How To Delete Rows CSV In Python

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 t

Solution 1:

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

Solution 2:

CSV is not a database format. It is read and written as a whole. You can't remove rows in the middle. So the only way to do this without creating a third file is to read in the file completely in memory and then write it out, without the offending rows.

But in general it's better to use a third file.

Solution 3:

As Lennart described, you can't modify a CSV file in-place as you iterate over it.

If you're really opposed to creating a third file, you might want to look into using a string buffer with StringIO, the idea being that you build up the new desired contents of file A in memory. At the end of your script, you can write the contents of the buffer over file A.

from cStringIO import StringIO


with open('fileB', 'rb') as file_b:
    new_a_buf = StringIO()
    writer = csv.writer(new_a_buf)
    reader2 = csv.reader(file_b)
    next(reader2)
    for row in reader2:
        if (row[0], row[2]) not in removal_list:
            writer.writerow(row)

# At this point, the contents (new_a_buf) exist in memory
with open('fileA', 'wb') as file_a:
    file_a.write(new_a_buf.getvalue())