# 6.7. `readline` — GNU readline interface

The `readline` module defines a number of functions to facilitate completion and reading/writing of history files from the Python interpreter. This module can be used directly or via the `rlcompleter` module. Settings made using this module affect the behaviour of both the interpreter’s interactive prompt and the prompts offered by the built-in `input()` function.

Note

On MacOS X the `readline` module can be implemented using the `libedit` library instead of GNU readline.

The configuration file for `libedit` is different from that of GNU readline. If you programmatically load configuration strings you can check for the text “libedit” in `readline.__doc__` to differentiate between GNU readline and libedit.

The `readline` module defines the following functions:

`readline.``parse_and_bind`(string)

  Parse and execute single line of a readline init file.

`readline.``get_line_buffer`()

  Return the current contents of the line buffer.

`readline.``insert_text`(string)

  Insert text into the command line.

`readline.``read_init_file`([filename])

  Parse a readline initialization file. The default filename is the last filename used.

`readline.``read_history_file`([filename])

  Load a readline history file. The default filename is `~/.history`.

`readline.``write_history_file`([filename])

  Save a readline history file. The default filename is `~/.history`.

`readline.``append_history_file`(nelements[, filename])

  Append the last nelements of history to a file. The default filename is `~/.history`. The file must already exist.

  New in version 3.5.

`readline.``clear_history`()

  Clear the current history. (Note: this function is not available if the installed version of GNU readline doesn’t support it.)

`readline.``get_history_length`()

  Return the desired length of the history file. Negative values imply unlimited history file size.

`readline.``set_history_length`(length)

  Set the number of lines to save in the history file. `write_history_file()` uses this value to truncate the history file when saving. Negative values imply unlimited history file size.

`readline.``get_current_history_length`()

  Return the number of lines currently in the history. (This is different from `get_history_length()`, which returns the maximum number of lines that will be written to a history file.)

`readline.``get_history_item`(index)

  Return the current contents of history item at index.

`readline.``remove_history_item`(pos)

  Remove history item specified by its position from the history.

`readline.``replace_history_item`(pos, line)

  Replace history item specified by its position with the given line.

`readline.``redisplay`()

  Change what’s displayed on the screen to reflect the current contents of the line buffer.

`readline.``set_startup_hook`([function])

  Set or remove the startup_hook function. If function is specified, it will be used as the new startup_hook function; if omitted or `None`, any hook function already installed is removed. The startup_hook function is called with no arguments just before readline prints the first prompt.

`readline.``set_pre_input_hook`([function])

  Set or remove the pre_input_hook function. If function is specified, it will be used as the new pre_input_hook function; if omitted or `None`, any hook function already installed is removed. The pre_input_hook function is called with no arguments after the first prompt has been printed and just before readline starts reading input characters.

`readline.``set_completer`([function])

  Set or remove the completer function. If function is specified, it will be used as the new completer function; if omitted or `None`, any completer function already installed is removed. The completer function is called as `function(text, state)`, for state in `0`, `1`, `2`, ..., until it returns a non-string value. It should return the next possible completion starting with text.

`readline.``get_completer`()

  Get the completer function, or `None` if no completer function has been set.

`readline.``get_completion_type`()

  Get the type of completion being attempted.

`readline.``get_begidx`()

  Get the beginning index of the readline tab-completion scope.

`readline.``get_endidx`()

  Get the ending index of the readline tab-completion scope.

`readline.``set_completer_delims`(string)

  Set the readline word delimiters for tab-completion.

`readline.``get_completer_delims`()

  Get the readline word delimiters for tab-completion.

`readline.``set_completion_display_matches_hook`([function])

  Set or remove the completion display function. If function is specified, it will be used as the new completion display function; if omitted or `None`, any completion display function already installed is removed. The completion display function is called as `function(substitution, [matches], longest_match_length)` once each time matches need to be displayed.

`readline.``add_history`(line)

  Append a line to the history buffer, as if it was the last line typed.

See also

Module `rlcompleter`
  Completion of Python identifiers at the interactive prompt.

## 6.7.1. Example

The following example demonstrates how to use the `readline` module’s history reading and writing functions to automatically load and save a history file named `.python_history` from the user’s home directory. The code below would normally be executed automatically during interactive sessions from the user’s `PYTHONSTARTUP` file.

    import atexit
    import os
    import readline

    histfile = os.path.join(os.path.expanduser("~"), ".python_history")
    try:
        readline.read_history_file(histfile)
    except FileNotFoundError:
        pass

    atexit.register(readline.write_history_file, histfile)

This code is actually automatically run when Python is run in interactive mode (see Readline configuration).

The following example achieves the same goal but supports concurrent interactive sessions, by only appending the new history.

    import atexit
    import os
    import readline
    histfile = os.path.join(os.path.expanduser("~"), ".python_history")

    try:
        readline.read_history_file(histfile)
        h_len = readline.get_history_length()
    except FileNotFoundError:
        open(histfile, 'wb').close()
        h_len = 0

    def save(prev_h_len, histfile):
        new_h_len = readline.get_history_length()
        readline.append_history_file(new_h_len - prev_h_len, histfile)
    atexit.register(save, h_len, histfile)

The following example extends the `code.InteractiveConsole` class to support history save/restore.

    import atexit
    import code
    import os
    import readline

    class HistoryConsole(code.InteractiveConsole):
        def __init__(self, locals=None, filename="<console>",
                     histfile=os.path.expanduser("~/.console-history")):
            code.InteractiveConsole.__init__(self, locals, filename)
            self.init_history(histfile)

        def init_history(self, histfile):
            readline.parse_and_bind("tab: complete")
            if hasattr(readline, "read_history_file"):
                try:
                    readline.read_history_file(histfile)
                except FileNotFoundError:
                    pass
                atexit.register(self.save_history, histfile)

        def save_history(self, histfile):
            readline.write_history_file(histfile)