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

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 

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

Argument takes a single value and stores it as a string