13. 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.
13.1. 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,
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,
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
13.2. 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
C-R starts an incremental reverse search;
a forward search.
13.3. 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
and options can be set with
set option-name value
# 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
character instead of Readline’s default filename completion function. If you
insist, you can override this by putting
~/.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: 
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
'.' 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,
os, which turn out to be needed in most sessions with the
# 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
13.4. 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.
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