Unpacking a zip object causing ValueError

Question

I created a zip object using the following line of code:

k=zip([1,2,3],['a','b','c'])

Converting this into a list gives the output:

[(1,'a'),(2,'b'),(3,'c')]

However, when I use this line of code

x,y=zip(*k)

it gives me this ValueError:

"ValueError: not enough values to unpack (expected 2, got 0)"

I've been trying to find out what the problem is but couldn't figure anything out.

it's working for me. did you get the error with same example you posted — deadshot, Jun 01 '20 at 17:47
There must be something else wrong with your code. I converted your problem into a one-liner and that works fine. Please run the one-liner and confirm whether it works for you or not: `zip(*list(zip([1,2,3],['a','b','c'])))` — Cerno, Jun 01 '20 at 17:49
Please take a look at your list conversion. You seem to apply zip(*) to k, which is not the list, but the original result of zip(). If you convert it to a list it will become a copy, so you will have to unzip your list, not k. — Cerno, Jun 01 '20 at 17:52
This works for me, apparently it also works if I use the line right after creating the zip object, but not if I use it for something else before that. As the comment before yours points out @Cerno. Yep your second comment also seems to work. Thanks! — random math student, Jun 01 '20 at 17:53
Can you post a full running example that raises the error? The error suggests that you are using the iterator twice - its empty the second time - but we are only guessing until you post it. BTW, try `list(k)` twice and you'll see the problem. — tdelaney, Jun 01 '20 at 18:13

score 4 · Accepted Answer · answered Jun 01 '20 at 17:49

Method zip returns a iterator, so when you print it, you consume it so after that k is empty

apply the second zip directly

k = zip([1,2,3],['a','b','c'])
x,y = zip(*k)
print(x, "/", y) # (1, 2, 3) / ('a', 'b', 'c')

wrap it in a list to use it multiple times

k = list(zip([1,2,3],['a','b','c']))
print(k)         # [(1, 'a'), (2, 'b'), (3, 'c')]
x,y = zip(*k)
print(x, "/", y) # (1, 2, 3) / ('a', 'b', 'c')

1 Answers1