CODEWITHSUNDEEP
python
Ch Y Duan
Ch Y Duan

Reputation: 155

Python ValueError: too many values to unpack (expected 2)

input

 2 4
 1 2 3 4
 1 0
 2 1
 2 3

I need to extract the pairs of numbers from third line to the end(only 2 numbers from the third line)
here is my function 

def read_nodes():
    n, r = map(int, input().split())
    n_list = []

    for i in range(2 , n):
        n1, n2 = map(int, input().split())
        n_list.append([n1, n2])
    return n_list
print(read_nodes())

I except [[1,0],[2,1],[2,3]] but says ValueError: too many values to unpack (expected 2)

Upvotes: 5

Views: 35634

Answers (2)

timgeb
timgeb

Reputation: 78650

@e4c5 already explained why the error occurs very well, so I am going to skip that part.

If you are using Python 3 and are only interested in the first two values, this is a good opportunity to use Extended Iterable Unpacking. Here are some short demos:

>>> n1, n2, *other = map(int, input().split())
1 2 3 4
>>> n1
1
>>> n2
2
>>> other
[3, 4]

other is the "wildcard" name that catches the remaining values. You can check whether the user provided exactly two values by checking the truthyness of other:

>>> n1, n2, *other = map(int, input().split())
1 2
>>> if not other: print('exactly two values')
... 
exactly two values

Note that this approach will still throw a ValueError if the user supplies less than two numbers because we need to unpack at least two from the list input().split() in order to assign the names n1 and n2.

Upvotes: 6

e4c5
e4c5

Reputation: 53734

There are two places where this can happen

n, r = map(int, input().split())

and

n1, n2 = map(int, input().split())

In both cases you are assuming that the input contains only two values. What if there are 3 or 20? Try something like

for x in map(int, input().split()):
    # code here

Or wrap the whole thing in try/except so that too many values will be handled cleanly.

Your for loop can probably just be

for i in range(2 , n):

    n_list.append(map(int, input().split())

Upvotes: 2

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