Python: Can someone help me debug this error? [newbie]

Question

I have a piece of code that's meant to compare two lists (which are read from CSV files) and return the items that are in A and not in B and vice versa. Here's what I've got:

import csv

#open CSV's and read first column with product IDs into variables pointing to lists
with open("A.csv", "rb") as f: 
    a = {row[0] if len(row) else default_value for row in csv.reader(f)}
with open("B.csv", "rb") as g: 
    b = {row[0] if len(row) else default_value for row in csv.reader(g)}


#create variables pointing to lists with unique product IDs in A and B respectively 

in_a_not_b = a-b 
in_b_not_a = b-a 

print len(in_a_not_b), " items in A missing from B", in_a_not_b
print len(in_b_not_a), " items in B missing from A", in_b_not_a


print "done!" 

It used to run just fine, until I got this error:

Traceback (most recent call last):
  File "C:/.../python - Comprare two lists", line 7, in <module>
    b = {row[0] if len(row) else default_value for row in csv.reader(g)}
  File "C:/.../python - Comprare two lists", line 7, in <setcomp>
    b = {row[0] if len(row) else default_value for row in csv.reader(g)}
NameError: global name 'default_value' is not defined

Can anyone please help ? Thanks!

Answer 1

You do not have a default_value variable.

just like your error says:

NameError: global name 'default_value' is not defined

You should put something like:

default_value = None

Above the code.

Answer 2

Your tables have now some empty rows, thus the condition

if len(row)

is now False and default_value is used but not found.