How to Delete Rows CSV in python

csv, delete-row, module, python, python-2.7

Solution

This solution uses `fileinput` with `inplace=True`, which writes to a temporary file and then automatically renames it at the end to your file name. You can't remove rows from a file but you can rewrite it with only the ones you want.

if the keyword argument `inplace=1` is passed to `fileinput.input()` or to the `FileInput` constructor, the file is moved to a backup file and standard output is directed to the input file (if a file of the same name as the backup file already exists, it will be replaced silently). This makes it possible to write a filter that rewrites its input file in place.

fileA

h1,h2,h3
a,b,c
d,e,f
g,h,i
j,k,l

fileB

h1,h2,h3
a,b,c
1,2,3
g,h,i
4,5,6

import fileinput, sys, csv

with open('fileB', 'rb') as file_b:
    r = csv.reader(file_b)
    next(r) #skip header
    seen = {(row[0], row[2]) for row in r}

f = fileinput.input('fileA', inplace=True) # sys.stdout is redirected to the file
print next(f), # write header as first line

w = csv.writer(sys.stdout) 
for row in csv.reader(f):
   if (row[0], row[2]) in seen: # write it if it's in B
       w.writerow(row)

fileA

h1,h2,h3
a,b,c    
g,h,i

Problem

I'm trying to compare two csv files (fileA and fileB), and remove any rows from fileA that are not found in fileB. I want to be able to do this without creating a third file. I thought I could do this using the csv writer module but now I'm second guessing myself. Currently, I'm using the following code to record my comparison data from file B: ``` removal_list = set() with open('fileB', 'rb') as file_b: reader1 = csv.reader(file_b) next(reader1) for row in reader1: removal_list.add((row[0], row[2])) ``` This is where I'm stuck and do not know how to delete the rows: ``` with open('fileA', 'ab') as file_a: with open('fileB', 'rb') as file_b: writer = csv.writer(file_a) reader2 = csv.reader(file_b) next(reader2) for row in reader2: if (row[0], row[2]) not in removal_list: # If row was not present in file B, Delete it from file A. #stuck here: writer.<HowDoIRemoveRow>(row) ```

Original source