How to Delete Rows CSV in python

Question

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)

Answer 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