Implement bash auto completion in Python

Splitting words

The Python script splits the shell command's full line to words using shlex. I see a few issues with this:

1. I'm not sure this will split the line exactly the same way as the shell would. Looking at help(shlex), I see "A lexical analyzer class for simple shell-like syntaxes", and I find that not very reassuring.

2. I think command line completion should be blazingly fast, so I look suspiciously at anything that needs to be import-ed, such as shlex.

3. Looking at the Programmable Completion section in man bash, it seems that Bash populates the COMP_WORDS array with the result of the split.

Therefore, it would be good to pass COMP_WORDS to the Python script, which would eliminate all the above concerns.

One way to achieve this would be to call the Python script with:

python3 script.py complete "$COMP_CWORD" "${COMP_WORDS[@]}"

And then change the Python script accordingly:

#!/usr/bin/env python3

import sys


def complete(comp_cword, *comp_words):
    prefixes = ("test", "test-中文", "test-한글")
    word = comp_words[int(comp_cword)]
    items = (prefix + "-level-" + comp_cword for prefix in prefixes)
    return (item for item in items if item.startswith(word))


if __name__ == "__main__":
    sys.stdout.write(" ".join(complete(*sys.argv[2:])))
    sys.stdout.flush()

Avoid converting generators to list when not needed

No lists were needed in the original script, everything could have been just generator expressions.

Keep it simple

I don't understand why the script imports os and uses os.sys and nothing else in os. You could just import sys instead.

I don't understand why sys.argv.__getitem__(slice(2, None)) was used instead of the simple and natural sys.argv[2:].

This line is complex, it takes attention to understand:

word = "" if len(split) - 1 < int(current_word) else split[int(current_word)]

This is a lot easier to understand:

if len(split) - 1 < int(current_word):
    word = ""
else:
    word = split[int(current_word)]

Looking further, word is used only in a filter .startswith(word). That filter will match every string. In which case, to maximize performance, it would be best to not create word, and not do any filtering, but return items directly:

if len(split) - 1 < int(current_word):
    return items

On even closer look, I don't see how COMP_CWORD can ever be an index out of range. So the check for bounds was unnecessary. (Strictly speaking, an index out of bounds may be possible when splitting words with shlex, since that might not be identical to the shell's own word splitting. Even then, it would be a highly unlikely case, therefore a more Pythonic way to handle the situation would be using a try-expect for a IndexError.)

Use better names

The name i is really best reserved for loop counters. (Even then, often you may find better names...)

In the suggested solution above I renamed the parameter names to match the shell variables they come from. I find this reduces the cognitive burden when reading the documentation of the variables in man bash, and the implementation of the completion code in Python.