I have made CSV file slicer. This is my first and biggest piece of code on Python. It takes one .csv file from current folder and then slices it to n parts and adds a first column, if provided.

import csv
import math
import os
import re
import sys

# Reading and returning files in current directory
def read_files():
    __location__ = os.path.realpath(
        os.path.join(os.getcwd(), os.path.dirname(__file__)))
    return [f for f in os.listdir(__location__) if os.path.isfile(f)]

# Removing files which match certain pattern
def remove_old_files():
    for f in read_files():
        if re.search("^[0-9]+\.csv$", str(f)) is not None:

# Getting file to split in current directory
def get_file_to_split():
    for f in read_files():
        if re.search(".*\.csv$", str(f)) is not None:
            split(f, int(sys.argv[1]))

# Split file into n pieces
def split(csv_file, pieces):
    first_col = None
    if len(sys.argv) > 2:
        first_col = sys.argv[2]

    with open(csv_file, 'r') as c:
        reader = csv.reader(c)
        data = list(reader)

    cols_to_write = math.ceil(data.__len__() / pieces)
    chunks = [data[x:x + cols_to_write] for x in range(0, len(data), cols_to_write)]

    for num_file in range(pieces):
        filename = str(num_file) + ".csv"

        with open(filename, 'w') as f:
            w = csv.writer(f)
            for i in range(cols_to_write):
                    if first_col is not None and i == 0:
                except IndexError:


if __name__ == "__main__":
    if int(sys.argv[1]) <= 0:
        raise SystemExit("Piece count must be natural number greater than zero.")


1 Answer 1


Here are some of the things I've noticed:

  • switching to argparse might make the argument parsing a bit more readable
  • the read_files() + remove_old_files() functions could make use of glob module with the **+recursive mode:

    for filename in glob.iglob('./**/[0-9]+.csv', recursive=True):
  • avoid calling "magic" methods like __len__() when not necessary - you can use len() function directly
  • you can define first_col in one line:

    first_col = sys.argv[2] if len(sys.argv) > 2 else None
  • c and f are not good variable names, think of something more descriptive - input_file and output_file?..
  • you can use an f-string to define the "filename" for a chunk
  • move the comments before the functions into proper docstrings

Also, what if you would slice the CSV in an iterative manner, something along these lines (other improvements applied):

import csv
import glob
import os
import sys
from itertools import islice

def remove_old_files():
    """Removing files which match certain pattern."""
    for filename in glob.iglob('./**/[0-9]+.csv', recursive=True):

def chunks(it, size):
    it = iter(it)
    return iter(lambda: tuple(islice(it, size)), ())

def split(csv_file, number_of_slices, first_column):
    """Split file into number_of_slices pieces."""
    with open(csv_file, 'r') as input_file:
        reader = csv.reader(input_file)

        for num_file, chunk in enumerate(chunks(reader, number_of_slices)):
            with open(f"{num_file}.csv", 'w') as output_file:
                writer = csv.writer(output_file)

                if first_column:
                    for row in chunk:
                        writer.writerow([first_column] + row)

if __name__ == "__main__":
    # TODO: argparse?
    if int(sys.argv[1]) <= 0:
        raise SystemExit("Piece count must be natural number greater than zero.")
    number_of_slices = int(sys.argv[1])
    first_column = sys.argv[2] if len(sys.argv) > 2 else None


    for filename in glob.iglob('./**/*.csv', recursive=True):
        split(filename, number_of_slices, first_column)
  • \$\begingroup\$ I personally prefer docopt, but either is good. \$\endgroup\$ Commented Oct 16, 2017 at 16:13
  • \$\begingroup\$ Thank You for suggestions. Now I'm applying them into code, but got some issues with glob. It returns 'path' as a result os.remove() does not remove files. Additionally './**/[0-9]+.csv' matches all files, which will possibly remove all .csv files into current directory. \$\endgroup\$
    – Katka
    Commented Oct 17, 2017 at 7:15
  • \$\begingroup\$ Said wrong in comment. Pattern does not match any of files, that's why it does not remove any of them. \$\endgroup\$
    – Katka
    Commented Oct 17, 2017 at 7:30
  • \$\begingroup\$ @eddga alright, quick check - what Python version are you using? Thanks. \$\endgroup\$
    – alecxe
    Commented Oct 17, 2017 at 11:44
  • \$\begingroup\$ @alecxe I tried to run this on Python 3.6.3 and 3.5.0 \$\endgroup\$
    – Katka
    Commented Oct 17, 2017 at 11:59

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