Python | How to Parse Command-Line Options

In this article, we will discuss how to write a Python program to parse options supplied on the command line (found in sys.argv).

Parsing command line arguments using Python argparse module

The argparse module can be used to parse command-line options. This module provides a very user-friendly syntax to define input of positional and keyword arguments.

Example: Sample program to take command line inputs using argparse module

'''
Hypothetical command-line tool for searching a 
collection of files for one or more text patterns.
'''
import argparse

parser = argparse.ArgumentParser(description ='Search some files')

parser.add_argument(dest ='filenames', metavar ='filename', nargs ='*')
parser.add_argument('-p', '--pat', metavar ='pattern', 
                    required = True, dest ='patterns', 
                    action ='append', 
                    help ='text pattern to search for')

parser.add_argument('-v', dest ='verbose',
                    action ='store_true', help ='verbose mode')
parser.add_argument('-o', dest ='outfile', 
                    action ='store', help ='output file')
parser.add_argument('--speed', dest ='speed', 
                    action ='store', choices = {'slow', 'fast'},
                    default ='slow', help ='search speed')
args = parser.parse_args()

The program mentioned above defines a command-line parser with the following usage:

usage: search.py [-h] [-p pattern] [-v] [-o OUTFILE] 
  [--speed {slow, fast}] [filename [filename ...]]

Search some files

positional arguments:
          filename

optional arguments:
-h, --help           show this help message and exit
-p pattern,    --pat pattern
                 text pattern to search for
-v                   verbose mode
-o OUTFILE           output file
--speed {slow, fast}  search speed

Note: Generally, argparse defines a --help option to print out all accepted arguments and their details. It will print all details about accepted arguments if we execute the script as follows:

python script_name.py --help

Code: The following session shows how data shows up in the program.

usage: search.py [-h] -p pattern [-v] [-o OUTFILE]
  [--speed {fast, slow}] [filename [filename ...]]

Input:

python3 search.py -v -p spam --pat = eggs foo.txt bar.txt

Output:

filenames = ['foo.txt', 'bar.txt']
patterns = ['spam', 'eggs']
verbose = True
outfile = None
speed = slow

• The argparse module is one of the largest modules in the standard library, and has a huge number of configuration options. This codes above show an essential subset that can be used and extended to get started.
• To parse options, you first create an ArgumentParser instance and add declarations for the options you want to support it using the add_argument() method.
• In each add_argument() call, the dest argument specifies the name of an attribute where the result of parsing will be placed.
• The metavar argument is used when generating help messages.
• The action argument specifies the processing associated with the argument and is often store for storing a value or append for collecting multiple argument values into a list.

Argument collects all the extra command-line arguments into a list. It’s being used to make a list of filenames 

parser.add_argument(dest = 'filenames',
                    metavar = 'filename', nargs = '*')

Argument sets a Boolean flag depending on whether the argument was provided 

parser.add_argument('-v', dest = 'verbose', 
                    action = 'store_true', 
                    help = 'verbose mode')

Argument takes a single value and stores it as a string

parser.add_argument('-o', dest = 'outfile', 
                    action = 'store', help = 'output file')