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A thin, practical wrapper around terminal capabilities in Python

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blessings
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MANIFEST.in
README.rst
fabfile.py
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README.rst

Blessings

Coding with Blessings looks like this...

from blessings import Terminal

t = Terminal()

print t.bold('Hi there!')
print t.bold_red_on_bright_green('It hurts my eyes!')

with t.location(0, t.height - 1):
    print 'This is at the bottom.'

Or, for byte-level control, you can drop down and play with raw terminal capabilities:

print '{t.bold}All your {t.red}bold and red base{t.normal}'.format(t=t)
print t.wingo(2)

The Pitch

Blessings lifts several of curses' limiting assumptions, and it makes your code pretty, too:

  • Use styles, color, and maybe a little positioning without clearing the whole screen first.
  • Leave more than one screenful of scrollback in the buffer after your program exits, like a well-behaved command-line app should.
  • Get rid of all those noisy, C-like calls to tigetstr and tparm, so your code doesn't get crowded out by terminal bookkeeping.
  • Act intelligently when somebody redirects your output to a file, omitting the terminal control codes the user doesn't want to see (optional).

Before And After

Without Blessings, this is how you'd print some underlined text at the bottom of the screen:

from curses import tigetstr, setupterm, tparm
from fcntl import ioctl
from os import isatty
import struct
import sys
from termios import TIOCGWINSZ

# If we want to tolerate having our output piped to other commands or
# files without crashing, we need to do all this branching:
if hasattr(sys.stdout, 'fileno') and isatty(sys.stdout.fileno()):
    setupterm()
    sc = tigetstr('sc')
    cup = tigetstr('cup')
    rc = tigetstr('rc')
    underline = tigetstr('smul')
    normal = tigetstr('sgr0')
else:
    sc = cup = rc = underline = normal = ''
print sc  # Save cursor position.
if cup:
    # tigetnum('lines') doesn't always update promptly, hence this:
    height = struct.unpack('hhhh', ioctl(0, TIOCGWINSZ, '\000' * 8))[0]
    print tparm(cup, height - 1, 0)  # Move cursor to bottom.
print 'This is {under}underlined{normal}!'.format(under=underline,
                                                  normal=normal)
print rc  # Restore cursor position.

Phew! That was long and full of incomprehensible trash! Let's try it again, this time with Blessings:

from blessings import Terminal

term = Terminal()
with term.location(0, term.height - 1):
    print 'This is', term.underline('pretty!')

Much better.

What It Provides

Blessings provides just one top-level object: Terminal. Instantiating a Terminal figures out whether you're on a terminal at all and, if so, does any necessary terminal setup. After that, you can proceed to ask it all sorts of things about the terminal. Terminal terminal terminal.

Simple Formatting

Lots of handy formatting codes ("capabilities" in low-level parlance) are available as attributes on a Terminal. For example:

from blessings import Terminal

term = Terminal()
print 'I am ' + term.bold + 'bold' + term.normal + '!'

You can also use them as wrappers so you don't have to say normal afterward:

print 'I am', term.bold('bold') + '!'

Or, if you want fine-grained control while maintaining some semblance of brevity, you can combine it with Python's string formatting, which makes attributes easy to access:

print 'All your {t.red}base {t.underline}are belong to us{t.normal}'.format(t=term)

Simple capabilities of interest include...

  • bold
  • reverse
  • underline
  • no_underline (which turns off underlining)
  • blink
  • normal (which turns off everything, even colors)
  • clear_eol (clear to the end of the line)
  • clear_bol (clear to beginning of line)
  • clear_eos (clear to end of screen)

Here are a few more which are less likely to work on all terminals:

  • dim
  • italic and no_italic
  • shadow and no_shadow
  • standout and no_standout
  • subscript and no_subscript
  • superscript and no_superscript
  • flash (which flashes the screen once)

Note that, while the inverse of underline is no_underline, the only way to turn off bold or reverse is normal, which also cancels any custom colors. This is because there's no way to tell the terminal to undo certain pieces of formatting, even at the lowest level.

You might notice that the above aren't the typical incomprehensible terminfo capability names; we alias a few of the harder-to-remember ones for readability. However, you aren't limited to these: you can reference any string-returning capability listed on the terminfo man page by the name under the "Cap-name" column: for example, term.rum.

Color

16 colors, both foreground and background, are available as easy-to-remember attributes:

from blessings import Terminal

term = Terminal()
print term.red + term.on_green + 'Red on green? Ick!' + term.normal
print term.bright_red + term.on_bright_blue + 'This is even worse!' + term.normal

You can also call them as wrappers, which sets everything back to normal at the end:

print term.red_on_green('Red on green? Ick!')
print term.yellow('I can barely see it.')

The available colors are...

  • black
  • red
  • green
  • yellow
  • blue
  • magenta
  • cyan
  • white

You can set the background color instead of the foreground by prepending on_, as in on_blue. There is also a bright version of each color: for example, on_bright_blue.

There is also a numerical interface to colors, which takes an integer from 0-15:

term.color(5) + 'Hello' + term.normal
term.on_color(3) + 'Hello' + term.normal

term.color(5)('Hello')
term.on_color(3)('Hello')

If some color is unsupported (for instance, if only the normal colors are available, not the bright ones), trying to use it will, on most terminals, have no effect: the foreground and background colors will stay as they were. You can get fancy and do different things depending on the supported colors by checking number_of_colors.

Compound Formatting

If you want to do lots of crazy formatting all at once, you can just mash it all together:

from blessings import Terminal

term = Terminal()
print term.bold_underline_green_on_yellow + 'Woo' + term.normal

Or you can use your newly coined attribute as a wrapper, which implicitly sets everything back to normal afterward:

print term.bold_underline_green_on_yellow('Woo')

This compound notation comes in handy if you want to allow users to customize the formatting of your app: just have them pass in a format specifier like "bold_green" on the command line, and do a quick getattr(term, that_option)('Your text') when you do your formatting.

I'd be remiss if I didn't credit couleur, where I probably got the idea for all this mashing.

Parametrized Capabilities

Some capabilities take parameters. Rather than making you dig up tparm() all the time, we simply make such capabilities into callable strings. You can pass the parameters right in:

from blessings import Terminal

term = Terminal()
print term.move(10, 1)

Here are some of interest:

move
Position the cursor elsewhere. Parameters are y coordinate, then x coordinate.
move_x
Move the cursor to the given column.
move_y
Move the cursor to the given row.

You can also reference any other string-returning capability listed on the terminfo man page by its name under the "Cap-name" column.

Height and Width

It's simple to get the height and width of the terminal, in characters:

from blessings import Terminal

term = Terminal()
height = term.height
width = term.width

These are newly updated each time you ask for them, so they're safe to use from SIGWINCH handlers.

Temporary Repositioning

Sometimes you need to flit to a certain location, print something, and then return: for example, when updating a progress bar at the bottom of the screen. Terminal provides a context manager for doing this concisely:

from blessings import Terminal

term = Terminal()
with term.location(0, term.height - 1):
    print 'Here is the bottom.'
print 'This is back where I came from.'

Parameters to location() are x and then y, but you can also pass just one of them, leaving the other alone. For example...

with term.location(y=10):
    print 'We changed just the row.'

If you want to reposition permanently, see move, in an example above.

Pipe Savvy

If your program isn't attached to a terminal, like if it's being piped to another command or redirected to a file, all the capability attributes on Terminal will return empty strings. You'll get a nice-looking file without any formatting codes gumming up the works.

If you want to override this--like if you anticipate your program being piped through less -r, which handles terminal escapes just fine--pass force_styling=True to the Terminal constructor.

In any case, there is an is_a_tty attribute on Terminal that lets you see whether the attached stream seems to be a terminal. If it's false, you might refrain from drawing progress bars and other frippery, since you're apparently headed into a pipe:

from blessings import Terminal

term = Terminal()
if term.is_a_tty:
    with term.location(0, term.height - 1):
        print 'Progress: [=======>   ]'
print term.bold('Important stuff')

Shopping List

There are decades of legacy tied up in terminal interaction, so attention to detail and behavior in edge cases make a difference. Here are some ways Blessings has your back:

  • Uses the terminfo database so it works with any terminal type
  • Provides up-to-the-moment terminal height and width, so you can respond to terminal size changes (SIGWINCH signals). (Most other libraries query the COLUMNS and LINES environment variables or the cols or lines terminal capabilities, which don't update promptly, if at all.)
  • Avoids making a mess if the output gets piped to a non-terminal
  • Works great with standard Python string templating
  • Provides convenient access to all terminal capabilities, not just a sugared few
  • Outputs to any file-like object, not just stdout
  • Keeps a minimum of internal state, so you can feel free to mix and match with calls to curses or whatever other terminal libraries you like

Blessings does not provide...

  • Native color support on the Windows command prompt. However, it should work when used in concert with colorama.

Bugs

Bugs or suggestions? Visit the issue tracker.

License

Blessings is under the MIT License. See the LICENSE file.

Version History

1.3
  • Add number_of_colors, which tells you how many colors the terminal supports.
  • Made color(n) and on_color(n) callable to wrap a string, like the named colors can. Also, make them both fall back to the setf and setb capabilities (like the named colors do) if the ANSI setaf and setab aren't available.
  • Allow color attr to act as an unparametrized string, not just a callable.
  • Make height and width examine any passed-in stream before falling back to stdout. (This rarely if ever affects actual behavior; it's mostly philosophical.)
  • Make caching simpler and slightly more efficient.
  • Get rid of a reference cycle between Terminals and FormattingStrings.
  • Update docs to reflect that terminal addressing (as in location()) is 0-based.
1.2
  • Added support for Python 3! We need 3.2.3 or greater, because the curses library couldn't decide whether to accept strs or bytes before that (http://bugs.python.org/issue10570).
  • Everything that comes out of the library is now unicode. This lets us support Python 3 without making a mess of the code, and Python 2 should continue to work unless you were testing types (and badly). Please file a bug if this causes trouble for you.
  • Changed to the MIT License for better world domination.
  • Added Sphinx docs.
1.1
  • Added nicely named attributes for colors.
  • Introduced compound formatting.
  • Added wrapper behavior for styling and colors.
  • Let you force capabilities to be non-empty, even if the output stream is not a terminal.
  • Added the is_a_tty attribute for telling whether the output stream is a terminal.
  • Sugared the remaining interesting string capabilities.
  • Let location() operate on just an x or y coordinate.
1.0
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