Python: Replace Item in Tuple Converted to List

Question

Recently came into a weird problem. I'm trying to change an item in a tuple I've converted to a list.

But what I had,

paths = [(1,2),(2,3),(2,0)]
plist = []

for pathlist in paths:
    for n in range(0, len(pathlist)):
        if pathlist[n] == 2:
            plist = list(pathlist)
            plist[n] = 4
            pathlist = tuple(plist)
            print(pathlist)
print(paths)

Didn't actually change the value in the list paths, i.e. it stayed the same as originally, even though I could tell from print(pathlist) that it had been modified correctly. And I can't remove it and append it because that would offset the for loop. Thanks for your help, guys.

What are you going for here? I'm not sure I understand what you expect `paths` to be... — mgilson, Dec 11 '13 at 02:56

score 1 · Accepted Answer · answered Dec 11 '13 at 03:09

the pathlist var is a new instance, it will not affect the paths list members.

try this:

for i in range(len(paths)):
    pathlist = paths[i]
    for n in range(0, len(pathlist)):
        if pathlist[n] == 2:
            plist = list(pathlist)
            plist[n] = 4
            paths[i] = tuple(plist)
            print(paths[i])

score 0 · Answer 2 · answered Dec 11 '13 at 03:16

You are only changing the list-copy you modified. You're not directly modifying paths.

Do it like this:

>>> for i, pl in enumerate(paths):
    for j, n in enumerate(pl):
        if n==2:
            pl = list(pl)
            pl[j] = 4
            paths[i] = pl


>>> paths
[[1, 4], [4, 3], [4, 0]]

score 0 · Answer 3 · answered Dec 11 '13 at 05:54

you should append the pathlist to some list in each turn...try this one

paths = [(1,2),(2,3),(2,0)]
flist = []
for pathlist in paths:
    for n in range(0, len(pathlist)):
        if pathlist[n] == 2:
            plist = list(pathlist)
            plist[n] = 4
            pathlist = tuple(plist)
            flist.append(pathlist)
print(flist)

the output will be

[(1, 4), (4, 3), (4, 0)]

score 0 · Answer 4 · answered Dec 11 '13 at 06:01

While initially pathlist points to an element of path, that link is lost when it is reassigned within the loop.

for pathlist in paths:
    ...
    pathlist = 'new value'

Try this simpler case with a list of integers. alist is not changed.

alist = [1,2,3]
for x in alist:
    x = x*2

But if the elements are lists, they can be changed without loosing that link to alist.

alist = [[1],[2],[3]]
for x in alist:
    x[0] = x[0]*2

Unfortunately, for these purposes, your tuples are more like integers than lists. You either have to use indexing to modify alist[i], or rebuild the list as in the list comprehension answer.

Python: Replace Item in Tuple Converted to List

4 Answers4