Type mapping

Some loose thoughts on bridging the “shell is strings, Python wants lists/dicts/integers/bools/etc” problem.


  • Explicit mapping, as with argparse: this particular flag turns into a list/boolean/int/whatever. Because we’re specifically mapping to function keyword arguments, a little of that complexity can be removed, but generally it’ll look very similar. E.g.:

    def mytask(foo):

    would turn this:

    $ invoke mytask --foo 7

    into 7, not "7".

  • Introspection-based mapping, i.e. introspecting the default values of a function signature and automatically transforming the CLI input. E.g.:

    def mytask(foo=5):

    invoked as:

    $ invoke mytask --foo 7

    results in the Python value 7 instead of "7", just as with the explicit example above.

  • Formatting-based mapping, i.e. having (optional) conventions in the string format of an incoming flag argument that cause transformations to occur. E.g. we could say that commas in an argument automatically trigger transformation into a list of strings; thus the invocation:

    $ invoke mytask --items a,b,c

    would on the Python end turn into a call like this:

    mytask(items=['a', 'b', 'c'])

What to do?

We haven’t decided exactly how many of these to use – we may end up using all three of them as appropriate, with some useful/sensible default and the option to enable/disable things for power users. The trick is to balance power/features with becoming overly complicated to understand or utilize.

Other types

Those examples cover integers/numbers, and lists/iterables. Strings are obviously easy/the default. What else is there?

  • Booleans: these are relatively simple too, either a flag exists (True) or is omitted (False).

    • Could also work in a --foo vs --no-foo convention to help with the inverse, i.e. values which should default to True and then need to be turned “off” on the command line. E.g.:

      def mytask(option=True):

      could result in having a flag called --no-option instead of --option. (Or possibly both.)

  • Dicts: these are tougher, but we could potentially use something like:

    $ invoke mytask --dictopt key1=val1,key2=val2

    resulting in:

    mytask(dictopt={'key1': 'val1', 'key2': 'val2'})

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