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Appendix A. Interactive Input Editing and History Substitution

COMMENT: Original Source

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. On Unix systems, 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 described below.

Line Editing on Unix Systems

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:A.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=/max/home/itamar/.pystartup" in bash.
#
# Note that PYTHONSTARTUP does *not* expand "~", so you have to put in the
# full path to your home directory.

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

Line Editing on Windows

On Windows, you can use doskey-style line editing. Use the arrow keys to move around on the line, and to bring back commands from the history buffer. The Escape key clears the current line, F7 displays the history buffer, and F8 can be used to search in the buffer.

IPython

While the standard interactive interpreter can do a lot of nice things, there’s a great replacement available, IPython.

Some of the key IPython features include:

  • Auto-complete (without having to muck about with PYTHONSTARTUP)
  • Fantastic introspection capabilities
  • Magic commands, including commands for mixing shell access with Python scripting
  • Jump-to-editor availability
  • Built-in macro system
  • Integration with matplotlib, and other mathematics/scientific libraries
  • Lots more!

The webpage has plenty of documentation and other feature listings, and it is highly recommended you check it out.


Footnotes

… file:A.1 Python will execute the contents of a file identified by the PYTHONSTARTUP environment variable when you start an interactive interpreter.