[issue26394] Have argparse provide ability to require a fallback value be present

[Michael Herold]

Michael Herold added the comment:

Thank you for clarifying my request. Callables sound like a great solution to me!

Looks like the fallback should be called in `take_action()` if argument_values is "empty" (None?). Per default it should be transparent to an `Action` if the values are based on a fallback call. However, it might be useful to call `Action` with two additional parameters like `commandline_values` and `fallback_values` such that a (custom) action 'append_with_fallback' can be realized. Calling the fallback each time (required for `fallback_values`) makes it some kind of early called hook for argparse.

I am attaching a sketch of what might be a typical use case. I noted that a callable supplied to fallback would usually rely on informations that could be provided as `action.long_option_names` and `action.is_positional` to avoid that this logic is implemented outside of argparse again.

Please let me know I can provide any help.

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Added file: http://bugs.python.org/file41984/fallback.py

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<http://bugs.python.org/issue26394>
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