Doc/howto/curses.rst

.. _curses-howto:

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

:Author: A.M. Kuchling, Eric S. Raymond
:Release: 2.04


.. topic:: Abstract

   This document describes how to use the :mod:`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 <https://pypi.org/project/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is no longer maintained, having been replaced by ncurses, which is an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 :mod:`curses`
module.  A ported version called `UniCurses
<https://pypi.org/project/UniCurses>`_ is available.  You could
also try `the Console module <http://effbot.org/zone/console-index.htm>`_
written by Fredrik Lundh, which doesn't
use the same API as curses but provides cursor-addressable text output
and full support for mouse and keyboard input.


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
:c:func:`addstr`, :c:func:`mvaddstr`, and :c:func:`mvwaddstr` into a single
:meth:`~curses.window.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 :func:`~curses.initscr` function, which will determine the
terminal type, send any required setup codes to the terminal, and
create various internal data structures.  If successful,
:func:`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
:func:`~curses.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
:const:`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
:func:`~curses.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 :func:`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 :func:`~curses.wrapper` function takes a callable object and does the
initializations described above, also initializing colors if color
support is present.  :func:`wrapper` then runs your provided callable.
Once the callable returns, :func:`wrapper` will restore the original
state of the terminal.  The callable is called inside a
:keyword:`try`...\ :keyword:`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 :func:`~curses.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
:func:`~curses.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
:data:`curses.LINES` and :data:`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
:meth:`~curses.window.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 :meth:`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 :meth:`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 :meth:`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.  :meth:`refresh` actually
does two things:

1) Calls the :meth:`~curses.window.noutrefresh` method of each window
   to update an underlying data structure representing the desired
   state of the screen.
2) Calls the function :func:`~curses.doupdate` function to change the
   physical screen to match the desired state recorded in the data structure.

Instead you can call :meth:`noutrefresh` on a number of windows to
update the data structure, and then call :func:`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,
:c:func:`addstr` displays a string at the current cursor location in
the ``stdscr`` window, while :c:func:`mvaddstr` moves to a given y,x
coordinate first before displaying the string. :c:func:`waddstr` is just
like :c:func:`addstr`, but allows specifying a window to use instead of
using ``stdscr`` by default. :c:func:`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
:meth:`~curses.window.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 :meth:`~curses.window.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 :attr:`encoding` attribute; this defaults to
the default system encoding as returned by
:func:`locale.getpreferredencoding`.

The :meth:`~curses.window.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, :const:`ACS_PLMINUS` is a +/-
symbol, and :const:`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 :func:`~curses.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                          |
+======================+======================================+
| :const:`A_BLINK`     | Blinking text                        |
+----------------------+--------------------------------------+
| :const:`A_BOLD`      | Extra bright or bold text            |
+----------------------+--------------------------------------+
| :const:`A_DIM`       | Half bright text                     |
+----------------------+--------------------------------------+
| :const:`A_REVERSE`   | Reverse-video text                   |
+----------------------+--------------------------------------+
| :const:`A_STANDOUT`  | The best highlighting mode available |
+----------------------+--------------------------------------+
| :const:`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 :func:`~curses.start_color` function soon
after calling :func:`~curses.initscr`, to initialize the default color set
(the :func:`curses.wrapper` function does this automatically).  Once that's
done, the :func:`~curses.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 :func:`~curses.color_pair`
function; this can be bitwise-OR'ed with other attributes such as
:const:`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 :func:`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 :mod:`curses`
module defines named constants for each of these colors:
:const:`curses.COLOR_BLACK`, :const:`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
:func:`~curses.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
:mod:`curses` module adds a basic text-input widget.  (Other libraries
such as `Urwid <https://pypi.org/project/urwid/>`_ have more extensive
collections of widgets.)

There are two methods for getting input from a window:

* :meth:`~curses.window.getch` refreshes the screen and then waits for
  the user to hit a key, displaying the key if :func:`~curses.echo` has been
  called earlier.  You can optionally specify a coordinate to which
  the cursor should be moved before pausing.

* :meth:`~curses.window.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
:meth:`~curses.window.nodelay` window method. After ``nodelay(True)``,
:meth:`getch` and :meth:`getkey` for the window become
non-blocking. To signal that no input is ready, :meth:`getch` returns
``curses.ERR`` (a value of -1) and :meth:`getkey` raises an exception.
There's also a :func:`~curses.halfdelay` function, which can be used to (in
effect) set a timer on each :meth:`getch`; if no input becomes
available within a specified delay (measured in tenths of a second),
curses raises an exception.

The :meth:`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 :const:`curses.KEY_PPAGE`,
:const:`curses.KEY_HOME`, or :const:`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 :mod:`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, :func:`curses.ascii.ctrl` returns the
control character corresponding to its argument.

There's also a method to retrieve an entire string,
:meth:`~curses.window.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 :mod:`curses.textpad` module supplies a text box that supports an
Emacs-like set of keybindings.  Various methods of the
:class:`~curses.textpad.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 :mod:`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 :mod:`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 :const:`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 <https://devguide.python.org/>`_ to
learn more about submitting patches to Python.

* `Writing Programs with NCURSES <http://invisible-island.net/ncurses/ncurses-intro.html>`_:
  a lengthy tutorial for C programmers.
* `The ncurses man page <https://linux.die.net/man/3/ncurses>`_
* `The ncurses FAQ <http://invisible-island.net/ncurses/ncurses.faq.html>`_
* `"Use curses... don't swear" <https://www.youtube.com/watch?v=eN1eZtjLEnU>`_:
  video of a PyCon 2013 talk on controlling terminals using curses or Urwid.
* `"Console Applications with Urwid" <http://www.pyvideo.org/video/1568/console-applications-with-urwid>`_:
  video of a PyCon CA 2012 talk demonstrating some applications written using
  Urwid.