Remove MIDI constructor defaults for midi_in and midi_out

[kevinjwalters]

A discussion has come up due to recent coding (in #3) and unintentional highlighting by sphinx-build-isms about the defaults for the midi_in and midi_out named parameters for the MIDI constructor. These two defaults have not changed (#9 does not change these values) and are present in the current library:

    def __init__(self, midi_in=usb_midi.ports[0], midi_out=usb_midi.ports[1], *,
                 in_channel=None, out_channel=0, in_buf_size=30, debug=False):

There's also the question about wasting memory if the user is not using usb_midi as it will always be imported. Is usb_midi frozen in?

@sommersoft has queried what the best practice is here.

This was partly noticed because sphinx-build has started choking on this and that error is picked up by Travis. This appears to be due to a sphinx-build change/issue as this code has not changed since it last passed. Other code may be subject to this bug. Discussion on this: #9 (comment) and #9 (comment)

Reminder:

• fix up any example code to ensure they still work if this is changed,
• update any learn guides in conjunction with changes here, see below.

TODOs:

• Update guide: Grand Central USB MIDI Controller in CircuitPython - this is probably best done in conjuction with updates required by Remove method based API for sending MIDI messages #12

[tannewt]

I don't think these should be the default values. The midi library should be agnostic to the transport.

I think the signature should be:

def __init__(self, *, midi_in=None, midi_out=None, in_channel=None, out_channel=0, in_buf_size=30, debug=False):

That would make midi_in and midi_out optional.

usb_midi is built-in so it won't impact memory use.

[kevinjwalters]

I made the code change to library. I'd suggest leaving this open until it's released and guide(s) are updated?

[kattni]

This appears to have been resolved.