Can you use Python to search a csv for a value?

Question

My objective is to search a small csv file (one column, a few hundred rows) for a value. The rows in the csv are all numbers. Here is my attempt:

import csv

#read csv file
csv_reader = csv.reader(open('test.csv', 'rU'))

length_list = len(list(csv_reader))
value = []

for line in csv_reader:
    value.append(line) #transfer contents of csv to list

def find_voucher(input_voucher, counter):
    while counter < length_list:
        check_voucher = input_voucher in value[counter]
        if check_voucher == True:
            print "TRUE"
        else:
            print "FALSE"
    counter = counter + 1
find_voucher(1000,1)

When I run the script I get this error:

check_voucher = input_voucher in value[counter]
IndexError: list index out of range

agf · Accepted Answer · 2012-03-21 07:06:07Z

4

csv.reader is an iterator -- after you go through it once, it's empty, unlike a list. You use it up when you do

length_list = len(list(csv_reader))

There are a number of ways you could simplify your program, but if you just do

length_list = len(value)

after the for loop instead of trying to find the length first, it should work.

An example of how you could simplify your program:

def find_voucher(input_voucher, filename):
    with open(filename, 'rU') as f:
        return ("%s\n" % input_voucher) in f

print find_voucher(1000, 'test.csv')

edited Mar 21, 2012 at 7:06

answered Mar 21, 2012 at 6:44

agf

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2 Comments

Brandon Dorris Over a year ago

First things first: your simplified script worked perfectly. I am still very new to python and I have not delved into "with".

agf Over a year ago

@BrandonDorris In this case, the with statement just automatically closes the file. The main point of that example was the return part, showing you how you can test if a line exists in a file very easily. There are lots of objects in Python that are iterators, so you're likely to hit the problem of consuming them accidentally again.

Alex L · Accepted Answer · 2012-03-21 07:23:12Z

I'd do something like this:

# value we're looking for
desired_value = 1000

# read csv file
csv_reader = csv.reader(open('test.csv', 'rU'))

# loop through csv file
for line_number, line in enumerate(csv_reader):
    # catch errors in converting to float
    try:
        # if desired matches first value on line
        if desired_value == float(line[0]): 
            # print line number and value
            print line_number, line[0]
    except ValueError as e:
        continue

This looks through the CSV once, converts the first value on each line to a float, and then if the converted float matches the desired_value, print the line number and value matched.

Test.csv:

10
20
1000
1500

Output:

2 1000

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