Doc/howto/curses.tex

\documentclass{howto}

\title{Curses Programming with Python}

\release{2.01}

\author{A.M. Kuchling, Eric S. Raymond}
\authoraddress{\email{amk@amk.ca}, \email{esr@thyrsus.com}}

\begin{document}
\maketitle

\begin{abstract}
\noindent
This document describes how to write text-mode programs with Python 2.x,
using the \module{curses} extension module to control the display.   

This document is available from the Python HOWTO page at
\url{http://www.python.org/doc/howto}.
\end{abstract}

\tableofcontents

\section{What is curses?}

The curses library supplies a terminal-independent screen-painting and
keyboard-handling facility for text-based terminals; such terminals
include VT100s, the Linux console, and the simulated terminal provided
by X11 programs such as xterm and rxvt.  Display terminals support
various control codes to perform common operations such as moving the
cursor, scrolling the screen, and erasing areas.  Different terminals
use widely differing codes, and often have their own minor quirks.

In a world of X displays, one might ask ``why bother''?  It's true
that character-cell display terminals are an obsolete technology, but
there are niches in which being able to do fancy things with them are
still valuable.  One is on small-footprint or embedded Unixes that 
don't carry an X server.  Another is for tools like OS installers
and kernel configurators that may have to run before X is available.

The curses library hides all the details of different terminals, and
provides the programmer with an abstraction of a display, containing
multiple non-overlapping windows.  The contents of a window can be
changed in various ways--adding text, erasing it, changing its
appearance--and the curses library will automagically figure out what
control codes need to be sent to the terminal to produce the right
output.

The curses library was originally written for BSD Unix; the later System V
versions of Unix from AT\&T added many enhancements and new functions.
BSD curses is no longer maintained, having been replaced by ncurses,
which is an open-source implementation of the AT\&T interface.  If you're
using an open-source Unix such as Linux or FreeBSD, your system almost
certainly uses ncurses.  Since most current commercial Unix versions
are based on System V code, all the functions described here will
probably be available.  The older versions of curses carried by some
proprietary Unixes may not support everything, though.

No one has made a Windows port of the curses module.  On a Windows
platform, try the Console module written by Fredrik Lundh.  The
Console module provides cursor-addressable text output, plus full
support for mouse and keyboard input, and is available from
\url{http://effbot.org/efflib/console}.

\subsection{The Python curses module}

Thy Python module is a fairly simple wrapper over the C functions
provided by curses; if you're already familiar with curses programming
in C, it's really easy to transfer that knowledge to Python.  The
biggest difference is that the Python interface makes things simpler,
by merging different C functions such as \function{addstr},
\function{mvaddstr}, \function{mvwaddstr}, into a single
\method{addstr()} method.  You'll see this covered in more detail
later.

This HOWTO is simply an introduction to writing text-mode programs
with curses and Python. It doesn't attempt to be a complete guide to
the curses API; for that, see the Python library guide's serction on
ncurses, and the C manual pages for ncurses.  It will, however, give
you the basic ideas.

\section{Starting and ending a curses application}

Before doing anything, curses must be initialized.  This is done by
calling the \function{initscr()} function, which will determine the
terminal type, send any required setup codes to the terminal, and
create various internal data structures.  If successful,
\function{initscr()} returns a window object representing the entire
screen; this is usually called \code{stdscr}, after the name of the
corresponding C
variable.

\begin{verbatim}
import curses
stdscr = curses.initscr()
\end{verbatim}

Usually curses applications turn off automatic echoing of keys to the
screen, in order to be able to read keys and only display them under
certain circumstances.  This requires calling the \function{noecho()}
function.

\begin{verbatim}
curses.noecho()
\end{verbatim}

Applications will also commonly need to react to keys instantly,
without requiring the Enter key to be pressed; this is called cbreak
mode, as opposed to the usual buffered input mode.

\begin{verbatim}
curses.cbreak()
\end{verbatim}

Terminals usually return special keys, such as the cursor keys or
navigation keys such as Page Up and Home, as a multibyte escape
sequence.  While you could write your application to expect such
sequences and process them accordingly, curses can do it for you,
returning a special value such as \constant{curses.KEY_LEFT}.  To get
curses to do the job, you'll have to enable keypad mode.

\begin{verbatim}
stdscr.keypad(1)
\end{verbatim}

Terminating a curses application is much easier than starting one.
You'll need to call 

\begin{verbatim}
curses.nocbreak(); stdscr.keypad(0); curses.echo()
\end{verbatim}

to reverse the curses-friendly terminal settings. Then call the
\function{endwin()} function to restore the terminal to its original
operating mode.

\begin{verbatim}
curses.endwin()
\end{verbatim}

A common problem when debugging a curses application is to get your
terminal messed up when the application dies without restoring the
terminal to its previous state.  In Python this commonly happens when
your code is buggy and raises an uncaught exception.  Keys are no
longer be echoed to the screen when you type them, for example, which
makes using the shell difficult.

In Python you can avoid these complications and make debugging much
easier by importing the module \module{curses.wrapper}.  It supplies a
function \function{wrapper} that takes a hook argument.  It does the
initializations described above, and also initializes colors if color
support is present.  It then runs your hook, and then finally
deinitializes appropriately.  The hook is called inside a try-catch
clause which catches exceptions, performs curses deinitialization, and
then passes the exception upwards.  Thus, your terminal won't be left
in a funny state on exception.

\section{Windows and Pads}

Windows are the basic abstraction in curses.  A window object
represents a rectangular area of the screen, and supports various
 methods to display text, erase it, allow the user to input strings,
and so forth.

The \code{stdscr} object returned by the \function{initscr()} function
is a window object that covers the entire screen.  Many programs may
need only this single window, but you might wish to divide the screen
into smaller windows, in order to redraw or clear them separately.
The \function{newwin()} function creates a new window of a given size,
returning the new window object.

\begin{verbatim}
begin_x = 20 ; begin_y = 7
height = 5 ; width = 40
win = curses.newwin(height, width, begin_y, begin_x)
\end{verbatim}

A word about the coordinate system used in curses: coordinates are
always passed in the order \emph{y,x}, and the top-left corner of a
window is coordinate (0,0).  This breaks a common convention for
handling coordinates, where the \emph{x} coordinate usually comes
first.  This is an unfortunate difference from most other computer
applications, but it's been part of curses since it was first written,
and it's too late to change things now.

When you call a method to display or erase text, the effect doesn't
immediately show up on the display.  This is because curses was
originally written with slow 300-baud terminal connections in mind;
with these terminals, minimizing the time required to redraw the
screen is very important.  This lets curses accumulate changes to the
screen, and display them in the most efficient manner.  For example,
if your program displays some characters in a window, and then clears
the window, there's no need to send the original characters because
they'd never be visible.  

Accordingly, curses requires that you explicitly tell it to redraw
windows, using the \function{refresh()} method of window objects.  In
practice, this doesn't really complicate programming with curses much.
Most programs go into a flurry of activity, and then pause waiting for
a keypress or some other action on the part of the user.  All you have
to do is to be sure that the screen has been redrawn before pausing to
wait for user input, by simply calling \code{stdscr.refresh()} or the
\function{refresh()} method of some other relevant window.

A pad is a special case of a window; it can be larger than the actual
display screen, and only a portion of it displayed at a time.
Creating a pad simply requires the pad's height and width, while
refreshing a pad requires giving the coordinates of the on-screen
area where a subsection of the pad will be displayed.  

\begin{verbatim}
pad = curses.newpad(100, 100)
#  These loops fill the pad with letters; this is
# explained in the next section
for y in range(0, 100):
    for x in range(0, 100):
        try: pad.addch(y,x, ord('a') + (x*x+y*y) % 26 )
        except curses.error: pass

#  Displays a section of the pad in the middle of the screen
pad.refresh( 0,0, 5,5, 20,75)
\end{verbatim}

The \function{refresh()} call displays a section of the pad in the
rectangle extending from coordinate (5,5) to coordinate (20,75) on the
screen;the upper left corner of the displayed section is coordinate
(0,0) on the pad.  Beyond that difference, pads are exactly like
ordinary windows and support the same methods.

If you have multiple windows and pads on screen there is a more
efficient way to go, which will prevent annoying screen flicker at
refresh time.  Use the methods \method{noutrefresh()} and/or
\method{noutrefresh()} of each window to update the data structure
representing the desired state of the screen; then change the physical
screen to match the desired state in one go with the function
\function{doupdate()}.  The normal \method{refresh()} method calls
\function{doupdate()} as its last act.

\section{Displaying Text}

{}From a C programmer's point of view, curses may sometimes look like
a twisty maze of functions, all subtly different.  For example,
\function{addstr()} displays a string at the current cursor location
in the \code{stdscr} window, while \function{mvaddstr()} moves to a
given y,x coordinate first before displaying the string.
\function{waddstr()} is just like \function{addstr()}, but allows
specifying a window to use, instead of using \code{stdscr} by default.
\function{mvwaddstr()} follows similarly.

Fortunately the Python interface hides all these details;
\code{stdscr} is a window object like any other, and methods like
\function{addstr()} accept multiple argument forms.  Usually there are
four different forms.

\begin{tableii}{|c|l|}{textrm}{Form}{Description}
\lineii{\var{str} or \var{ch}}{Display the string \var{str} or
character \var{ch}}
\lineii{\var{str} or \var{ch}, \var{attr}}{Display the string \var{str} or
character \var{ch}, using attribute \var{attr}}
\lineii{\var{y}, \var{x}, \var{str} or \var{ch}}
{Move to position \var{y,x} within the window, and display \var{str}
or \var{ch}}
\lineii{\var{y}, \var{x}, \var{str} or \var{ch}, \var{attr}}
{Move to position \var{y,x} within the window, and display \var{str}
or \var{ch}, using attribute \var{attr}}
\end{tableii}

Attributes allow displaying text in highlighted forms, such as in
boldface, underline, reverse code, or in color.  They'll be explained
in more detail in the next subsection.

The \function{addstr()} function takes a Python string as the value to
be displayed, while the \function{addch()} functions take a character,
which can be either a Python string of length 1, or an integer.  If
it's a string, you're limited to displaying characters between 0 and
255.  SVr4 curses provides constants for extension characters; these
constants are integers greater than 255.  For example,
\constant{ACS_PLMINUS} is a +/- symbol, and \constant{ACS_ULCORNER} is
the upper left corner of a box (handy for drawing borders).

Windows remember where the cursor was left after the last operation,
so if you leave out the \var{y,x} coordinates, the string or character
will be displayed wherever the last operation left off.  You can also
move the cursor with the \function{move(\var{y,x})} method.  Because
some terminals always display a flashing cursor, you may want to
ensure that the cursor is positioned in some location where it won't
be distracting; it can be confusing to have the cursor blinking at
some apparently random location.  

If your application doesn't need a blinking cursor at all, you can
call \function{curs_set(0)} to make it invisible.  Equivalently, and
for compatibility with older curses versions, there's a
\function{leaveok(\var{bool})} function.  When \var{bool} is true, the
curses library will attempt to suppress the flashing cursor, and you
won't need to worry about leaving it in odd locations.

\subsection{Attributes and Color}

Characters can be displayed in different ways.  Status lines in a
text-based application are commonly shown in reverse video; a text
viewer may need to highlight certain words.  curses supports this by
allowing you to specify an attribute for each cell on the screen.

An attribute is a integer, each bit representing a different
attribute.  You can try to display text with multiple attribute bits
set, but curses doesn't guarantee that all the possible combinations
are available, or that they're all visually distinct.  That depends on
the ability of the terminal being used, so it's safest to stick to the
most commonly available attributes, listed here.

\begin{tableii}{|c|l|}{constant}{Attribute}{Description}
\lineii{A_BLINK}{Blinking text}
\lineii{A_BOLD}{Extra bright or bold text}
\lineii{A_DIM}{Half bright text}
\lineii{A_REVERSE}{Reverse-video text}
\lineii{A_STANDOUT}{The best highlighting mode available}
\lineii{A_UNDERLINE}{Underlined text}
\end{tableii}

So, to display a reverse-video status line on the top line of the
screen,
you could code:

\begin{verbatim}
stdscr.addstr(0, 0, "Current mode: Typing mode",
	      curses.A_REVERSE)
stdscr.refresh()
\end{verbatim}

The curses library also supports color on those terminals that
provide it, The most common such terminal is probably the Linux
console, followed by color xterms.

To use color, you must call the \function{start_color()} function
soon after calling \function{initscr()}, to initialize the default
color set (the \function{curses.wrapper.wrapper()} function does this
automatically).  Once that's done, the \function{has_colors()}
function returns TRUE if the terminal in use can actually display
color.  (Note from AMK:  curses uses the American spelling
'color', instead of the Canadian/British spelling 'colour'.  If you're
like me, you'll have to resign yourself to misspelling it for the sake
of these functions.)

The curses library maintains a finite number of color pairs,
containing a foreground (or text) color and a background color.  You
can get the attribute value corresponding to a color pair with the
\function{color_pair()} function; this can be bitwise-OR'ed with other
attributes such as \constant{A_REVERSE}, but again, such combinations
are not guaranteed to work on all terminals.

An example, which displays a line of text using color pair 1:

\begin{verbatim}
stdscr.addstr( "Pretty text", curses.color_pair(1) )
stdscr.refresh()
\end{verbatim}

As I said before, a color pair consists of a foreground and
background color.  \function{start_color()} initializes 8 basic
colors when it activates color mode.  They are: 0:black, 1:red,
2:green, 3:yellow, 4:blue, 5:magenta, 6:cyan, and 7:white.  The curses
module defines named constants for each of these colors:
\constant{curses.COLOR_BLACK}, \constant{curses.COLOR_RED}, and so
forth.

The \function{init_pair(\var{n, f, b})} function changes the
definition of color pair \var{n}, to foreground color {f} and
background color {b}.  Color pair 0 is hard-wired to white on black,
and cannot be changed.  

Let's put all this together. To change color 1 to red
text on a white background, you would call:

\begin{verbatim}
curses.init_pair(1, curses.COLOR_RED, curses.COLOR_WHITE)
\end{verbatim}

When you change a color pair, any text already displayed using that
color pair will change to the new colors.  You can also display new
text in this color with:

\begin{verbatim}
stdscr.addstr(0,0, "RED ALERT!", curses.color_pair(1) )
\end{verbatim}

Very fancy terminals can change the definitions of the actual colors
to a given RGB value.  This lets you change color 1, which is usually
red, to purple or blue or any other color you like.  Unfortunately,
the Linux console doesn't support this, so I'm unable to try it out,
and can't provide any examples.  You can check if your terminal can do
this by calling \function{can_change_color()}, which returns TRUE if
the capability is there.  If you're lucky enough to have such a
talented terminal, consult your system's man pages for more
information.

\section{User Input}

The curses library itself offers only very simple input mechanisms.
Python's support adds a text-input widget that makes up some of the
lack.

The most common way to get input to a window is to use its
\method{getch()} method. that pauses, and waits for the user to hit
a key, displaying it if \function{echo()} has been called earlier.
You can optionally specify a coordinate to which the cursor should be
moved before pausing.

It's possible to change this behavior with the method
\method{nodelay()}. After \method{nodelay(1)}, \method{getch()} for
the window becomes non-blocking and returns ERR (-1) when no input is
ready.  There's also a \function{halfdelay()} function, which can be
used to (in effect) set a timer on each \method{getch()}; if no input
becomes available within the number of milliseconds specified as the
argument to \function{halfdelay()}, curses throws an exception.

The \method{getch()} method returns an integer; if it's between 0 and
255, it represents the ASCII code of the key pressed.  Values greater
than 255 are special keys such as Page Up, Home, or the cursor keys.
You can compare the value returned to constants such as
\constant{curses.KEY_PPAGE}, \constant{curses.KEY_HOME}, or
\constant{curses.KEY_LEFT}.  Usually the main loop of your program
will look something like this:

\begin{verbatim}
while 1:
    c = stdscr.getch()
    if c == ord('p'): PrintDocument()
    elif c == ord('q'): break  # Exit the while()
    elif c == curses.KEY_HOME: x = y = 0
\end{verbatim}

The \module{curses.ascii} module supplies ASCII class membership
functions that take either integer or 1-character-string
arguments; these may be useful in writing more readable tests for
your command interpreters.  It also supplies conversion functions 
that take either integer or 1-character-string arguments and return
the same type.  For example, \function{curses.ascii.ctrl()} returns
the control character corresponding to its argument.

There's also a method to retrieve an entire string,
\constant{getstr()}.  It isn't used very often, because its
functionality is quite limited; the only editing keys available are
the backspace key and the Enter key, which terminates the string.  It
can optionally be limited to a fixed number of characters.

\begin{verbatim}
curses.echo()            # Enable echoing of characters

# Get a 15-character string, with the cursor on the top line 
s = stdscr.getstr(0,0, 15)  
\end{verbatim}

The Python \module{curses.textpad} module supplies something better.
With it, you can turn a window into a text box that supports an
Emacs-like set of keybindings.  Various methods of \class{Textbox}
class support editing with input validation and gathering the edit
results either with or without trailing spaces.   See the library
documentation on \module{curses.textpad} for the details.

\section{For More Information}

This HOWTO didn't cover some advanced topics, such as screen-scraping
or capturing mouse events from an xterm instance.  But the Python
library page for the curses modules is now pretty complete.  You
should browse it next.

If you're in doubt about the detailed behavior of any of the ncurses
entry points, consult the manual pages for your curses implementation,
whether it's ncurses or a proprietary Unix vendor's.  The manual pages
will document any quirks, and provide complete lists of all the
functions, attributes, and \constant{ACS_*} characters available to
you.

Because the curses API is so large, some functions aren't supported in
the Python interface, not because they're difficult to implement, but
because no one has needed them yet.  Feel free to add them and then
submit a patch.  Also, we don't yet have support for the menus or
panels libraries associated with ncurses; feel free to add that.

If you write an interesting little program, feel free to contribute it
as another demo.  We can always use more of them!

The ncurses FAQ: \url{http://dickey.his.com/ncurses/ncurses.faq.html}

\end{document}