
Interactive Input Editing and History Substitution
**************************************************

Some versions of the Python interpreter support editing of the current
input line and history substitution, similar to facilities found in
the Korn shell and the GNU Bash shell.  This is implemented using the
GNU Readline library, which supports Emacs-style and vi-style editing.
This library has its own documentation which I won't duplicate here;
however, the basics are easily explained.  The interactive editing and
history described here are optionally available in the Unix and Cygwin
versions of the interpreter.

This chapter does *not* document the editing facilities of Mark
Hammond's PythonWin package or the Tk-based environment, IDLE,
distributed with Python. The command line history recall which
operates within DOS boxes on NT and some other DOS and Windows flavors
is yet another beast.


Line Editing
============

If supported, input line editing is active whenever the interpreter
prints a primary or secondary prompt.  The current line can be edited
using the conventional Emacs control characters.  The most important
of these are: ``C-A`` (Control-A) moves the cursor to the beginning of
the line, ``C-E`` to the end, ``C-B`` moves it one position to the
left, ``C-F`` to the right.  Backspace erases the character to the
left of the cursor, ``C-D`` the character to its right. ``C-K`` kills
(erases) the rest of the line to the right of the cursor, ``C-Y``
yanks back the last killed string. ``C-underscore`` undoes the last
change you made; it can be repeated for cumulative effect.


History Substitution
====================

History substitution works as follows.  All non-empty input lines
issued are saved in a history buffer, and when a new prompt is given
you are positioned on a new line at the bottom of this buffer. ``C-P``
moves one line up (back) in the history buffer, ``C-N`` moves one
down.  Any line in the history buffer can be edited; an asterisk
appears in front of the prompt to mark a line as modified.  Pressing
the ``Return`` key passes the current line to the interpreter.
``C-R`` starts an incremental reverse search; ``C-S`` starts a forward
search.


Key Bindings
============

The key bindings and some other parameters of the Readline library can
be customized by placing commands in an initialization file called
``~/.inputrc``.  Key bindings have the form

   key-name: function-name

or

   "string": function-name

and options can be set with

   set option-name value

For example:

   # I prefer vi-style editing:
   set editing-mode vi

   # Edit using a single line:
   set horizontal-scroll-mode On

   # Rebind some keys:
   Meta-h: backward-kill-word
   "\C-u": universal-argument
   "\C-x\C-r": re-read-init-file

Note that the default binding for ``Tab`` in Python is to insert a
``Tab`` character instead of Readline's default filename completion
function.  If you insist, you can override this by putting

   Tab: complete

in your ``~/.inputrc``.  (Of course, this makes it harder to type
indented continuation lines if you're accustomed to using ``Tab`` for
that purpose.)

Automatic completion of variable and module names is optionally
available.  To enable it in the interpreter's interactive mode, add
the following to your startup file: [1]

   import rlcompleter, readline
   readline.parse_and_bind('tab: complete')

This binds the ``Tab`` key to the completion function, so hitting the
``Tab`` key twice suggests completions; it looks at Python statement
names, the current local variables, and the available module names.
For dotted expressions such as ``string.a``, it will evaluate the
expression up to the final ``'.'`` and then suggest completions from
the attributes of the resulting object.  Note that this may execute
application-defined code if an object with a ``__getattr__()`` method
is part of the expression.

A more capable startup file might look like this example.  Note that
this deletes the names it creates once they are no longer needed; this
is done since the startup file is executed in the same namespace as
the interactive commands, and removing the names avoids creating side
effects in the interactive environment.  You may find it convenient to
keep some of the imported modules, such as ``os``, which turn out to
be needed in most sessions with the interpreter.

   # Add auto-completion and a stored history file of commands to your Python
   # interactive interpreter. Requires Python 2.0+, readline. Autocomplete is
   # bound to the Esc key by default (you can change it - see readline docs).
   #
   # Store the file in ~/.pystartup, and set an environment variable to point
   # to it:  "export PYTHONSTARTUP=~/.pystartup" in bash.

   import atexit
   import os
   import readline
   import rlcompleter

   historyPath = os.path.expanduser("~/.pyhistory")

   def save_history(historyPath=historyPath):
       import readline
       readline.write_history_file(historyPath)

   if os.path.exists(historyPath):
       readline.read_history_file(historyPath)

   atexit.register(save_history)
   del os, atexit, readline, rlcompleter, save_history, historyPath


Alternatives to the Interactive Interpreter
===========================================

This facility is an enormous step forward compared to earlier versions
of the interpreter; however, some wishes are left: It would be nice if
the proper indentation were suggested on continuation lines (the
parser knows if an indent token is required next).  The completion
mechanism might use the interpreter's symbol table.  A command to
check (or even suggest) matching parentheses, quotes, etc., would also
be useful.

One alternative enhanced interactive interpreter that has been around
for quite some time is IPython, which features tab completion, object
exploration and advanced history management.  It can also be
thoroughly customized and embedded into other applications.  Another
similar enhanced interactive environment is bpython.

-[ Footnotes ]-

[1] Python will execute the contents of a file identified by the
    ``PYTHONSTARTUP`` environment variable when you start an
    interactive interpreter.  To customize Python even for non-
    interactive mode, see *The Customization Modules*.
