Python Prompt Toolkit impressions

30 Sep 2014

Last night I discovered Jonathan Slenders' Python Terminal Toolkit: a fancy interface to the Python interpreter very much like bpython-curtsies, the project I’ve been working on on and off for a year. I was simultaneously very excited to see this work and a little depressed - on first glance many aspects of the project seem better than bpython, and appears to include a terminal wrapper at least as nice as my curtsies library. I think there’s potential for code reuse though, and it would be really fun to talk to Jonathan about the project. If the toolkit turns out to be flexible enough and we can add the necessary features, it might make for a great frontend for bpython.

Jonathan says Python Prompt Toolkit isn’t done, and I don’t mean this as a final product review. I just wanted to get out my reaction to it right now. I haven’t even looked at the code yet, so I’m just talking about functionality. These are my raw, unedited thoughts mostly so I can take stock of them.

Things I love

  • vi and emacs readline keybindings - tos9 wants to add vi keybindings to bpython, but what Jonathan is written is more powerful than I had imagined.
  • Multiline command editing done well!
  • Doesn’t execute on syntax error
  • History preservation + fancy terminal graphics; this is what bpython-curtsies was originally all about.
  • The ability to interface with IPython suggests an external Python process being communicated with, which I think is a great idea.
  • Asyncronous autocompletion - I’ve wanted to make bpython’s autocompletion asyncronous for a long time! This is terrific.
  • Using jedi for autocompletion instead of inspecting objects ourselves - interesting! This gives completion within unexecuted code, and additionally is sometimes better autocompletion than bpython
  • reverse incremental search w/ highlighting - this looks just like what I implemented a month ago. It’s great!
  • Using the normal shell for raw_input - smart! Bpython has to be its own shell to manage linewrapping of the input, but Python Terminal Toolkit appears not to care - and this works just fine! I think it might be an issue with fancier resize coping techniques.
  • Support for wide terminal characters! bpython “support” doesn’t work once a line wraps, which is why it can hardly be called support

Things I don’t love

(all of which are perfectly changeable or matters of opinion)

  • Docstrings not displayed, and function parameters are less prominent than bpython
  • IPython style prompt (useful for IPython history magicks, but ugly. Same with spacing vs the vanilla Python prompt
  • Doesn’t suspend/background (neither does bpython-curtsies right now, I’m interested to see how this gets handled.)
  • I don’t like the autocompletion colorscheme (wow really nitpicking now - can you tell I’m a little sad I didn’t write this myself? :)
  • Autocomplete doesn’t feel as snappy - not sure if this is because it’s an external process and we’re doing IPC (haven’t looked at code yet) or if it’s display code because lines aren’t being cached, but I’ve been worried about this happening if we move autocompletion to an external process. I think it’s worth it, but it feels different.
  • autocomplete doesn’t re-complete on delete - is this a Jedi thing?
  • Literal completion - I wrote my own completion system instead of using Jedi, and at rare times it feels nicer, times like 10.0.
  • Decreasing the window width wrecks havok with history. This is an issue with bpython-curtsies as well that I want to address, and I’m not yet sure that it’s worth the hassle. I’m working on terminal emulator testing tools so I can try different wrap behaviors, and have written some things to predict how different terminals will resize the screen to better compensate for this effect.
  • No obvious way to go back whole commands vs lines - the Julia repl uses ctrl-p/n to go back multiline history entries, arrows to move up and down lines - and ctrl-j to run commands, ctrl-m to go down a line. (The Julia REPL and an OCaml REPL called utop should both be checked out for inspiration if you’re writing a REPL.)

Features in bpython not currently in ptpython

Only because the author asked - I don’t think all these things are necessary, and a while ago I mentioned how nice it might be to break compatibility.

  • Rewind - I personally love this feature. There’s some history rewriting involved, but I think it’s probably worth it.
  • Send session to editor - same as rewind.
  • Reload modules and rerun session - clears modules out, in the future will hopefully use a new Python process. Same infrastructure as rewind.
  • Autoreload - same as above plus file watching. Meh.
  • I’m using blessings instead of hardcoding vt100 escape sequences - and I don’t know what the distribution of terminal is, so I don’t know how much compatibility this buys me. I bet not much.
  • No windows support - this is something we got “for free” by using curses (with a ton of hard work from Brandon Navra), but in bpython-curtsies it’s not there. If it were me I’d leave this for later too.
  • Autocompletion above the cursor - I wouldn’t worry about this, and we have it off by default in bpython-curtsies - but it can be nice for preserving your view of history interspersed with reading long docstrings
  • Fish-style right arrow completion. I’m mixed, sometimes I love it, sometimes it feels pretty silly. Same with arrow keys doing reverse incremental search -ish things with the current line.

A stated goal of Python Prompt Toolkit is to be merged into IPython. I think this is a worthy goal, but last time I talked to Fernando Perez about this he was saying they didn’t want something that didn’t work on Windows in the general IPython codebase. That doesn’t mean that if everyone was crying out for this it wouldn’t be added, but something to be aware of (and perhaps a reason to use a terminal compatibility layer like blessings if they care about that too).