Python - Initializing Multiple Lists/Line

88
votes

This is terribly ugly:

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

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

7
pythonlistinitialization
I don't think it's ugly, especially if you do psData = [] # Some comment explaining what's in there - extraneon
Ugliness or not, beware with those names so similar one to each other: psData and pdata, mAData and msData... - Francesco
-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
It shouldn't be baffling if the coder includes a comment to explain what the line of code does, like # initialize list variables - CCKx

7 Answers

203
votes
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!

29
votes
psData,nsData,msData,ckData,mAData,RData,pData = [],[],[],[],[],[],[]
10
votes

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

1
votes

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 not defined in Python3.

0
votes

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")
-1
votes

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()
-5
votes

Something along the lines of

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

would appear to be the most elegant solution.