Allow a type to customize the prompt

As of Click 7, click.Choice appears only to support choice prompting by string, i.e. the user must specify the exact string (modulo case sensitivity) from the choices. It would be nice if Click could support two additional modes of selection:

Numeric:

Choose from:
[1] Choice A...
[2] Choice B...
[3] Choice C...
Enter the index for one of the above choices: 2
"Choice B..." was selected

or partial matches as long as what the user enters is unique:

Choose from:
This thing
That thing
Those things
Enter choice: thi
"This thing" was selected

(“thi” is unique amongst the three strings so is chosen)

I thought it would be easy to implement this by subclassing click.Choice but unfortunately not: click.Choice does not provide the initial prompt message (only the prompt after an invalid choice) - this is provided by click.prompt from the specified choices list. The prompt message in at least the first example above would require to be changed to add numeric indexes. It seems to me therefore that this functionality requires more structural changes to click and so I thought I’d submit this issue.

[StephenCarboni]

Selecting the choice would be much nicer. Like this: https://github.com/Mckinsey666/bullet#bullet-lists-and-checkboxes

[SeanDS]

I implemented numeric choices as a subclass of click.Choice:

class NumericChoice(click.Choice):
    def __init__(self, choices, **kwargs):
        choicepairs = []
        choicestrs = []
        for i, choice in enumerate(choices, start=1):
            choicepairs.append((str(i), choice))
            choicestrs.append("[{}] {}".format(i, choice))
        self.choicemap = dict(choicepairs)
        super().__init__(choicestrs, **kwargs)

    def convert(self, value, param, ctx):
        try:
            return self.choicemap[value]
        except KeyError as e:
            self.fail('invalid choice: %s. (choose from %s)' %
                      (value, ', '.join(self.choices)), param, ctx)

It works for me, but I’m targeting Python 3.6+ only. If this might be useful I can clean it up and make a pull request at some point. It might be best to add this as an option to the existing click.Choice, though.

For partial string matching, perhaps difflib would be useful. I haven’t thought how to do that yet.