Curses Programming with Python
******************************

Author:
   A.M. Kuchling, Eric S. Raymond

Release:
   2.04


Abstract
^^^^^^^^

This document describes how to use the "curses" extension module to
control text-mode displays.


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 various programs.  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 graphical 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 niche is on small-footprint or embedded
Unixes that don’t run an X server.  Another is tools such as OS
installers and kernel configurators that may have to run before any
graphical support is available.

The curses library provides fairly basic functionality, providing the
programmer with an abstraction of a display containing multiple non-
overlapping windows of text.  The contents of a window can be changed
in various ways—adding text, erasing it, changing its appearance—and
the curses library will figure out what control codes need to be sent
to the terminal to produce the right output.  curses doesn’t provide
many user-interface concepts such as buttons, checkboxes, or dialogs;
if you need such features, consider a user interface library such as
Urwid.

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 BSD curses is no longer maintained, having been replaced by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.

The Windows version of Python doesn’t include the "curses" module.  A
ported version called UniCurses is available.


The Python curses module
------------------------

The 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 "addstr()", "mvaddstr()", and
"mvwaddstr()" into a single "addstr()" method.  You’ll see this
covered in more detail later.

This HOWTO is 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 section on
ncurses, and the C manual pages for ncurses.  It will, however, give
you the basic ideas.


Starting and ending a curses application
========================================

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

   import curses
   stdscr = curses.initscr()

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 "noecho()" function.

   curses.noecho()

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.

   curses.cbreak()

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 "curses.KEY_LEFT".  To get curses to
do the job, you’ll have to enable keypad mode.

   stdscr.keypad(True)

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

   curses.nocbreak()
   stdscr.keypad(False)
   curses.echo()

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

   curses.endwin()

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 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 "curses.wrapper()" function and using it like
this:

   from curses import wrapper

   def main(stdscr):
       # Clear screen
       stdscr.clear()

       # This raises ZeroDivisionError when i == 10.
       for i in range(0, 11):
           v = i-10
           stdscr.addstr(i, 0, '10 divided by {} is {}'.format(v, 10/v))

       stdscr.refresh()
       stdscr.getkey()

   wrapper(main)

The "wrapper()" function takes a callable object and does the
initializations described above, also initializing colors if color
support is present.  "wrapper()" then runs your provided callable.
Once the callable returns, "wrapper()" will restore the original state
of the terminal.  The callable is called inside a "try"…"except" that
catches exceptions, restores the state of the terminal, and then re-
raises the exception.  Therefore your terminal won’t be left in a
funny state on exception and you’ll be able to read the exception’s
message and traceback.


Windows and Pads
================

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

The "stdscr" object returned by the "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
"newwin()" function creates a new window of a given size, returning
the new window object.

   begin_x = 20; begin_y = 7
   height = 5; width = 40
   win = curses.newwin(height, width, begin_y, begin_x)

Note that the coordinate system used in curses is unusual. Coordinates
are always passed in the order *y,x*, and the top-left corner of a
window is coordinate (0,0).  This breaks the normal convention for
handling coordinates where the *x* coordinate 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.

Your application can determine the size of the screen by using the
"curses.LINES" and "curses.COLS" variables to obtain the *y* and *x*
sizes.  Legal coordinates will then extend from "(0,0)" to
"(curses.LINES - 1, curses.COLS - 1)".

When you call a method to display or erase text, the effect doesn’t
immediately show up on the display.  Instead you must call the
"refresh()" method of window objects to update the screen.

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 was very important.  Instead curses
accumulates changes to the screen and displays them in the most
efficient manner when you call "refresh()".  For example, if your
program displays some text in a window and then clears the window,
there’s no need to send the original text because they’re never
visible.

In practice, explicitly telling curses to redraw a window 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 first calling "stdscr.refresh()" or the "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 the pad displayed at a time.
Creating a pad 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.

   pad = curses.newpad(100, 100)
   # These loops fill the pad with letters; addch() is
   # explained in the next section
   for y in range(0, 99):
       for x in range(0, 99):
           pad.addch(y,x, ord('a') + (x*x+y*y) % 26)

   # Displays a section of the pad in the middle of the screen.
   # (0,0) : coordinate of upper-left corner of pad area to display.
   # (5,5) : coordinate of upper-left corner of window area to be filled
   #         with pad content.
   # (20, 75) : coordinate of lower-right corner of window area to be
   #          : filled with pad content.
   pad.refresh( 0,0, 5,5, 20,75)

The "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 update the screen and prevent annoying screen flicker
as each part of the screen gets updated.  "refresh()" actually does
two things:

1. Calls the "noutrefresh()" method of each window to update an
   underlying data structure representing the desired state of the
   screen.

2. Calls the function "doupdate()" function to change the physical
   screen to match the desired state recorded in the data structure.

Instead you can call "noutrefresh()" on a number of windows to update
the data structure, and then call "doupdate()" to update the screen.


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,
"addstr()" displays a string at the current cursor location in the
"stdscr" window, while "mvaddstr()" moves to a given y,x coordinate
first before displaying the string. "waddstr()" is just like
"addstr()", but allows specifying a window to use instead of using
"stdscr" by default. "mvwaddstr()" allows specifying both a window and
a coordinate.

Fortunately the Python interface hides all these details.  "stdscr" is
a window object like any other, and methods such as "addstr()" accept
multiple argument forms.  Usually there are four different forms.

+-----------------------------------+-------------------------------------------------+
| Form                              | Description                                     |
|===================================|=================================================|
| *str* or *ch*                     | Display the string *str* or character *ch* at   |
|                                   | the current position                            |
+-----------------------------------+-------------------------------------------------+
| *str* or *ch*, *attr*             | Display the string *str* or character *ch*,     |
|                                   | using attribute *attr* at the current position  |
+-----------------------------------+-------------------------------------------------+
| *y*, *x*, *str* or *ch*           | Move to position *y,x* within the window, and   |
|                                   | display *str* or *ch*                           |
+-----------------------------------+-------------------------------------------------+
| *y*, *x*, *str* or *ch*, *attr*   | Move to position *y,x* within the window, and   |
|                                   | display *str* or *ch*, using attribute *attr*   |
+-----------------------------------+-------------------------------------------------+

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

The "addstr()" method takes a Python string or bytestring as the value
to be displayed.  The contents of bytestrings are sent to the terminal
as-is.  Strings are encoded to bytes using the value of the window’s
"encoding" attribute; this defaults to the default system encoding as
returned by "locale.getencoding()".

The "addch()" methods take a character, which can be either a string
of length 1, a bytestring of length 1, or an integer.

Constants are provided for extension characters; these constants are
integers greater than 255.  For example, "ACS_PLMINUS" is a +/-
symbol, and "ACS_ULCORNER" is the upper left corner of a box (handy
for drawing borders).  You can also use the appropriate Unicode
character.

Windows remember where the cursor was left after the last operation,
so if you leave out the *y,x* coordinates, the string or character
will be displayed wherever the last operation left off.  You can also
move the cursor with the "move(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 "curs_set(False)" to make it invisible.  For compatibility with
older curses versions, there’s a "leaveok(bool)" function that’s a
synonym for "curs_set()".  When *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.


Attributes and Color
--------------------

Characters can be displayed in different ways.  Status lines in a
text-based application are commonly shown in reverse video, or 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 an 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.

+------------------------+----------------------------------------+
| Attribute              | Description                            |
|========================|========================================|
| "A_BLINK"              | Blinking text                          |
+------------------------+----------------------------------------+
| "A_BOLD"               | Extra bright or bold text              |
+------------------------+----------------------------------------+
| "A_DIM"                | Half bright text                       |
+------------------------+----------------------------------------+
| "A_REVERSE"            | Reverse-video text                     |
+------------------------+----------------------------------------+
| "A_STANDOUT"           | The best highlighting mode available   |
+------------------------+----------------------------------------+
| "A_UNDERLINE"          | Underlined text                        |
+------------------------+----------------------------------------+

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

   stdscr.addstr(0, 0, "Current mode: Typing mode",
                 curses.A_REVERSE)
   stdscr.refresh()

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 "start_color()" function soon after
calling "initscr()", to initialize the default color set (the
"curses.wrapper()" function does this automatically).  Once that’s
done, the "has_colors()" function returns TRUE if the terminal in use
can actually display color.  (Note: curses uses the American spelling
‘color’, instead of the Canadian/British spelling ‘colour’.  If you’re
used to the British spelling, 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
"color_pair()" function; this can be bitwise-OR’ed with other
attributes such as "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:

   stdscr.addstr("Pretty text", curses.color_pair(1))
   stdscr.refresh()

As I said before, a color pair consists of a foreground and background
color. The "init_pair(n, f, b)" function changes the definition of
color pair *n*, to foreground color f and background color b.  Color
pair 0 is hard-wired to white on black, and cannot be changed.

Colors are numbered, and "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:
"curses.COLOR_BLACK", "curses.COLOR_RED", and so forth.

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

   curses.init_pair(1, curses.COLOR_RED, curses.COLOR_WHITE)

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:

   stdscr.addstr(0,0, "RED ALERT!", curses.color_pair(1))

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 "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.


User Input
==========

The C curses library offers only very simple input mechanisms.
Python’s "curses" module adds a basic text-input widget.  (Other
libraries such as Urwid have more extensive collections of widgets.)

There are two methods for getting input from a window:

* "getch()" refreshes the screen and then waits for the user to hit a
  key, displaying the key if "echo()" has been called earlier.  You
  can optionally specify a coordinate to which the cursor should be
  moved before pausing.

* "getkey()" does the same thing but converts the integer to a string.
  Individual characters are returned as 1-character strings, and
  special keys such as function keys return longer strings containing
  a key name such as "KEY_UP" or "^G".

It’s possible to not wait for the user using the "nodelay()" window
method. After "nodelay(True)", "getch()" and "getkey()" for the window
become non-blocking. To signal that no input is ready, "getch()"
returns "curses.ERR" (a value of -1) and "getkey()" raises an
exception. There’s also a "halfdelay()" function, which can be used to
(in effect) set a timer on each "getch()"; if no input becomes
available within a specified delay (measured in tenths of a second),
curses raises an exception.

The "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 "curses.KEY_PPAGE",
"curses.KEY_HOME", or "curses.KEY_LEFT".  The main loop of your
program may look something like this:

   while True:
       c = stdscr.getch()
       if c == ord('p'):
           PrintDocument()
       elif c == ord('q'):
           break  # Exit the while loop
       elif c == curses.KEY_HOME:
           x = y = 0

The "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 such loops.  It also
supplies conversion functions  that take either integer or 1
-character-string arguments and return the same type.  For example,
"curses.ascii.ctrl()" returns the control character corresponding to
its argument.

There’s also a method to retrieve an entire string, "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.

   curses.echo()            # Enable echoing of characters

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

The "curses.textpad" module supplies a text box that supports an
Emacs-like set of keybindings.  Various methods of the "Textbox" class
support editing with input validation and gathering the edit results
either with or without trailing spaces.  Here’s an example:

   import curses
   from curses.textpad import Textbox, rectangle

   def main(stdscr):
       stdscr.addstr(0, 0, "Enter IM message: (hit Ctrl-G to send)")

       editwin = curses.newwin(5,30, 2,1)
       rectangle(stdscr, 1,0, 1+5+1, 1+30+1)
       stdscr.refresh()

       box = Textbox(editwin)

       # Let the user edit until Ctrl-G is struck.
       box.edit()

       # Get resulting contents
       message = box.gather()

See the library documentation on "curses.textpad" for more details.


For More Information
====================

This HOWTO doesn’t cover some advanced topics, such as reading the
contents of the screen or capturing mouse events from an xterm
instance, but the Python library page for the "curses" module is now
reasonably complete.  You should browse it next.

If you’re in doubt about the detailed behavior of the curses
functions, 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 ACS_* characters available to you.

Because the curses API is so large, some functions aren’t supported in
the Python interface.  Often this isn’t because they’re difficult to
implement, but because no one has needed them yet.  Also, Python
doesn’t yet support the menu library associated with ncurses. Patches
adding support for these would be welcome; see the Python Developer’s
Guide to learn more about submitting patches to Python.

* Writing Programs with NCURSES: a lengthy tutorial for C programmers.

* The ncurses man page

* The ncurses FAQ

* “Use curses… don’t swear”: video of a PyCon 2013 talk on controlling
  terminals using curses or Urwid.

* “Console Applications with Urwid”: video of a PyCon CA 2012 talk
  demonstrating some applications written using Urwid.
