How Do I Pass a List as an Argument to a Command-Line Program Using argparse?

Passing Lists as Arguments in argparse

When attempting to pass a list as an argument to a command-line program using argparse, it's essential to understand the available options for representing lists within the parser.

Not Recommended: Using type=list

Avoid using type=list with argparse as it can lead to incorrect results. It will return a list of lists, not a single list containing the desired elements.

Use nargs for Required Arguments

The nargs parameter allows you to specify the number of arguments an option accepts. To pass a list as a required argument, use:

<code class="python">parser.add_argument('-l', '--list', nargs='+', help='Set flag', required=True)</code>

Use action='append' for Optional Arguments

For optional arguments, use action='append' to allow providing multiple instances of the argument.

<code class="python">parser.add_argument('-l', '--list', action='append', help='Set flag')</code>

Syntax for Invoking These Options

For nargs, the arguments should be provided together without spaces, such as:

<code class="bash">python test.py -l 12345678</code>

For action='append', the argument should be provided multiple times, such as:

<code class="bash">python test.py -l 1234 -l 5678</code>

Additional Considerations

• Don't use quotes when passing arguments to argparse on the command line.
• If you want the list elements to be type-converted (e.g., to integers), use type=int within the nargs setting.
• nargs with or * allows 1 or more arguments, respectively, and ? allows 0 or 1 arguments. For a specific number of arguments, provide an integer to nargs.

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