Python - Initializing Multiple Lists/Line

Question

This is terribly ugly:

psData = []
nsData = []
msData = []
ckData = []
mAData = []
RData = []
pData = []

Is there a way to declare these variables on a single line?

I don't think it's ugly, especially if you do psData = [] # Some comment explaining what's in there — extraneon, Mar 08 '10 at 16:08
Ugliness or not, beware with those names so similar one to each other: psData and pdata, mAData and msData... — Francesco, Mar 08 '10 at 16:11
-1: Don't replace this with some "code golf" construction that uses the minimum number of keystrokes. This is nice. Anything more terse will baffle and frustrate folks who want to maintain this program. Indeed, anything more obscure than this will likely get rewritten to this. — S.Lott, Mar 08 '10 at 19:41
It shouldn't be baffling if the coder includes a comment to explain what the line of code does, like # initialize list variables — CCKx, Jan 20 '17 at 15:01

score 220 · Accepted Answer · edited Jun 30 '16 at 09:05

220

alist, blist, clist, dlist, elist = ([] for i in range(5))

The downside of above approach is, you need to count the number of names on the left of = and have exactly the same number of empty lists (e.g. via the range call, or more explicitly) on the right hand side.

The main thing is, don't use something like

alist, blist, clist, dlist, elist = [[]] * 5

nor

alist = blist = clist = dlist = elist = []

which would make all names refer to the same empty list!

edited Jun 30 '16 at 09:05

aerin

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answered Mar 08 '10 at 16:04

Alex Martelli

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@thenickname, always glad to help! – Alex Martelli Mar 08 '10 at 16:08
4

Great answer, I was trying to do the C style of `alist, blist, clist = []` and failing untill I found this – Bas Jansen Apr 09 '13 at 16:06
1

Does it apply to dictionaries too? – rom Oct 21 '13 at 13:24
3

Yes, it does apply to dictionaries too. That's what I was looking for until I came across this question and answer. – CCKx Jan 20 '17 at 14:55
Thanks for the explanation; the first line is what I was looking for. – Luke Davis Mar 02 '17 at 20:28

YOU · Answer 2 · 2010-03-09T04:01:16.193

31

psData,nsData,msData,ckData,mAData,RData,pData = [],[],[],[],[],[],[]

edited Mar 09 '10 at 04:01

answered Mar 08 '10 at 16:05

YOU

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score 11 · Answer 3 · answered Mar 08 '10 at 16:07

11

Depending on your needs, you could consider using a defaultdict with a list factory. Something like:

my_lists = collections.defaultdict(list)

and then you can directly append to my_lists["psData"] and so on. This is the relevant doc page: http://docs.python.org/library/collections.html#collections.defaultdict

answered Mar 08 '10 at 16:07

Francesco

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Thanks for that! I love that python lets me write useful code as a beginner, but also has so many advanced features that I can keep learning to improve my code. – vorpal Oct 04 '17 at 03:13
That's one of its strong points and a testament to the ingenuity of Guido and of the other core contributors to the language design! – Francesco Oct 04 '17 at 03:27

Benyamin Jafari · Answer 4 · 2022-04-21T09:40:47.070

3

A bit more efficient approach:

alist, blist, clist, dlist, elist = ([] for _ in xrange(5))

[NOTE]:

xrange() is more optimal than range() in Python2. ^(Ref)
The i variable was unusable so using _ is better. ^(Ref)
xrange() is no longer in Python3 — range() is the same with xrange().

edited Apr 21 '22 at 09:40

answered Oct 20 '18 at 07:28

Benyamin Jafari

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How can something be "more optimal" than something else? :) – bers Dec 05 '19 at 10:27

score 0 · Answer 5 · answered Jan 13 '17 at 11:05

Bare in mind that, tidiness may come with consequences of performance. The range function call will slow down the init process slightly. Beware if you have some process that need to reinit the variable many time.

import time
def r_init():
    st=time.time()
    alist, blist, clist, dlist, elist = ([] for i in range(5))
    et=time.time()
    print("{:.15f}".format(et-st))

def p_init():
    st=time.time()
    alist=[];blist=[];clist=[];dlist=[];elist=[]
    et=time.time()
    print("{:.15f}".format(et-st))

for x in range(1,10):
    r_init()
    p_init()
    print("\n")

score -1 · Answer 6 · edited Jul 11 '12 at 20:05

-1

You can use a class to initialize/store the data, it would take more lines, but could be easier to read, and more object oriented.

Like:

class Data:
    def __init__(self):
        self.var1=[]
        <etc.>
    def zeroize(self):
        self.var1=[]
        <etc.>

Then in main near the beginning:

data=Data()

Then in your loops or anywhere in main post declaration you can use the class.

data.var1.append(varN)
if(something):
    data.zeroize()

edited Jul 11 '12 at 20:05

Spectre87

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answered Jul 11 '12 at 17:12

prince

1

this is essentially just a dict, also - it does not solve the OP's question, nor is it simpler, the OP clearly wanted less code, and less hassle, this is not that. however, this is a different type of solution to the problem, that does merit something, but it can be done much better, also, you have used some strange names. – Inbar Rose Oct 11 '12 at 13:22

score -5 · Answer 7 · answered Sep 14 '18 at 16:31

-5

Something along the lines of

alist, blist, clist, dlist, elist = ([],)*5

would appear to be the most elegant solution.

answered Sep 14 '18 at 16:31

Bryson S.

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`alist.append("oops"); print(blist)`: `['oops']`. – bers Dec 05 '19 at 10:28

Python - Initializing Multiple Lists/Line

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