split_csv.py

# Original CSV file
original_csv_file = 'filename.csv'
# Columns that need to be separated
# For this example, we have 3 subfiles, with these columns
# File 1: 0, 2, 4
# File 2: 1, 3, 5
# File 3: 0, 5
subfile_columns = [
    [0, 2, 4],
    [1, 3, 5],
    [0, 5],
]
num_files = len(subfile_columns)

# 1. Open the new subfiles in "append" mode -- that way we can add new lines to each file one-by-one
subfiles = []
for idx in range(num_files):
    # Create new file name
    filename = original_csv_file[:-4] + str(idx) + '.csv'
    # Create file handle in append mode ('a')
    file_handle = open(filename, 'a')
    subfiles.append(file_handle)

# 2. Open the original file, and write subfiles in the process
with open(original_csv_file, 'r') as csv_file_handle:
    for line in csv_file_handle:        # 2.1. Read the file one line at a time
        split_line = line.split(',')    # 2.2. Split the line by comma
        for idx in range(num_files):    # 2.3. Iterate over all files
            # 2.4. Get the relevant columns
            columns = subfile_columns[idx]
            # 2.5. Create a new line by combining the relevant columns
            subline = ','.join([split_line[col] for col in columns]) + '\n'  # Don't forget new line character
            # 2.6. Get the relevant file
            fh = subfiles[idx]
            # 2.7. Add the subline to the subfile
            fh.write(subline)

# 3. Close all the open subfiles
for sf in subfiles:
    sf.close()