collapse csv rows by a column value

collapse_by_column.py

'''you have a csv file with something like:
        col1, col2
        x,    1
        x,    2
        y,    1
        z,    4
        z,    8
    and you want to output something like:
        col1, col2
        x,    1;2
        y,    1
        z,    4;8
        
Use itertools groupby
'''
import csv
from itertools import groupby

ifile = './path_to_csv.csv'
ofile = './path_to_output.csv'

with open(ifile, 'r') as i:
    with open(ofile, 'w') as o:
        reader = csv.reader(i)
        writer = csv.writer(o)
        header = next(reader)
        writer.writerow(['col1', 'col2'])
        grouped = groupby(reader, lambda x: x[0])
        for key, group in grouped:
            vals = list(group)
            row = [key, ';'.join([v[2] for v in vals])]
            writer.writerow(row)