Why do I get a ValueError when trying to split a line of input and assign to multiple variables?

Question

I tried some code like this to read question and answer pairs from a file:

questions_list = []
answers_list = []

with open('qanda.txt', 'r') as questions_file:
    for line in questions_file:
        line = line.strip()
        questions, answers = line.split(':')
        questions_list.append(questions)
        answers_list.append(answers)

But sometimes I get an exception from the questions, answers = line.split(':') line, that says:

(Python 2.x)
builtins.ValueError: need more than 1 value to unpack

(Python 3.x)
ValueError: not enough values to unpack (expected 2, got 1)

Alternatively, the opposite problem:
ValueError: too many values to unpack (expected 2)

Why does this occur? How can I fix or work around the problem?

Asked By: user3046660

||

Source

Answer 1

line.split(':') apparently returns a list with one element, not two.

Hence that’s why it can’t unpack the result into questions and answers. Example:

>>> line = 'this-line-does-not-contain-a-colon'
>>> question, answers = line.split(':')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 1 value to unpack

Answered By: Simeon Visser

Answer 2

This is probably because when you’re doing the splitting, there is no :, so the function just returns one argument, and not 2. This is probably caused by the last line, meaning that you’re last line has nothing but empty spaces. Like so:

>>> a = '   '
>>> a = a.strip()
>>> a
''
>>> a.split(':')
['']

As you can see, the list returned from .split is just a single empty string. So, just to show you a demo, this is a sample file:

a: b
c: d
e: f

g: h

We try to use the following script (val.txt is the name of the above file):

with open('val.txt', 'r') as v:
    for line in v:
        a, b = line.split(':')
        print a, b

And this gives us:

Traceback (most recent call last):
a  b

c  d
  File "C:/Nafiul Stuff/Python/testingZone/28_11_13/val.py", line 3, in <module>

    a, b = line.split(':')
e  f
ValueError: need more than 1 value to unpack

When trying to look at this through a debugger, the variable line becomes n, and you can’t split that.

However, a simple logical ammendment, would correct this problem:

with open('val.txt', 'r') as v:
    for line in v:
        if ':' in line:
            a, b = line.strip().split(':')
            print a, b

Answered By: Games Brainiac

Answer 3

Try:

question, answers = line.split(':', maxsplit=1)
question, __, answers = line.partition(':')

Also in Python 3 you can do something else:

question, *many_answers = line.split(':')

which looks like:

temp = line.split(':')
question = temp[0]
many_answers = tuple(temp[1:])

Answered By: Kos

Answer 4

The reason why this happens could be a few, as already covered in the other answers. Empty line, or maybe a line only have a question and no colon. If you want to parse the lines even if they don’t have the colon (for example if some lines only have the question), you can change your split to the following:

questions, answers, garbage = (line+'::').split(':', maxsplit=2)

This way, the values for questions and answers will be filled if they are there, and will be empty if the original file doesn’t have them. For all intents and purposes, ignore the variable garbage.

Answered By: msb

Why do I get a ValueError when trying to split a line of input and assign to multiple variables?

Question:

Answers: