Baffled by readline module

[Eryk Sun]

On 3/9/23, Greg Ewing via Python-list <python-list at python.org> wrote:
> On 10/03/23 4:00 pm, 2QdxY4RzWzUUiLuE at potatochowder.com wrote:
>> My ~/.pythonrc contains the following:
>>
>>      import readline
>>      import rlcompleter
>>      readline.parse_and_bind( 'tab: complete' )
>
> I don't have a ~/.pythonrc, so that's not what's doing it
> for me.

If it's available, the readline and rlcompleter modules are imported
if stdin is a tty, [I]solated mode isn't enabled, and either [i]nspect
is enabled or the REPL is being run. For example:

    $ python -c "import sys;print('readline' in sys.modules)"
    False

    $ python -ic "import sys;print('readline' in sys.modules)"
    True
    >>>

    $ python -ic "import sys;print('readline' in sys.modules)" 0</dev/null
    False
    >>>

    $ python -Iic "import sys;print('readline' in sys.modules)"
    False
    >>>

This is determined by the following function in "Modules/main.c:"

    pymain_import_readline(const PyConfig *config)
    {
        if (config->isolated) {
            return;
        }
        if (!config->inspect && config_run_code(config)) {
            return;
        }
        if (!isatty(fileno(stdin))) {
            return;
        }

        PyObject *mod = PyImport_ImportModule("readline");
        if (mod == NULL) {
            PyErr_Clear();
        }
        else {
            Py_DECREF(mod);
        }
        mod = PyImport_ImportModule("rlcompleter");
        if (mod == NULL) {
            PyErr_Clear();
        }
        else {
            Py_DECREF(mod);
        }
    }

The hook for input() is handled by PyOS_Readline() in
"Parser/myreadline.c". It depends on the exported function pointer
PyOS_ReadlineFunctionPointer. When imported, the readline extension
module in "Modules/readline.c" sets PyOS_ReadlineFunctionPointer to
its call_readline() function.

---

On Windows, Python is not distributed with a readline module. Instead,
Python relies on the console host's builtin support for command-line
editing, history, and aliases.

The Windows console API function ReadConsoleW() has a parameter that
supports implementing custom command-line completion. This is how the
CMD shell implements tab completion for file paths. However, no one
has developed a version of rlcompleter for the Windows console.
Instead, there's a third-party "pyreadline3" package (note the "3";
don't install "pyreadline" by mistake) that implements readline for
the Windows console, which supports rlcompleter.