Argparse: Way to include default values in '--help'?

Question

Suppose I have the following argparse snippet:

diags.cmdln_parser.add_argument( '--scan-time',                      action  = 'store',                      nargs   = '?',                      type    = int,                      default = 5,                      help    = "Wait SCAN-TIME seconds between status checks.") 

Currently, --help returns:

usage: connection_check.py [-h]                              [--version] [--scan-time [SCAN_TIME]]            Test the reliability/uptime of a connection.    optional arguments: -h, --help            show this help message and exit --version             show program's version number and exit --scan-time [SCAN_TIME]                     Wait SCAN-TIME seconds between status checks. 

I would prefer something like:

--scan-time [SCAN_TIME]                     Wait SCAN-TIME seconds between status checks.                     (Default = 5) 

Peeking at the help formatter code revealed limited options. Is there a clever way to get argparse to print the default value for --scan-time in a similar fashion, or should I just subclass the help formatter?

Answer 1

Use the argparse.ArgumentDefaultsHelpFormatter formatter:

parser = argparse.ArgumentParser(     # ... other options ...     formatter_class=argparse.ArgumentDefaultsHelpFormatter) 

To quote the documentation:

The other formatter class available, ArgumentDefaultsHelpFormatter, will add information about the default value of each of the arguments.

Note that this only applies to arguments that have help text defined; with no help value for an argument, there is no help message to add information about the default value to.

The exact output for your scan-time option then becomes:

  --scan-time [SCAN_TIME]                         Wait SCAN-TIME seconds between status checks.                         (default: 5) 

Answer 2

Add '%(default)s' to the help parameter to control what is displayed.

parser.add_argument("--type", default="toto", choices=["toto","titi"],                               help = "type (default: %(default)s)") 

Notes:

• It is %+ default in parenthesis + format characters (not to be confused with curly brackets {default} we find in format or f-string)
• Don't forget to add the "specifier character" for the type representation at the end (i.e. s for strings, d for integers, f for floats, etc.)
• You can also add the usual "printf" format specifiers (like number of digits for floats, leading zeros, etc.)

You can refer to printf documentation for more details.