gh-133390: Extend completion for .commands in sqlite3

[StanFromIreland]

@tanloong @encukou

• Issue: Support completion for sqlite3 command-line interface #133390

[tanloong]

Only commands with a dot at the line beginning is considered dot commands. text is the non-whitespace chars under wherever the cursor is at. This means using text.startswith() enables dot command completion for not only actual dot commands but also any inline word that starts with dot:

sqlite> SELECT .<tab>
.help 
.quit 
.version 
sqlite> SELECT .

Using readline.get_line_buffer() instead is more reasonable. This returns the whole current line and can be used to avoid completion for dot-started inline word:

origline = readline.get_line_buffer()
if origline.startswith("."):
    ...

[StanFromIreland]

sqlite3 identifiers cannot start with . (OperationalError (SQLITE_ERROR): near ".": syntax error). Calling readline would complicate our imports a bit more, I'm not sure it is worth it. Not much changes, it is invalid just like it was before.

[tanloong]

I actually also made a local change to add dot commands completion. This is the idea on my local branch, just sharing it for discussion:

1. Complete without dot if there is any number of whitespaces between the dot and the cursor.

sqlite> . |
help
version
quit

2. Complete with the dot if cursor is immediately at the right handside of the dot.

sqlite> .|
.help
.version
.quit

3. Complete nothing if there is any non-whitespace chars between the dot and the cursor (as we currently don't have dot commands that receive arguments).

sqlite> . help |
# no candidates

COMMANDS = ("quit", "help", "version")


def _complete(text, state):
    global _completion_matches

    if state == 0:
        import readline

        origline = readline.get_line_buffer()
        if origline.startswith("."):
            if origline == text:
                text_lstrip = text.lstrip(".")
                _completion_matches = [f".{cmd}" for cmd in COMMANDS
                                       if cmd.startswith(text_lstrip)]
            elif origline.removeprefix(".").lstrip(" ") == text:
                _completion_matches = [cmd for cmd in COMMANDS
                                       if cmd.startswith(text)]
            else:
                _completion_matches.clear()
        else:
            text_upper = text.upper()
            _completion_matches = [c for c in SQLITE_KEYWORDS if c.startswith(text_upper)]
    try:
        return _completion_matches[state] + " "
    except IndexError:
        return None

[encukou]

Thank you!
@tanloong, additional improvements are welcome :)