CSV/JSON converter

Question

This is my first Python script and I would like to hear the opinion of more experienced users, so I can do it right from the beginning instead of trying to correct my mistakes after months (years?) of coding.

#!/usr/bin/env python

import argparse
import csv
import json
import os
import sys
from pprint import pprint

__author__  = 'RASG'
__version__ = '2018.02.15.1843'

# ------------------------------------------------------------------------------
# Argumentos
# ------------------------------------------------------------------------------

arg_parser = argparse.ArgumentParser(
    description     = 'csv <-> json converter',
    epilog          = 'Output files: /tmp/rag-*',
    formatter_class = lambda prog: argparse.RawTextHelpFormatter(prog, max_help_position=999)
)

arg_parser.add_argument('-v', '--version', action='version', version=__version__)

argslist = [
    ('-f',  '--file',   '(default: stdin) Input file',             dict(type=argparse.FileType('r'), default=sys.stdin)),
    ('-ph', '--header', '(default: %(default)s) Print csv header', dict(action='store_true')),
]

for argshort, arglong, desc, options in argslist: arg_parser.add_argument(argshort, arglong, help=desc, **options)

args = arg_parser.parse_args()

# ------------------------------------------------------------------------------
# Funcoes
# ------------------------------------------------------------------------------

def get_file_ext(f):

    file_name, file_ext = os.path.splitext(f.name)
    return file_ext.lower()


def csv_to_json(csv_file):

    csv_reader  = csv.DictReader(csv_file)
    output_file = open('/tmp/rag-parsed.json', 'wb')

    data = []
    for row in csv_reader: data.append(row)
    json_data = json.dumps(data, indent=4, sort_keys=True)
    output_file.write(json_data + '\n')


def json_to_csv(json_file):

    json_reader = json.loads(json_file.read())
    output_file = open('/tmp/rag-parsed.csv', 'wb')

    campos     = json_reader[0].keys()
    csv_writer = csv.DictWriter(output_file, fieldnames=campos, extrasaction='ignore', lineterminator='\n')

    if args.header: csv_writer.writeheader()

    for j in json_reader: csv_writer.writerow(j)


# ------------------------------------------------------------------------------
# Executar
# ------------------------------------------------------------------------------

for ext in [get_file_ext(args.file)]:

    if ext == '.csv':
        csv_to_json(args.file)
        break

    if ext == '.json':
        json_to_csv(args.file)
        break

    sys.exit('File type not allowed')

Answer 1

Before any criticism, great job. This is mature-looking code.

Alright, now some criticism ;)

• Single-line if and for loops are not good style in Python: always break the body out into its own indented block, even if it's only a single line; for example:

  for argshort, arglong, desc, options in argslist:
      arg_parser.add_argument(argshort, arglong, help=desc, **options)
  

• Add docstrings to your functions, to document what they do and their inputs/outputs (I prefer the Google format, but you're free to choose whatever works best for you):

  def get_file_ext(f):
      """Retrieves the file extension for a given file path
  
      Args:
          f (file): The filepath to get the extension of
  
      Returns:
          str: The lower-case extension of that file path
      """
      file_name, file_ext = os.path.splitext(f.name)
      return file_ext.lower()
  

• There are a couple more built-ins you could use. For example, get_file_ext(f) could be replaced by Path(f.name).suffix (using pathlib, if you're using an up-to-date Python)

• Unless there's some other good reason, use json.dump(open(...), data, ...) and json.load(open(...)) rather than reading/writing the file yourself, like json.loads(open(...).read()). This way, you never need to store the JSON text in memory, it can get saved/read to/from disk lazily by the parser. It's also just cleaner. (Also, you don't need 'wb' mode, just 'w', since JSON is text, not an arbitrary byte stream.)

• When you do want to manually open a file, it's better practice to use it as a context manager, which will automatically close the file at the proper time:

  with open(...) as output_file:
      output_file.write(...)
  

• Wrap the body of your script in a __main__ block:

  if __name__ == '__main__':
      for ext in [...]: ...
  

  or

  def main():
      for ext in [...]: ...
  
  if __name__ == '__main__':
      main()
  

That's more the style that's popular and standard in the Python community. You're clearly familiar with good coding, though, and it shows in your style. Good job, and welcome to Python!