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How it works...

As described in step 1 in the How to do it… section, the arguments are added to the parser through .add_arguments. Once all arguments are defined, calling parse_args() returns an object that contains the results (or exits if there's an error).

Each argument should add a help description, but their behavior can change greatly:

If an argument starts with a -, it is considered an optional parameter, like the -c argument in step 4. If not, it's a positional argument, like the number argument in step 1.

For clarity, always define a default value for optional parameters. It will be None if you don't, but this may be confusing.

Remember to always add a help parameter with a description of the parameter; help is automatically generated, as shown in step 2.
If a type is present, it will be validated, for example, number in step 3. By default, the type will be string.
The actions store_true and store_false can be used to generate flags, arguments that don't require any extra parameters. Set the corresponding default value as the opposite Boolean. This is demonstrated in the U argument in steps 6 and 7.
The name of the property in the args object will be, by default, the name of the argument (without the dash, if it's present). You can change it with dest. For example, in step 6, the command-line argument -U is described as uppercase.

Changing the name of an argument for internal usage is very useful when using short arguments, such as single letters. A good command-line interface will use -c, but internally it's probably a good idea to use a more verbose label, such as configuration_file. Explicit is better than implicit!

Some arguments can work in coordination with others, as shown in step 3. Perform all required operations to pass the main function as clear and concise parameters. For example, in step 3, only two parameters are passed, but one may have been modified.