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)