CSV to JSON in Python

csv_to_json.py

import csv
import json
import sys

#Expects argument 1 to be a .csv filename. Matching file ending in .json is written.
if sys.argv[1] is not None and len(sys.argv[1])>0:
    infile = open(sys.argv[1],'r')
    outfile = open(sys.argv[1].replace(".csv",".json"),'w')
    reader = csv.reader(infile)
    colnames = []
    output_array = []
    for row in reader:
        out_dict = {}
        if reader.line_num == 1: #Assume the columns have names.
            colnames = row
        else:
            for colnum in range(len(row)):
                out_dict[colnames[colnum]] = row[colnum]
            output_array.append(out_dict)
    outfile.write(json.dumps(output_array))