How can I unpack four variables from a list?

Question

I have this list:

varlist = ['1', 'King', 't', '1']

I'm trying to unpack these four variables in a for loop;

for var1, var2, var3, var4 in varlist:
    post += len(var1)-len(var3)
    post *= len(var2)/len(var4)

When I run this loop, I get the following:

ValueError: need more than 1 value to unpack

How can I fix this?

Answer 1

Unpacking is different from iteration. You need to do this:

var1, var2, var3, var4 = varlist
post += len(var1)-len(var3)
post *= len(var2)/len(var4)

No loop is needed here.

Answer 2

You can't actually do this. There's a quick hack to make it work the way you think it will, but ultimately what you're trying to do doesn't work. Here's the quick hack:

varlist = ['1', 'King', 't', '1']
for var1, var2, var3, var4 in [varlist]: # note the brackets
    post += len(var1)-len(var3)
    post *= len(var2)\len(var4) # note that this line fails, because \ isn't /

The reason you can't do this is that a for loop iterates through each element in an iterable and assigns that value to whatever you tell it to. In this case, the first element in varlist is '1' , not ('1','King','t','1') . By doing for <stuff> in [varlist] you wrap your current varlist in another list, and iterate on that instead. The first element of THAT is ['1','King','t','1'] , which is unpackable into var1, var2, var3, var4 .

I'd strongly recommend rethinking your design, however, if this is really what you want to do. This sounds great if you have a lot of varlist s, otherwise just do:

var1, var2, var3, var4 = varlist # unpack without the loop
post += len(var1) - len(var3)
post *= len(var2) / len(var4)

Answer 3

I don't think you don't want unpack, you want a way to group the list, and access the individual elements of the grouping which can be done with grouper :

from itertools import izip_longest
def grouper(iterable, n, fillvalue=None):
    "Collect data into fixed-length chunks or blocks"
    # grouper('ABCDEFG', 3, 'x') --> ABC DEF Gxx
    args = [iter(iterable)] * n
    return izip_longest(fillvalue=fillvalue, *args)

post = 0

varlist = ['1', 'King', 't', '1', 'lol', 'newbs', 'money', 'plox']

print list(grouper(varlist, 4))

# Prints: [('1', 'King', 't', '1'), ('lol', 'newbs', 'money', 'plox')]

for var1, var2, var3, var4 in grouper(varlist, 4):
    post += len(var1)-len(var3)
    post *= len(var2)/len(var4)

print post

# Prints: -2

Answer 4

varlist = ['1', 'King', 't', '1']

for i,j,k,l in zip(*[iter(varlist)]*4):
    print i,j,k,l
1 King t 1

The *4 is the amount of elements you want, if you had a list like varlist=[1, 2, 3, 4, 5, 6, 7, 8,9,10]

It will zip four elements in a step of 4 giving the output:

1 2 3 4
5 6 7 8

The elements in your list must be a multiple of the variables you are assigning them to or you will not get all the elements as shown in the ouput above.