How to rename variables in a loop in Python

Question

I want to run a program in Python which loops several times, creating a NEW array each time - i.e. no data is overwritten - with the array named with a reference to the loop number, so that I can call it in subsequent loops. For instance, I might want to create arrays x0, x1, x2, ..., xi in a loop running from 0 to i, and then call each of these in another loop running over the same variables. (Essentially the equivalent of being able to put a variable into a string as 'string %d %(x)').

The best way is not to do this... use a list (or dictionary) instead — jamylak, Apr 17 '13 at 10:30
Your problem is that you are using local variables where you should be using a list or dictionary *instead*. — Martijn Pieters, Apr 17 '13 at 10:32

score 8 · Answer 1 · answered Apr 17 '13 at 10:35

8

You can access the globals() dictionary to introduce new variables. Like:

for i in range(0,5):
    globals()['x'+str(i)] = i

After this loop you get

>>> x0, x1, x2, x3, x4
(0, 1, 2, 3, 4)

Note, that according to the documentation, you should not use the locals() dictionary, as changes to this one may not affect the values used by the interpreter.

answered Apr 17 '13 at 10:35

MartinStettner

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Except that it doesn't work for `locals()` – John La Rooy Apr 17 '13 at 10:36
Yes, I already changed my answer. Although it seems to work, if you only introduce new variables (and don't change their value afterwards) – MartinStettner Apr 17 '13 at 10:38
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Note to anyone reading this answer : PLEASE DONT DO SUCH A STUPID THING. The correct solution is to use a dict or list. – bruno desthuilliers Apr 17 '13 at 10:50
I didn't suggest that this is a good way to do anything. But I believe it is a correct answer to a perfectly valid original question. Perhaps you might want to reread the SO section on downvoting ... – MartinStettner Apr 17 '13 at 10:55
1

@brunodesthuilliers There are a couple of scenarios where it might make sense to modify the global environment (there might be a reason for the presence of the `globals()` dict after all...) Without knowing anything more, you cannot say, if this is the case here. – MartinStettner Apr 17 '13 at 11:03

score 2 · Answer 2 · answered Apr 17 '13 at 10:40

2

Relying on variables's names and changing them is not the best way to go.

As people already pointed out in comments, it would be better to use a dict or a list instead.

answered Apr 17 '13 at 10:40

glglgl

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StarlitGhost · Accepted Answer · 2013-04-17T10:55:09.463

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Using a dict:

arraysDict = {}
for i in range(0,3):
    arraysDict['x{0}'.format(i)] = [1,2,3]

print arraysDict
# {'x2': [1, 2, 3], 'x0': [1, 2, 3], 'x1': [1, 2, 3]}
print arraysDict['x1']
# [1,2,3]

Using a list:

arraysList = []
for i in range(0,3):
    arraysList.append([1,2,3])

print arraysList
# [[1, 2, 3], [1, 2, 3], [1, 2, 3]]
print arraysList[1]
# [1, 2, 3]

edited Apr 17 '13 at 10:55

answered Apr 17 '13 at 10:42

StarlitGhost

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score 0 · Answer 4 · answered Apr 17 '13 at 10:34

0

You can create an object for this, and then use the setattr function for this. Basically, it will work like this:

setattr(myobject, 'string', value)

It is the same as doing : myobject.string = value

answered Apr 17 '13 at 10:34

Paco

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How to rename variables in a loop in Python

4 Answers4