# 16.8. `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, which supports completion of Python identifiers at the interactive prompt. Settings made using this module affect the behaviour of both the interpreter’s interactive prompt and the prompts offered by the `raw_input()` and `input()` built-in functions.

Note

The underlying Readline library API may be implemented by the `libedit` library instead of GNU readline. On MacOS X the `readline` module detects which library is being used at run time.

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.

Readline keybindings may be configured via an initialization file, typically `.inputrc` in your home directory. See Readline Init File in the GNU Readline manual for information about the format and allowable constructs of that file, and the capabilities of the Readline library in general.

## 16.8.1. Init file

The following functions relate to the init file and user configuration:

`readline.``parse_and_bind`(string)

  Execute the init line provided in the string argument. This calls `rl_parse_and_bind()` in the underlying library.

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

  Execute a readline initialization file. The default filename is the last filename used. This calls `rl_read_init_file()` in the underlying library.

## 16.8.2. Line buffer

The following functions operate on the line buffer:

`readline.``get_line_buffer`()

  Return the current contents of the line buffer (`rl_line_buffer` in the underlying library).

`readline.``insert_text`(string)

  Insert text into the line buffer at the cursor position. This calls `rl_insert_text()` in the underlying library, but ignores the return value.

`readline.``redisplay`()

  Change what’s displayed on the screen to reflect the current contents of the line buffer. This calls `rl_redisplay()` in the underlying library.

## 16.8.3. History file

The following functions operate on a history file:

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

  Load a readline history file, and append it to the history list. The default filename is `~/.history`. This calls `read_history()` in the underlying library.

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

  Save the history list to a readline history file, overwriting any existing file. The default filename is `~/.history`. This calls `write_history()` in the underlying library.

`readline.``get_history_length`()
`readline.``set_history_length`(length)

  Set or return the desired number of lines to save in the history file. The `write_history_file()` function uses this value to truncate the history file, by calling `history_truncate_file()` in the underlying library. Negative values imply unlimited history file size.

## 16.8.4. History list

The following functions operate on a global history list:

`readline.``clear_history`()

  Clear the current history. This calls `clear_history()` in the underlying library. The Python function only exists if Python was compiled for a version of the library that supports it.

  New in version 2.4.

`readline.``get_current_history_length`()

  Return the number of items 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.)

  New in version 2.3.

`readline.``get_history_item`(index)

  Return the current contents of history item at index. The item index is one-based. This calls `history_get()` in the underlying library.

  New in version 2.3.

`readline.``remove_history_item`(pos)

  Remove history item specified by its position from the history. The position is zero-based. This calls `remove_history()` in the underlying library.

  New in version 2.4.

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

  Replace history item specified by its position with line. The position is zero-based. This calls `replace_history_entry()` in the underlying library.

  New in version 2.4.

`readline.``add_history`(line)

  Append line to the history buffer, as if it was the last line typed. This calls `add_history()` in the underlying library.

## 16.8.5. Startup hooks

> New in version 2.3.

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

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

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

  Set or remove the function invoked by the `rl_pre_input_hook` callback of the underlying library. If function is specified, it will be used as the new hook function; if omitted or `None`, any function already installed is removed. The hook is called with no arguments after the first prompt has been printed and just before readline starts reading input characters. This function only exists if Python was compiled for a version of the library that supports it.

## 16.8.6. Completion

The following functions relate to implementing a custom word completion function. This is typically operated by the Tab key, and can suggest and automatically complete a word being typed. By default, Readline is set up to be used by `rlcompleter` to complete Python identifiers for the interactive interpreter. If the `readline` module is to be used with a custom completer, a different set of word delimiters should be set.

`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.

  The installed completer function is invoked by the entry_func callback passed to `rl_completion_matches()` in the underlying library. The text string comes from the first parameter to the `rl_attempted_completion_function` callback of the underlying library.

`readline.``get_completer`()

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

  New in version 2.3.

`readline.``get_completion_type`()

  Get the type of completion being attempted. This returns the `rl_completion_type` variable in the underlying library as an integer.

  New in version 2.6.

`readline.``get_begidx`()
`readline.``get_endidx`()

  Get the beginning or ending index of the completion scope. These indexes are the start and end arguments passed to the `rl_attempted_completion_function` callback of the underlying library.

`readline.``set_completer_delims`(string)
`readline.``get_completer_delims`()

  Set or get the word delimiters for completion. These determine the start of the word to be considered for completion (the completion scope). These functions access the `rl_completer_word_break_characters` variable in the underlying library.

`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. This sets or clears the `rl_completion_display_matches_hook` callback in the underlying library. The completion display function is called as `function(substitution, [matches], longest_match_length)` once each time matches need to be displayed.

  New in version 2.6.

## 16.8.7. 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 `.pyhist` from the user’s home directory. The code below would normally be executed automatically during interactive sessions from the user’s `PYTHONSTARTUP` file.

    import os
    import readline
    histfile = os.path.join(os.path.expanduser("~"), ".pyhist")
    try:
        readline.read_history_file(histfile)
        # default history len is -1 (infinite), which may grow unruly
        readline.set_history_length(1000)
    except IOError:
        pass
    import atexit
    atexit.register(readline.write_history_file, histfile)
    del os, histfile

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

    import code
    import readline
    import atexit
    import os

    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 IOError:
                    pass
                atexit.register(self.save_history, histfile)

        def save_history(self, histfile):
            readline.set_history_length(1000)
            readline.write_history_file(histfile)