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, 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 "input()" function.

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.


6.7.1. 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.


6.7.2. 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.


6.7.3. 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.append_history_file(nelements[, filename])

   Append the last *nelements* items of history to a file.  The
   default filename is "~/.history".  The file must already exist.
   This calls "append_history()" in the underlying library.  This
   function only exists if Python was compiled for a version of the
   library that supports it.

   New in version 3.5.

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.


6.7.4. 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.

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.)

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.

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.

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.

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.

readline.set_auto_history(enabled)

   Enable or disable automatic calls to "add_history()" when reading
   input via readline.  The *enabled* argument should be a Boolean
   value that when true, enables auto history, and that when false,
   disables auto history.

   New in version 3.6.

   **CPython implementation detail:** Auto history is enabled by
   default, and changes to this do not persist across multiple
   sessions.


6.7.5. Startup hooks
====================

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.


6.7.6. 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.

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.

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.


6.7.7. 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 ".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)
       # 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
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_current_history_length()
   except FileNotFoundError:
       open(histfile, 'wb').close()
       h_len = 0

   def save(prev_h_len, histfile):
       new_h_len = readline.get_current_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 "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)
