In python create a list with a variable in the name

Question

I'm trying to generate a series of (empty) lists using a for loop in python, and I want the name of the list to include a variable. e.g. y0, y1, y2 etc. Ideally looking something like this:

for a in range (0,16777216):
global y(a)=[]

You don't want to do that. See the answers for what to actually do. — Winston Ewert, Aug 30 '12 at 19:00
Creating empty data structures you're not immediately going to use is unpythonic. Python is capable of lazy evaluation, especially in its iterative data structures. Take advantage of that! — kojiro, Aug 30 '12 at 19:03
My final intention was to fake a table by creating a bunch of lists as the y values and the positions as the x values. Having to use another list seems a little kludgey. — schwal, Aug 30 '12 at 19:10
@schwal: I think most Python programmers will think you've got that exactly backwards. If you use a `dict` or a list of lists, then that *is* your table-- no fakery required. In fact, the local variables you want to create would simply live in their *own* `locals()` dictionary. So under the hood you'd be using the same data structure, just far less conveniently, which doesn't make much sense. — DSM, Aug 30 '12 at 19:27

score 9 · Answer 1 · edited May 23 '17 at 12:33

9

why wouldn't you do a dictionary of lists?

y = {}
for a in range (0,16777216):
    y[a] = []

also for brevity: https://stackoverflow.com/a/1747827/884453

y = {a : [] for a in range(0,16777216)}

edited May 23 '17 at 12:33

Community

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answered Aug 30 '12 at 18:59

Francis Yaconiello

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Except all the keys are integers, so perhaps a list of lists or sparse array? – kojiro Aug 30 '12 at 19:00
i typo'd that a, it wasn't supposed to be a string. fixed. – Francis Yaconiello Aug 30 '12 at 19:02
added dict comprehension – Francis Yaconiello Sep 11 '12 at 14:24

score 6 · Answer 2 · answered Aug 30 '12 at 18:59

6

Not sure if this is quite what you want but couldn't you emulate the same behavior by simply creating a list of lists? So your_list[0] would correspond to y0.

answered Aug 30 '12 at 18:59

Tejas Sharma

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score 1 · Answer 3 · edited May 23 '17 at 12:22

1

The answer is: don't. Instead create a list so that you access the variables with y[0], y[1], etc. See the accepted answer here for info.

edited May 23 '17 at 12:22

Community

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answered Aug 30 '12 at 19:00

BrenBarn

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You should mark this question as a duplicate to the one you linked. – Burhan Khalid Aug 30 '12 at 19:04

score 1 · Answer 4 · answered Aug 30 '12 at 19:14

Maybe you should check out defaultdict

form collection import defaultdict

# This will lazily create lists on demand
y = defaultdict(list)

Also if you want a constraint on the key override the default __getitem__ function like the following...

def __getitem__(self, item):
    if isintance(item, int) and 0 < item < 16777216:
         return defaultdict.__getitem__(self, item)
    else:
         raise KeyError

In python create a list with a variable in the name

4 Answers4