Get tab completion in custom comint mode

Question

I usually try to start CLI's (might not be defining it properly) using commands like:

(make-comint-in-buffer "PUTTY" nil "./plink.exe" nil "dbserver")
(make-comint-in-buffer "Python" nil "c:/Python26/python.exe" nil "-i")
(make-comint-in-buffer "Python" nil "c:/Python27/python.exe" nil "-i")
(make-comint-in-buffer "git" nil "C:/Program Files (x86)/Git/bin/sh.exe" nil "--login" "-i")

Starting the above specially the first and the last in command prompt provides the facility of tab completion. But in comint-mode I don't seem to get this feature.

Entering tab adds a simple tab character under the point instead of passing it to the process and getting back the completion candidates. comint-dynamic-complete and C-qtab doesn't produce the desired result.

How can I utilize in above said feature in comint-mode running different processes?

Answer 1

Short answer

You need to write your own completion functions inside Emacs. These may pull info in from the external program, but you can't have direct access to the tab-completion provided by python or git etc.

Long answer:

When you run make-comint-in-buffer, you are setting up the most basic setup for interactively sending and receiving data from an external process. Whenever you press enter, Emacs sends the current line to the underlying process, and then prints back the output from the process. The external process doesn't know what you're doing until Emacs sends a full line to it - no single characters are passed directly.

This is different from running the external program directly in a terminal. When you run python in a terminal, it interprets every character as you type it (or at least it can). So when you hit tab it knows to try to complete something. When you run python inside Emacs, Emacs is processing all of your input until you hit enter. Which means when you hit tab, Emacs just inserts a tab character, and python doesn't know to try and complete something.

Emacs can provide tab-completion for comint modes, but you need to build the completion targets on the Emacs side, you can't rely on the native completion of the external program. To get an idea of how this works, take a look at the completion functions in shell.el, which you can open with M-x find-library shell.

This isn't something you can do with a line or two of elisp. It's an important part of major-modes written to interact with external processes - things like ESS for R http://ess.r-project.org/

That said, if you dig into the guts of shell.el you may find enough code you can modify to get you started.