UPDATE: 2016
If you are happy to use the helpful more_itertools
external library:
from more_itertools import unique_everseen
with open('1.csv', 'r') as f, open('2.csv', 'w') as out_file:
out_file.writelines(unique_everseen(f))
A more efficient version of @IcyFlame’s solution
with open('1.csv', 'r') as in_file, open('2.csv', 'w') as out_file:
seen = set() # set for fast O(1) amortized lookup
for line in in_file:
if line in seen: continue # skip duplicate
seen.add(line)
out_file.write(line)
To edit the same file in-place you could use this (Old Python 2 code)
import fileinput
seen = set() # set for fast O(1) amortized lookup
for line in fileinput.FileInput('1.csv', inplace=1):
if line in seen: continue # skip duplicate
seen.add(line)
print line, # standard output is now redirected to the file