattening lists

<from-20221011210859@ram.dialup.fu-berlin.de>

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Newsgroups: comp.lang.python
Subject: Re: flattening lists
Date: 11 Oct 2022 20:09:45 GMT
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 by: Stefan Ram - Tue, 11 Oct 2022 20:09 UTC

ram@zedat.fu-berlin.de (Stefan Ram) writes:
>. I never understood "yield from" until just now, when I was
>thinking, "Maybe this could be the piece that fits in here!"

PS: If I'm starting to think about it: Having succeeded
after using it by trial in one case does not mean that I
have understood it!

Re: flattening lists

<mailman.655.1665519121.20444.python-list@python.org>

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Subject: Re: flattening lists
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 by: David Lowry-Duda - Tue, 11 Oct 2022 20:02 UTC

On Tue, Oct 11, 2022 at 12:32:23PM -0700, SquidBits _ wrote:
>Does anyone else think there should be a flatten () function, which just turns a multi-dimensional list into a one-dimensional list in the order it's in. e.g.
>
>[[1,2,3],[4,5,6,7],[8,9]] becomes [1,2,3,4,5,6,7,8,9].
>
>I have had to flatten lists quite a few times and it's quite tedious to
>type out. It feels like this should be something built in to python,
>anyone else think this way?

I typically don't mind things that are one liners, especially if the one
liner is a list comprehension.

def flatten1(inlist):
return [l for sublist in inlist for l in sublist]

givenlist = [[1, 2, 3], [4, 5, 6, 7], [8, 9]]
print(flatten1(givenlist))

def flatten2(inlist):
return sum(inlist, [])

print(flatten2(givenlist))

Looking up "flatten" in python's source reveals the (not at all obvious
to me as I don't use chain) alternative

import itertools
def flatten3(inlist):
return list(itertools.chain.from_iterable)

print(flatten3(givenlist))

I notice that "flatten" appears many times in python's source. I didn't
check how many times it's used with the same meaning, though.

- DLD

Re: flattening lists

<mailman.657.1665520038.20444.python-list@python.org>

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 by: Thomas Passin - Tue, 11 Oct 2022 20:27 UTC

Is this what you usually do?

l1 = [[1,2,3],[4,5,6,7],[8,9]]
l2 = []
for lz in l1:
l2.extend(lz)

print(l2) # [1,2,3,4,5,6,7,8,9]

Not all that "tedious", perhaps... I tend to accumulate little utilities
like this in a file and point to it with a .pth file. Of course, you
have to remember to include it if you share your program with someone else.

On 10/11/2022 3:32 PM, SquidBits _ wrote:
> Does anyone else think there should be a flatten () function, which just turns a multi-dimensional list into a one-dimensional list in the order it's in. e.g.
>
> [[1,2,3],[4,5,6,7],[8,9]] becomes [1,2,3,4,5,6,7,8,9].
>
> I have had to flatten lists quite a few times and it's quite tedious to type out. It feels like this should be something built in to python, anyone else think this way?

Re: flattening lists

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 by: dn - Tue, 11 Oct 2022 21:08 UTC

On 12/10/2022 08.32, SquidBits _ wrote:
> Does anyone else think there should be a flatten () function, which just turns a multi-dimensional list into a one-dimensional list in the order it's in. e.g.
>
> [[1,2,3],[4,5,6,7],[8,9]] becomes [1,2,3,4,5,6,7,8,9].
>
> I have had to flatten lists quite a few times and it's quite tedious to type out. It feels like this should be something built in to python, anyone else think this way?

There is a flatten function!

First though, are we ONLY talking about 'flattening' a 2D list (per
example, above) or might the requirements extend to multiple-dimensions?
The solutions vary accordingly!

Two-dimensions:
(don't think this method previously-mentioned, but very readable)

>>> l = [[1,2,3],[4,5,6,7],[8,9]]
>>> flattened = list()
>>> for element in l:
.... flattened += element
....
>>> flattened
[1, 2, 3, 4, 5, 6, 7, 8, 9]

(NB if "l" were three-dimensional, "flattened" would become 2D)

Multi-dimensional:
Reach for itertools:
(https://docs.python.org/3/library/itertools.html#itertools.chain)

>>> import itertools as it
>>> iter_flattened = it.chain( *l )
>>> list( iter_flattened )
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Wrt "I have had to flatten lists quite a few times and it's quite
tedious to type out.", isn't this a "code-smell"?

Certainly motivation to generalise and write a solution as a function.
Do it once, and do it right!

Hence advice elsewhere to build a list-processing utility-library.

On the other hand, has it already been done for us?

An exercise for the reader:
is reaching for itertools 'over-kill' in 2D?
- speed-comparison between loading the itertools library and then
employing the speedy method, or using a (built-in) for-loop at
Python-speed with no import-overhead?
(results will vary significantly according to len( l ), but do they
remain consistently in-favor or one method or the other?)
--
Regards,
=dn

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 by: MRAB - Tue, 11 Oct 2022 23:26 UTC

On 2022-10-11 21:09, Stefan Ram wrote:
> ram@zedat.fu-berlin.de (Stefan Ram) writes:
>>. I never understood "yield from" until just now, when I was
>>thinking, "Maybe this could be the piece that fits in here!"
>
> PS: If I'm starting to think about it: Having succeeded
> after using it by trial in one case does not mean that I
> have understood it!
>
>
This:

yield from iterable

is equivalent to:

for item in iterable:
yield item

but is more efficient.

That's really all you need to know!

Re: flattening lists

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 by: Dan Stromberg - Wed, 12 Oct 2022 02:46 UTC

On Tue, Oct 11, 2022 at 12:48 PM SquidBits _ <giladsmum@gmail.com> wrote:

> Does anyone else think there should be a flatten () function, which just
> turns a multi-dimensional list into a one-dimensional list in the order
> it's in. e.g.
>
> [[1,2,3],[4,5,6,7],[8,9]] becomes [1,2,3,4,5,6,7,8,9].
>
> I have had to flatten lists quite a few times and it's quite tedious to
> type out. It feels like this should be something built in to python, anyone
> else think this way?
>

I think the usual argument against putting something like this in the
standard library (I hope it won't be built in), is that there are many ways
to define "flatten". That is, should it only go one level deep? All the
way to the bottom? n levels deep? Should it do something special with
lists, dicts, tuples, sets?

This looks like a nice URL on the topic:
https://www.pythonpool.com/flatten-list-python/

Re: flattening lists

<mailman.670.1665593143.20444.python-list@python.org>

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https://www.novabbs.com/devel/article-flat.php?id=19843&group=comp.lang.python#19843

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 by: Antoon Pardon - Wed, 12 Oct 2022 16:45 UTC

Op 11/10/2022 om 21:32 schreef SquidBits _:
> Does anyone else think there should be a flatten () function, which just turns a multi-dimensional list into a one-dimensional list in the order it's in. e.g.
>
> [[1,2,3],[4,5,6,7],[8,9]] becomes [1,2,3,4,5,6,7,8,9].
>
> I have had to flatten lists quite a few times and it's quite tedious to type out. It feels like this should be something built in to python, anyone else think this way?

Depending on what you exactly mean by "flatten", it already is easy to
flatten a list in python:

>>> lst = [[1,2,3],[4,5,6,7],[8,9]]
>>> list(itertools.chain.from_iterable(lst))
[1, 2, 3, 4, 5, 6, 7, 8, 9]

--
Antoon Pardon

1
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