One-liner for nearly redundant list comprehensions

Question

Consider two list comprehensions gamma and delta with nearly redundant code. The difference being the sliced lists alpha and beta, namely

gamma = [alpha[i:i+30] for i in range(0,49980,30)]
delta = [beta[i:i+30] for i in range(0,49980,30)]

Is there a pythonic way to write this as a one liner (say gamma,delta = ... )?

I have a few other pieces of code that are similar in nature, and I'd like to simplify the code's seeming redundancy.

@TimPietzcker Just commented the same thing on coldspeed's answer — Joe Iddon, Dec 31 '17 at 09:29
Thanks for the comments! Didn't notice that OP wanted 2D lists. — cs95, Dec 31 '17 at 09:42
Also see http://stackoverflow.com/questions/434287/what-is-the-most-pythonic-way-to-iterate-over-a-list-in-chunks and http://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-evenly-sized-chunks-in-python — PM 2Ring, Dec 31 '17 at 09:43
Double zip? `gamma, delta = zip(values[i:i+30] for i in range(0,50000,30) for values in [list(zip(alpha, beta))])` — Peter Wood, Dec 31 '17 at 10:09
@Daniel I hadn't, could you say more? I eventually want to `vstack` these chunks. — sunspots, Dec 31 '17 at 10:13
with `alpha = np.array(alpha)` you can write `gamma = alpha.reshape(30, -1)`. — Daniel, Dec 31 '17 at 10:16
@sunspots Hmm, if you had tagged this numpy, you'd have had your answer marked duplicate, because reshape is a common idiom ;-) — cs95, Dec 31 '17 at 10:21
@PaulPanzer terrific point (I'll edit this). I'd been using 49980. Cheers. — sunspots, Dec 31 '17 at 11:21
Btw. is there anything wrong with `gamma, delta = ([x[i:i+30] for i in range(0,49980,30)] for x in (alpha, beta))`? — Paul Panzer, Dec 31 '17 at 11:27

score 8 · Answer 1 · answered Dec 31 '17 at 09:35

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Although one-line list-comprehensions are really useful, they aren't always the best choice. So here since you're doing the same chunking to both lists, if you wanted to change the chunking, you would have to modify both lines.

Instead, we could use a function that would chunk any given list and then use a one-line assignment to chunk gamma and delta.

def chunk(l):
    return [l[i:i+30] for i in range(0, len(l), 30)]

gamma, delta = chunk(gamma), chunk(delta)

answered Dec 31 '17 at 09:35

Joe Iddon

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More directly, if you want to reuse them, put them in a function. – Dan D. Dec 31 '17 at 09:47
I prefer this: `def chunk(l, size): return [l[i:i+size] for i in range(0, len(l), size)]` – RoadRunner Dec 31 '17 at 11:32
@RoadRunner Yeah I was considering whether or not to make it general purpose, could go either way **:)** – Joe Iddon Dec 31 '17 at 11:35
Yeah not that important for this question, I still upvoted regardless. – RoadRunner Dec 31 '17 at 11:36

Moinuddin Quadri · Accepted Answer · 2017-12-31T10:10:57.907

4

As far as your question related to combining both the list comprehension expression above is concerned, you can get gamma and delta by using zip with single list comprehension as:

gamma, delta = zip(*[(alpha[i:i+30], beta[i:i+30]) for i in range(0,50000,30)])

Sample example to show how zip works:

>>> zip(*[(i, i+1) for i in range(0, 10, 2)])
[(0, 2, 4, 6, 8), (1, 3, 5, 7, 9)]

Here our list comprehension will return the list of tuples:

>>> [(i, i+1) for i in range(0, 10, 2)]
[(0, 1), (2, 3), (4, 5), (6, 7), (8, 9)]

Then we are unpacking this list using * and using zip we are aggregating the element from each of the iterables:

>>> zip(*[(i, i+1) for i in range(0, 10, 2)])
[(0, 2, 4, 6, 8), (1, 3, 5, 7, 9)]

As an alternative, for dividing the list into evenly sized chunks, please take a look at "How do you split a list into evenly sized chunks?"

edited Dec 31 '17 at 10:10

answered Dec 31 '17 at 09:11

Moinuddin Quadri

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@Daniel how does this compromise readability? And are you referring to the slicing or the latter zip? – sunspots Dec 31 '17 at 09:24
@Daniel Performance wise usage of `zip` in above example will performa better as compared to the two *list comprehensions* – Moinuddin Quadri Dec 31 '17 at 09:32
@MoinuddinQuadri Have you tested that? The separate comprehensions are faster on my machine. – Dunes Dec 31 '17 at 10:21
@MoinuddinQuadri: your zip-variant is about 50% slower than two LCs. – Daniel Dec 31 '17 at 10:21
The result isn't quite right, since you produce tuples instead of lists. – Stefan Pochmann Dec 31 '17 at 11:56

Stefan Pochmann · Answer 3 · 2017-12-31T12:30:53.193

1

Just another way...

gamma, delta = ([src[i:i+30] for i in range(0,49980,30)] for src in (alpha, beta))

It's a bit faster than the accepted zip solution:

genny 3.439506340350704
zippy 4.3039169818228515

Code:

from timeit import timeit
alpha = list(range(60000))
beta = list(range(60000))
def genny():
    gamma, delta = ([src[i:i+30] for i in range(0,49980,30)] for src in (alpha, beta))
def zippy():
    gamma, delta = zip(*[(alpha[i:i+30], beta[i:i+30]) for i in range(0,50000,30)])
n = 1000
print('genny', timeit(genny, number=n))
print('zippy', timeit(zippy, number=n))

edited Dec 31 '17 at 12:30

answered Dec 31 '17 at 11:33

Stefan Pochmann

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This is what Paul Panzer put in the comments and Peter Wood had a similar idea with zip (earlier in the comments). – sunspots Dec 31 '17 at 11:42
I hadn't seen the timing portion before, it's a nice addition. – sunspots Dec 31 '17 at 12:41

score 0 · Answer 4 · answered Dec 31 '17 at 09:13

0

You can you lambda expression:

g = lambda l: [l[i:i+30] for i in range(0,50000, 30)]
gamma, delta = g(alpha), g(beta)

answered Dec 31 '17 at 09:13

Noa

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If you're naming a lambda, then there's no point to it being a lambda. – cs95 Dec 31 '17 at 09:13
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Please do _not_ use `lambda` for named functions. `lambda` is supposed to be for anonymous functions. If you want a named function, use the `def` syntax. However, I do agree that it's probably a good idea to use a function for this task. – PM 2Ring Dec 31 '17 at 09:14
I agree with the comments and I also think that using a function is a better idea for this. It is difficult sometimes to say which is a more pythonic solution. But this seems to be a legit use case for function. – SRC Dec 31 '17 at 09:20
I've seen in lots of places that this a legit way to define a one-liner function... why is this wrong? – Noa Dec 31 '17 at 09:21
It's not technically wrong, it's just bad style. It only saves a couple of bytes of typing, so what's the benefit of using a `lambda` instead of a proper named function? – PM 2Ring Dec 31 '17 at 09:25
@PM2Ring Gotcha – Noa Dec 31 '17 at 09:26
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All functions have a `__name__` attribute. This can be handy when debugging, among other situations. (So if you print a function, or convert it to a string, the name attribute is part of that string representation). But a lambda's name is set to `''`, which isn't very informative. ;) – PM 2Ring Dec 31 '17 at 09:30

One-liner for nearly redundant list comprehensions

4 Answers4