Defer computing merged_track

[akx]

Follows up on #470.

There's no point in computing merged_track (quite expensively) until it's needed.

However, the comment #470 (comment) should be addressed (separately): it's not clear that merged_track is a cached object, and changes to tracks or their contents will not be reflected automatically.

[rdoursenaud]

Thanks, looks good on a surface level but…

Doesn't this defeat the purpose of pre-computing merged_track which was to avoid the delay before playback?

merged_track is still an utterly broken kludge anyway:

• it doesn't update when a track is modified and becomes out of sync with the object
• it seems to only be used for naive playback purposes where a better data structure or process would alleviate its need

I'm planning on getting rid of it for mido 2.x.

[akx]

Doesn't this defeat the purpose of pre-computing merged_track which was to avoid the delay before playback?

That moved the delay to all instantiations of MidiFile, even if they didn't use merged_track (or iterate over the MidiFile), which is painful for downstream users (and energy bills thereof) of the library who don't need merged_track at all; see YatingMusic/miditoolkit#44 that speeds up that library's test suite by quite a lot. (check_msgdict is still rather slow, but that's a different can of worms.)

[rdoursenaud]

Yeah, it wasn't a smart decision on my part to accept this PR as-is.
I'm just trying to be cautious and aware of the repercussions.

[rdoursenaud]

I believe your approach is the better tradeoff for now.
I'll have to think and find a better approach that fits both use cases for the next release.

[akx]

@rdoursenaud I absolutely know the feeling of trying to be cautious (being the maintainer of babel) 😄

As for 2.0, I think __iter__ should probably be removed altogether (since #537 would necessitate a different API anyway); I think MidiFile.iter_events(all_tracks=True) sounds pretty good at first thought, etc... :)

[rdoursenaud]

I'm new to the maintaining a popular library thing and still learning the ropes.
Thanks for your invaluable insights!
Feel free to stick around ;)

[by-nari]

This is a feature that I really need. I'm trying to remove a track from the event stream when playing but without success. I tried merging the remaining tracks and then playing them, but a small number of MIDI files would fail with this method. The best way is to know which track the event comes from, then remove them right there