Doc/library/readline.rst

:mod:`readline` --- GNU readline interface
==========================================

.. module:: readline
   :platform: Unix
   :synopsis: GNU readline support for Python.
.. sectionauthor:: Skip Montanaro <skip@pobox.com>


The :mod:`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 :mod:`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 built-in :func:`input`
function.

.. note::

  The underlying Readline library API may be implemented by
  the ``libedit`` library instead of GNU readline.
  On MacOS X the :mod:`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 :const:`readline.__doc__`
  to differentiate between GNU readline and libedit.


Init file
---------

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


.. function:: parse_and_bind(string)

   Execute the init line provided in the *string* argument. This calls
   :c:func:`rl_parse_and_bind` in the underlying library.


.. function:: read_init_file([filename])

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


Line buffer
-----------

The following functions operate on the line buffer:


.. function:: get_line_buffer()

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


.. function:: insert_text(string)

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


.. function:: redisplay()

   Change what's displayed on the screen to reflect the current contents of the
   line buffer.  This calls :c:func:`rl_redisplay` in the underlying library.


History file
------------

The following functions operate on a history file:


.. function:: read_history_file([filename])

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


.. function:: write_history_file([filename])

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


.. function:: append_history_file(nelements[, filename])

   Append the last *nelements* items of history to a file.  The default filename is
   :file:`~/.history`.  The file must already exist.  This calls
   :c:func:`append_history` in the underlying library.

   .. versionadded:: 3.5


.. function:: get_history_length()
              set_history_length(length)

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


History list
------------

The following functions operate on a global history list:


.. function:: clear_history()

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


.. function:: get_current_history_length()

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


.. function:: get_history_item(index)

   Return the current contents of history item at *index*.  The item index
   is one-based.  This calls :c:func:`history_get` in the underlying library.


.. function:: remove_history_item(pos)

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


.. function:: replace_history_item(pos, line)

   Replace history item specified by its position with *line*.
   The position is zero-based.  This calls :c:func:`replace_history_entry`
   in the underlying library.


.. function:: add_history(line)

   Append *line* to the history buffer, as if it was the last line typed.
   This calls :c:func:`add_history` in the underlying library.


Startup hooks
-------------


.. function:: set_startup_hook([function])

   Set or remove the function invoked by the :c:data:`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.


.. function:: set_pre_input_hook([function])

   Set or remove the function invoked by the :c:data:`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.


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 :mod:`rlcompleter` to complete Python identifiers for
the interactive interpreter.  If the :mod:`readline` module is to be used
with a custom completer, a different set of word delimiters should be set.


.. function:: 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 :c:func:`rl_completion_matches` in the underlying library.
   The *text* string comes from the first parameter to the
   :c:data:`rl_attempted_completion_function` callback of the
   underlying library.


.. function:: get_completer()

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


.. function:: get_completion_type()

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


.. function:: get_begidx()
              get_endidx()

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


.. function:: set_completer_delims(string)
              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 :c:data:`rl_completer_word_break_characters`
   variable in the underlying library.


.. function:: 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
   :c:data:`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.


.. _readline-example:

Example
-------

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

   import atexit
   import os
   import readline

   histfile = os.path.join(os.path.expanduser("~"), ".python_history")
   try:
       readline.read_history_file(histfile)
       # default history len is -1 (infinite), which may grow unruly
       readline.set_history_length(1000)
   except FileNotFoundError:
       pass

   atexit.register(readline.write_history_file, histfile)

This code is actually automatically run when Python is run in
:ref:`interactive mode <tut-interactive>` (see :ref:`rlcompleter-config`).

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.set_history_length(1000)
       readline.append_history_file(new_h_len - prev_h_len, histfile)
   atexit.register(save, h_len, histfile)

The following example extends the :class:`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.set_history_length(1000)
           readline.write_history_file(histfile)