我有一段代码用于比较两个列表（从 CSV 文件中读取）并返回 A 中而不是 B 中的项目，反之亦然。这是我所拥有的：
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!"
它曾经运行得很好，直到我收到这个错误：
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
有人可以帮忙吗？谢谢！