Error : too many values to unpack (expected 2) - Python

Question

First of all below are the list not tuple also would like to append the data in both the list and above all that need to know the reason of error along with solution

Trying to compare two list basically, here "ref" is the reference list and "obt" is product list, while running code(below) an error is generated.

What is the solution for it?

Also would like to apply the same to the data frames in pandas, what should be the code for it?

ref =[1,2,3,4]
obt =[0.5,1,1.5,5]
i,j=0
for i,j in obt,ref:
    global i,j
    if (obt[i] <= ref[j]):
        print ("all ok")
    else:
        print("error")
    i=i+1
    j=j+1

This isn't the problem, but you don't need `global` within a for loop — OneCricketeer, Apr 29 '16 at 13:39
You should look at the answer that uses `zip` I believe that is the best way to iterate over two lists, you also don't need any import statements — northsideknight, Apr 29 '16 at 13:57

score 1 · Answer 1 · answered Apr 29 '16 at 13:41

I think you would be good with zip:

for o, r in zip(obt, ref):
   if o <= r:
      print ("all ok")
   else:
      print("error")

Here is what zip(obt, ref) will produce:

[(0.5, 1), (1, 2), (1.5, 3), (5, 4)]

And, while looping over it you can compare the values from the tuple.

score 1 · Answer 2 · answered Apr 29 '16 at 13:43

1

This assignment statement should not work:

i,j=0

Try this instead:

>>> i, j = 0, 0
>>> i
0
>>> j
0

But if you wanted to compare two different lists here's how you do it:

import itertools
for i, j in itertools.zip_longest(list1, list2):
    # do stuff with i and j
    # if either i or j is none then
    # they are not the same lengths

answered Apr 29 '16 at 13:43

Games Brainiac

80,178
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141
199

Using `i, j = 0, 0` doesn't completely solve the problem. It still throws the same error on the line `for i,j in obt,ref:` – hostingutilities.com Apr 29 '16 at 13:57

score 0 · Answer 3 · answered Apr 29 '16 at 13:39

0

You could also use a try, except statement if you are running into issues with lists of different length Like:

try:
   for i, j in obt, ref:
   # Compare lists
except: # Enter error after except
   # Do something else

answered Apr 29 '16 at 13:39

Sven Hakvoort

3,543
2
17
34

score 0 · Answer 4 · answered Apr 29 '16 at 13:42

0

You will need to use other variables then i and j. These will be altered by the for loop. It is better to use for x in range(len(list) for the indexing

answered Apr 29 '16 at 13:42

Sven Hakvoort

3,543
2
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The for loop will reset the variables after each loop. That's not the problem – OneCricketeer Apr 29 '16 at 13:42
That's true, but i and j will get values out of the list and if you use these as an index later this can cause some issues like index errors. So that's why i also suggested this – Sven Hakvoort Apr 29 '16 at 13:44
But they are not indexes, they are values in the list. There would first be an IndexError because floats cant be used as an index – OneCricketeer Apr 29 '16 at 13:47
Oh yeah, you're right. Didn't see the first float. – Sven Hakvoort Apr 29 '16 at 13:48
Anyways, the error mention in the title is specifically on this line `i,j=0` – OneCricketeer Apr 29 '16 at 13:53

hostingutilities.com · Answer 5 · 2016-04-29T14:49:19.573

Python for loops are actually foreach loops. You could do

ref =[1,2,3,4]
obt =[0.5,1,1.5,5]

for i in range(len(ref)):
    if (obt[i] <= ref[i]):
        print ("all ok")
    else:
        print("error")

The range function will iterate through the numbers 0 to the length of your array, range(len(ref)) = [0,1,2,3], you can then use for which in Python is actually a foreach to grab these numbers.

The enumerate function will give you both the index and the value, so you can also do

ref =[1,2,3,4]
obt =[0.5,1,1.5,5]

for i,r in enumerate(ref):
    if (obt[i] <= r):
        print ("all ok")
    else:
        print("error")

The enumerate function gives back the index and ref[i], which is being stored in i and r. This example better shows how Python is behaving like a foreach loop and not a for loop.

The reason for the error

On the line i,j=0, you have to give j a value as well, so you would do i,j=0,0. Now what happens here is that 0,0 is that same thing as (0,0). It's a tuple. The tuple then has to be unpacked. When a tuple is unpacked the first variable, i, gets the first value 0. The second variable, j, gets the second value. And if there where more values in the tuple this would continue until every value in the tuple is unpacked to some variable.

Once this is fixed we can proceed to the next line. for i,j in obt,ref. Once again, you will see the exact same error. This time python is expecting to iterate over something in the form of [(2 value tuple), (2 value tuple), (2 value tuple), (2 value tuple)].

The zip function can be used to create this structure, as has been suggested above, or if you prefer using the indexes in a for loop, you could use for i in range(len(ref)) to iterate over the list of indexes [0,1,2,3]. This only work if ref and obt are the same length, otherwise you have to use zip.

Both ways work, using zip, or the method I mentioned, it's up to you how you want to solve the problem. If one list is longer than the other, zip will truncate the longer lists. The itertools.zip_longest solution that Games Braniac mentioned will extend the shortest length list so that they are the same size, and then zips them up.

Error : too many values to unpack (expected 2) - Python

5 Answers5