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.

• mgilson: What are you going for here? I'm not sure I understand what you expect paths to be...

Answer 1

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])

Answer 2

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]]

Answer 3

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)]

Answer 4

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.