Question

I’m trying to load two sets of CSV files and do some calculations on both such as difference of each set, mean absolute error set1 - set2 exc.

I’m trying to load both sets like this:

import glob    
for a, b in (glob.glob("*a.csv"), glob.glob("*b.csv")):

And I get the error:

ValueError: too many values to unpack

Answer 1

The Problem

You are getting the ValueError because you are trying to assign more items to the tuple than the number of target variables you provide. The for loop tuple unpacking syntax will loop through each list in your tuple and attempt to assign each value in the tuple to your targets (a and b). For instance, this would work:

for a,b in (['0a.csv', '1a.csv'], ['0b.csv', '1b.csv']):
    print a,b

It assigns the first value of each list to a and the second value to b. The code above prints:

0a,csv 1a.csv
0b.csv 1b.csv

Thus, you are getting the ValueError because the results from at least one of your glob.glob calls is a list longer than two elements.

A Solution

Based on what you are trying to do, I think you want to use zip.

import glob

for a,b in zip(glob.glob("*a.csv"), glob.glob("*b.csv")):
    # whatever

That will take pairs of files matching the pattern you gave and assign them to a and b. For example, if you have files 0a.csv, 1a.csv, 2a.csv, 0b.csv, 1b.csv, and 2b.csv doing

for a,b in zip(glob.glob("*a.csv"), glob.glob("*b.csv")):
    print a, b

results in

0a.csv 0b.csv
1a.csv 1b.csv
2a.csv 2b.csv