how to write back to open csv file in python

Question

I'm trying to go through a csv file, validate zip codes and write the city and state to the last column in the csv file.

I managed to get the csv data and get the city and state, but I don't understand how to write the new data to the last column. SO examples like this show how to create a csv, but not work with an existing csv.

here is my code so far:

with open("propertyOutput.csv", "rbw") as fp:
    reader = csv.DictReader(fp, skipinitialspace=True)
    table = [row for row in reader]
    for rows in table:
        stringNeed = rows['zip']
        if not stringNeed.isdigit(): 
            print"not number"  #would like to write this to the column
            pass
        else:
            if not len(stringNeed) == 5:  
                print"string not 5 long"  # would like to write this to the column
            else:
                x = getCityState(stringNeed)
                print x  # would like to write this to the column

I guess it is better to write everything in new file with added info than editing the open file. and I don't think their is any way to insert in the middle except rewrite whole file. — Ananta, Dec 02 '13 at 19:45

user1251007 · Accepted Answer · 2013-12-02T19:58:24.927

You need to do it in two steps:

Read the csv file and store information

import csv

with open("propertyOutput.csv", "rbw") as fp:
    reader = csv.DictReader(fp, skipinitialspace=True)
    table = [row for row in reader]
    header = reader.fieldnames

Write the information to an new file or replace old file

with open("propertyOutput.csv", "wb") as fp:
    writer = csv.DictWriter(fp, header)
    for row in table:
        if not stringNeed.isdigit(): 
            rows['zip'] = "not number"
        # even more stuff to check and edit here
        # write the edited row
        writer.writerow(row)

score 0 · Answer 2 · answered Dec 02 '13 at 19:54

Files, in general, do not support the concept of "insertion". You can only delete them, overwrite them (that is, either replace the file entirely, or replace a certain number of bytes with exactly the same number of (different) bytes), or append to them.

So to change the contents of a CSV file, you will have to overwrite it (the good news is, the CSV module makes it pretty easy).

how to write back to open csv file in python

2 Answers2

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