Parsing command-line arguments is common in programming. In Bash, one can use getopts. In Perl, one can use the module Getopt::Long. In Python, we can use the package argparse.
The major steps of using the package include 3 steps:
- Create a parser.
- Add arguments.
- Parse the arguments.
Here I will describe each step in details, particularly the options of the functions.
Create a parser
Below is a simple example of creating a parser object.
parser = argparse.ArgumentParser(description="what the program does")In addition to ‘description’, the other major options for argparse.ArgumentParser() are:
| option | example value | meaning |
|---|---|---|
| prog | “myapp.py” | the name of the program, default is sys.argv[0] |
| usage | “usage” | a string describing one-line usage, default is generated from added arguments (see below) |
| epilog | “text after argument help” | text after help output |
| parents | [p1, p2] | a list of ArgumentParser objects whose arguments will also be added |
| prefix_chars | ‘-+’ | the characters prefix optional arguments, default is ‘-’ |
By default, the values for ‘description’ and ‘epilog’ are line-wrapped, so newlines are removed when formatted. This can be changed by assigning ‘RawDescriptionHelpFormatter’ or ‘RawTextHelpFormatter’ to the option ‘formatter_class’, which will show the description string as assigned. The difference between the two values is that the latter will replaces multiple concatenated newlines with one.
Add arguments
With the created parser, one can add arguments, like
parser.add_argument("pos_arg", type=str, help="positional arg")
parser.add_argument("--optional", type=int, default=5, help="optional argument")
...The major options for the function add_argument are:
| option | example value | meaning |
|---|---|---|
| type | int | the type of returned value, default is string |
| help | “help message” | explanation of the argument |
| action | ‘store’ | how to process the option |
| default | 100 | the default value if not provided |
| dest | ‘myvar’ | the attribute name used for extracting the value later |
| required | False | whether the option is required |
| nargs | 3 | the number of values expected for an argument, can also be ‘?’, ‘+’, ’*’ |
When providing options, one can abbreviate the options, as long as the prefix matching returns a unique option.
In default, the parser groups positional and optional arguments separately when displaying them. One can group the arguments manually by creating argument groups, like:
group = parser.add_argument_group("group_name", "group_desc")
group_add_argument('--foo', help="foo help")
group_add_argument('bar', help="bar help")One can use parser.print_help() to show what the help message will look like eventually.
Parse arguments
With the parser set up, we can parse the arguments as follows:
args=parser.parse_args();The above call creates an object with arguments stored in attributes. For example, to access the argument ‘pos_arg’, one can use args.pos_arg, similarly, args.optional for the optional argument.
Advantages
The package will generate usage output automatically when user runs the program without any arguments or with the option ‘-h’ or ‘–help’.
References
argparse manual, https://docs.python.org/3.3/library/argparse.html
a tutorial, https://docs.python.org/3.3/howto/argparse.html
Last modified on 2018-08-16