6. Commandline argument parsing

6.1. argparse

6.1.1. Parser parameters

prog The name of the program (default: sys.argv[0])
usage A usage message (default: auto-generated from arguments)
description A description of what the program does
epilog Text following the argument descriptions
parents Parsers whose arguments should be copied into this one
formatter_class HelpFormatter class for printing help messages
prefix_chars Characters that prefix optional arguments
fromfile_prefix_chars Characters that prefix files containing additional arguments
argument_default The default value for all arguments
conflict_handler String indicating how to handle conflicts
add_help Add a -h/-help option
allow_abbrev Allow long options to be abbreviated unambiguously

6.1.2. Argument parameters

parameter description
name or flags Either a name or a list of option strings, e.g. foo or -f, --foo
action The basic type of action to be taken when this argument is encountered at the command line
nargs The number of command-line arguments that should be consumed
const A constant value required by some action and nargs selections
default The value produced if the argument is absent from the command line
type The type to which the command-line argument should be converted
choices A container of the allowable values for the argument
required Whether or not the command-line option may be omitted (optionals only)
help A brief description of what the argument does
metavar A name for the argument in usage messages
dest The name of the attribute to be added to the object returned by parse_args()

6.1.3. Simple parsing

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('--numbers', nargs='*', default=[1, 2.5, 3.0], type=float)

args = parser.parse_args()


6.1.4. Advanced parameter parsing

import argparse

parser = argparse.ArgumentParser(
    prog='PROGRAM NAME',
    description='A foo that bars',
    epilog="And that's how you'd foo a bar")

parser.add_argument('--sum', dest='accumulate', action='store_const', const=sum,
                    default=max, help='sum the integers (default: find the max)')

parser.add_argument('--foo', nargs='?', help='foo help')
parser.add_argument('--bar', nargs='+', help='bar help')
parser.add_argument('--foobar', nargs='*', default=[1, 2, 3], help='BAR!')

parser.add_argument('--integers', metavar='int',
    type=int, choices=range(10), nargs='+',
    help='an integer in the range 0..9')

parser.add_argument('--baz', nargs='?', type=int, default=42,
    help='the bar to %(prog)s (default: %(default)s)')

parser.add_argument('--move', choices=['rock', 'paper', 'scissors'])

parser.add_argument('--length', default=10, type=int, required=True)
parser.add_argument('--width', default=10.5, type=float)

script_arguments = parser.parse_args()

6.1.5. File handling

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('--input', default='input.csv', type=argparse.FileType('r'))
parser.add_argument('--output', default='output.c', type=argparse.FileType('w'))

args = parser.parse_args()

with args.input as input, args.output as output:
    content = input.read()
    # do conversion

6.2. Example

import argparse
import sys
import logging

def read(filename):
        with open(filename) as file:
            return file.read()

    except FileNotFoundError:
        logging.critical('File does not exists')

parser = argparse.ArgumentParser()
parser.add_argument('--file', default='/tmp/input.csv', type=read)
args = parser.parse_args()
import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--file', default='/tmp/input.csv', type=argparse.FileType('r'))

    args = parser.parse_args()

except SystemExit:
    print('File does not exists')


6.3. docopt

"""Naval Fate.

  naval_fate.py ship new <name>...
  naval_fate.py ship <name> move <x> <y> [--speed=<kn>]
  naval_fate.py ship shoot <x> <y>
  naval_fate.py mine (set|remove) <x> <y> [--moored | --drifting]
  naval_fate.py test (true|false)
  naval_fate.py (-h | --help)
  naval_fate.py --version

  -h --help     Show this screen.
  --version     Show version.
  --speed=<kn>  Speed in knots [default: 10].
  --moored      Moored (anchored) mine.
  --drifting    Drifting mine.

from docopt import docopt

if __name__ == '__main__':
    arguments = docopt(__doc__, version='Naval Fate 2.0')

    test = arguments.get('test', None)

# python doc.py test on

# {'--drifting': False,
#  '--help': False,
#  '--moored': False,
#  '--speed': '10',
#  '--version': False,
#  '<name>': [],
#  '<x>': None,
#  '<y>': None,
#  'mine': False,
#  'move': False,
#  'new': False,
#  'off': False,
#  'on': True,
#  'remove': False,
#  'set': False,
#  'ship': False,
#  'shoot': False,
#  'test': True}

6.4. Assignments

6.4.1. Argument parsing

  • Filename: argparse_avg.py
  • Lines of code to write: 5 lines
  • Estimated time of completion: 10 min
  1. Napisz parser parametrów linii poleceń

  2. Ma przyjmować tylko int i float

  3. Dla parametrów ma uruchomić funkcje avg() z listingu poniżej:

    def avg(*args):
        return sum(args) / len(args)
  4. Uruchamianie python argparse_avg.py --numbers 5 10 100 32 -90 27.5