Best way to reverse lines in a csv file python

Question

I want to know the best way to reverse the lines of a big csv file (+50000 lines) in python 2.7 and rewrite it, avoiding the first line.

input:
A;B;C
1;2;3
4;5;6

output
A;B;C
4;5;6
1;2;3

I need to know how to do it in a efficient way in python 2.7.

Thank you guys,

menchopez

Answer 1

read the csv file using csv module and open the output also using csv module. Now you're working with lists as rows.

Use next to write the title line as-is. Now that the first line is consumed, convert the rest of the data into a list to read it fully and apply writerows on the reversed list:

import csv

with open("in.csv") as fr, open("out.csv","wb") as fw:
    cr = csv.reader(fr,delimiter=";")
    cw = csv.writer(fw,delimiter=";")
    cw.writerow(next(cr))  # write title as-is
    cw.writerows(reversed(list(cr)))

writerows is the fastest way of doing it, because it involves no python loops.

Python 3 users have to open the output file using open("out.csv","w",newline="") instead.