The way to Create a Python curses-enabled Software


Within the first a part of this programming tutorial collection, we discovered learn how to set up and setup the Python curses module, which is said to the C ncurses library. Immediately, we’ll proceed that dialogue as we create our first “Hiya, World” instance utilizing the curses library.

If you happen to missed the primary a part of this collection, you’ll be able to learn it right here: Python curses: Drawing with Textual content.

Making a Hiya, World! Software with Python curses

With the entire formalities concluded, it’s now time to create a easy program that can display fundamental ncurses performance through a Python curses-enabled program. The code beneath will write a customary “Hiya, world!” message to the terminal:

# demo-ncurses-hello-world.py

import curses
import sys

def important(argv):
  # BEGIN ncurses startup/initialization...
  # Initialize the curses object.
  stdscr = curses.initscr()

  # Don't echo keys again to the consumer.
  curses.noecho()

  # Non-blocking or cbreak mode... don't await Enter key to be pressed.
  curses.cbreak()

  # Flip off blinking cursor
  curses.curs_set(False)

  # Allow coloration if we are able to...
  if curses.has_colors():
    curses.start_color()

  # Elective - Allow the keypad. This additionally decodes multi-byte key sequences
  # stdscr.keypad(True)

  # END ncurses startup/initialization...

  caughtExceptions = ""
  strive:
    # Coordinates begin from prime left, within the format of y, x.
    stdscr.addstr(0, 0, "Hiya, world!")
    screenDetailText = "This display screen is [" + str(curses.LINES) + "] excessive and [" + str(curses.COLS) + "] throughout."
    startingXPos = int ( (curses.COLS - len(screenDetailText))/2 )
    stdscr.addstr(3, startingXPos, screenDetailText)
    stdscr.addstr(5, curses.COLS - len("Press a key to give up."), "Press a key to give up.")

    # Truly attracts the textual content above to the positions specified.
    stdscr.refresh()

    # Grabs a price from the keyboard with out Enter having to be pressed (see cbreak above)
    stdscr.getch()
  besides Exception as err:
   # Simply printing from right here won't work, as this system continues to be set to
   # use ncurses.
   # print ("Some error [" + str(err) + "] occurred.")
   caughtExceptions = str(err)

  # BEGIN ncurses shutdown/deinitialization...
  # Flip off cbreak mode...
  curses.nocbreak()

  # Flip echo again on.
  curses.echo()

  # Restore cursor blinking.
  curses.curs_set(True)

  # Flip off the keypad...
  # stdscr.keypad(False)

  # Restore Terminal to authentic state.
  curses.endwin()

  # END ncurses shutdown/deinitialization...

  # Show Errors if any occurred:
  if "" != caughtExceptions:
   print ("Received error(s) [" + caughtExceptions + "]")

if __name__ == "__main__":
  important(sys.argv[1:])


The primary line in important, stdscr = curses.initscr(), reveals that the curses treats the display screen as a curses window object that occurs to cowl the whole display screen. The entire different features that write textual content to the display screen are members of the curses object. Nonetheless, stdscr = curses.initscr() goes additional by initializing the ncurses module in order that it will possibly do its work on the terminal.

Textual content Positioning with curses in Python

The code above makes use of ncurses’ positioning grid to put the textual content on the display screen. ncurses makes use of a zero-indexed grid system, represented by X and Y values, to place components on the display screen:

Python ncurses text positioning

The 2 values, curses.COLS and curses.LINES seek advice from the utmost variety of columns within the terminal and the utmost variety of traces within the terminal, respectively.

The “Hiya, World!” program above makes use of three completely different coordinate positions within the terminal in an effort to show textual content. The primary place, 0, 0, merely writes “Hiya, World!” to the top-left nook of the terminal. Whereas ncurses, generally, could be very delicate to writing textual content exterior of its containing window, the code makes the idea that the terminal is broad sufficient to accommodate the textual content. Bear in mind that operating the “Hiya, World!” program above with a really slender house (lower than the size of “Hiya, World!”) will trigger an exception.

The second place, which is calculated based mostly on the width of a string, is an approximation of the middle of the terminal on a set line. Be aware that, in contrast to a very graphical program, the place is at all times going to be both 1 over, 1 much less, or precisely on the width of the terminal. This variance is as a result of the terminal width have to be an integer, as cursor positions can’t be fractional. The code even casts the results of the calculation to an integer for this very cause.

The third place right-justifies the textual content literal with the directions to press a (any) key to give up. As with the second place, the beginning X coordinate is calculated relative to the curses.COLS, besides that there isn’t a division by 2.

Be aware: the exception messages returned by the Python curses module resulting from incorrect sizing of strings, home windows, or different objects typically make no point out of a measurement drawback. One option to mitigate that is to verify all of those calculations earlier than passing any values into any curses object, and, if the maths doesn’t permit for a match, then prematurely elevate an exception with an appropriate error message.

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The way to Draw or Place Textual content with Python curses

Because the remark above the stdscr.refresh() code signifies, that is the place all of the textual content is definitely drawn to the display screen. This suggests that, ought to alternate textual content have to be positioned at a location wherein textual content already exists, then one other name to stdscr.refresh() is important. If the textual content that replaces present textual content isn’t lengthy sufficient to utterly cowl the present textual content, then areas will have to be appended to the brand new textual content in an effort to cowl up the present textual content. Calls to stdscr.addstr(…) typically don’t overwrite present textual content.

Person Enter and Python curses

The stdscr.getch() code works in tandem with the curses.cbreak(), curses.curs_set(False), and curses.noecho() calls above it. With out curses.cbreak(), it will be essential to press Enter after urgent some other key. For this instance, that might not be the specified operation. With out curses.noecho(), the worth of no matter key was pressed can be echoed again to the terminal. That echoing would have the potential to undesirably change the textual content on the display screen. Lastly, with out curses.curs_set(False), the blinking cursor would nonetheless be displayed on the display screen, and this may doubtlessly confuse customers in purposes with extra advanced interfaces.

Home windows and Python curses

As 99% of the “promoting level” of ncurses is the power to show home windows in a text-based terminal interface it begs the purpose of really creating some. And, why not make this a little bit extra attention-grabbing by including some colours to the output too?

The code beneath makes additional use of the Python curses.window object, together with the colours outlined within the curses module to create three randomly generated home windows on the terminal. Now, a extra seasoned developer would possibly name out the “non-usage” of object-oriented code right here, as this code can be use case for that, however for the needs of an introductory demonstration, it’s simpler to focus extra on the curses objects themselves, versus how Python calls them, even when it makes for longer code:

# demo-3-windows2.py

# Makes use of the curses library to create 3 randomly sized home windows with completely different coloration
# backgrounds on the display screen.

# Be aware that this isn't essentially the most environment friendly option to code this, however I need to get away
# the person objects in order that it's simpler to hint what's going on.

import curses
import math
import random
import sys

# A set of layouts, to be randomly chosen.
layouts = ['2 top, 1 bottom', '2 left, 1 right', '1 top, 2 bottom', '1 left, 2 right']

def important (argv):
  # Initialize the curses object.
  stdscr = curses.initscr()

  # Don't echo keys again to the consumer.
  curses.noecho()

  # Non-blocking or cbreak mode... don't await Enter key to be pressed.
  curses.cbreak()

  # Flip off blinking cursor
  curses.curs_set(False)

  # Allow coloration if we are able to...
  if curses.has_colors():
    curses.start_color()

  # Elective - Allow the keypad. This additionally decodes multi-byte key sequences
  # stdscr.keypad(True)

  # Starting of Program... 
  # Create an inventory of all the colours apart from black and white. These will server as 
  # the background colours for the home windows. As a result of these constants are outlined in 
  # ncurses,
  # we will not create the listing till after the curses.initscr name:
  bgColors = [curses.COLOR_BLUE, curses.COLOR_CYAN, curses.COLOR_GREEN, 
   curses.COLOR_MAGENTA, curses.COLOR_RED, curses.COLOR_YELLOW]
  colours = random.pattern(bgColors, 3)

  # Create 3 ncurses coloration pair objects.
  curses.init_pair(1, curses.COLOR_WHITE, colours[0])
  curses.init_pair(2, curses.COLOR_WHITE, colours[1])
  curses.init_pair(3, curses.COLOR_WHITE, colours[2])

  caughtExceptions = ""
  strive:
   # Be aware that print statements don't work when utilizing ncurses. If you wish to write
   # to the terminal exterior of a window, use the stdscr.addstr methodology and specify
   # the place the textual content will go. Then use the stdscr.refresh methodology to refresh the 
   # show.
   #stdscr.addstr(0, 0, "Gonna make some home windows.")
   #stdscr.refresh()

   # The lists beneath will finally maintain 4 values, the X and Y coordinates of the 
   # top-left nook relative to the display screen itself, and the variety of characters
   # going proper and down, respectively.
   window1 = []
   window2 = []
   window3 = []

   # The variables beneath will finally include the window objects.
   window1Obj = ""
   window2Obj = ""
   window3Obj = ""

   # The variables beneath will correspond roughly to the X, Y coordinates of the 
   # of every window.
   window1 = []
   window2 = []
   window3 = []

   # There's going to be a caption on the backside left of the display screen, but it surely must
   # go within the correct window.
   window1Caption = ""
   window2Caption = ""
   window3Caption = ""


   # The randomly sized home windows that do not take up one aspect of the display screen should not 
   # be lower than 1/3 the display screen measurement, or a couple of third of the display screen measurement on 
   # both edge.
   minWindowWidth = math.flooring(curses.COLS * 1.0/3.0)
   maxWindowWidth = math.flooring(curses.COLS * 2.0/3.0)
   minWindowHeight = math.flooring(curses.LINES * 1.0/3.0)
   maxWindowHeight = math.flooring(curses.LINES * 2.0/3.0)
   # Choose a format. The random.randrange command will return a price between 0 and three.
   chosenLayout = layouts[random.randrange(0,4)]
   if '2 prime, 1 backside' == chosenLayout:
    # Home windows 1 and a couple of would be the prime, Window 3 would be the backside.
    window1Width = random.randrange(minWindowWidth, maxWindowWidth)
    window1Height = random.randrange(minWindowHeight, maxWindowHeight)
    window1 = [0, 0, window1Width, window1Height]

    window2Width = curses.COLS - window1Width
    window2Height = window1Height
    window2 = [window1Width, 0, window2Width, window2Height]

    window3 = [0, window1Height, curses.COLS, curses.LINES - window1Height]
    window3Caption = chosenLayout + " - Press a key to give up."

   elif '2 left, 1 proper' == chosenLayout:
    # Home windows 1 and a couple of will probably be on the left, Window 3 will probably be on the correct.
    window1Width = random.randrange(minWindowWidth, maxWindowWidth)
    window1Height = random.randrange(minWindowHeight, maxWindowHeight)
    window1 = [0, 0, window1Width, window1Height]

    window2Width = window1Width
    window2Height = curses.LINES - window1Height
    window2 = [0, window1Height, window2Width, window2Height]
    window2Caption = chosenLayout + " - Press a key to give up."

    window3Width = curses.COLS - window1Width
    window3Height = curses.LINES
    window3 = [window1Width, 0, window3Width, window3Height]

   elif '1 prime, 2 backside' == chosenLayout:
    # Window 1 will probably be on the highest, Home windows 2 and three will probably be on the underside.
    window1Width = curses.COLS
    window1Height = random.randrange(minWindowHeight, maxWindowHeight)
    window1 = [0, 0, window1Width, window1Height]

    window2Width = random.randrange(minWindowWidth, maxWindowWidth)
    window2Height = curses.LINES - window1Height
    window2 = [0, window1Height, window2Width, window2Height]
    window2Caption = chosenLayout + " - Press a key to give up."

    window3Width = curses.COLS - window2Width
    window3Height = window2Height
    window3 = [window2Width, window1Height, window3Width, window3Height]

   elif '1 left, 2 proper' == chosenLayout:
    # Window 1 will probably be on the left, Home windows 2 and three will probably be on the correct.
    window1Width = random.randrange(minWindowWidth, maxWindowWidth)
    window1Height = curses.LINES
    window1 = [0, 0, window1Width, window1Height]
    window1Caption = chosenLayout + " - Press a key to give up."

    window2Width = curses.COLS - window1Width
    window2Height = random.randrange(minWindowHeight, maxWindowHeight)
    window2 = [window1Width, 0, window2Width, window2Height]

    window3Width = window2Width
    window3Height = curses.LINES - window2Height
    window3 = [window1Width, window2Height, window3Width, window3Height]

   # Create and refresh every window. Put the caption 2 traces up from backside
   # in case it wraps. Placing it on the final line with no room to wrap (if
   # the window is simply too slender for the textual content) will trigger an exception.

   window1Obj = curses.newwin(window1[3], window1[2], window1[1], window1[0])
   window1Obj.bkgd(' ', curses.color_pair(1))
   # Calculate tough heart...
   window1Center = [math.floor(window1[2]/2.0), math.flooring(window1[3]/2.0)]
   # Add the string to the middle, with BOLD flavoring.
   window1Obj.addstr(window1Center[1], window1Center[0] - 4, "Window 1", 
    curses.color_pair(1) | curses.A_BOLD)
   if "" != window1Caption:
    window1Obj.addstr(curses.LINES - 2, 0, window1Caption, 
     curses.color_pair(1) | curses.A_BOLD)
   window1Obj.refresh()

   window2Obj = curses.newwin(window2[3], window2[2], window2[1], window2[0])
   window2Obj.bkgd(' ', curses.color_pair(2))
   # Calculate tough heart...
   window2Center = [math.floor(window2[2]/2.0), math.flooring(window2[3]/2.0)]
   # Add the string to the middle, with BOLD flavoring.
   window2Obj.addstr(window2Center[1], window2Center[0] - 4, "Window 2", 
    curses.color_pair(2) | curses.A_BOLD)
   if "" != window2Caption:
    # The "Y coordinate" right here is the underside of the *window* and never the display screen.
    window2Obj.addstr(window2[3] - 2, 0, window2Caption, 
     curses.color_pair(2) | curses.A_BOLD)
   window2Obj.refresh()

   window3Obj = curses.newwin(window3[3], window3[2], window3[1], window3[0])
   window3Obj.bkgd(' ', curses.color_pair(3))
   # Calculate tough heart...
   window3Center = [math.floor(window3[2]/2.0), math.flooring(window3[3]/2.0)]
   # Add the string to the middle, with BOLD flavoring.
   window3Obj.addstr(window3Center[1], window3Center[0] - 4, "Window 3", 
    curses.color_pair(3) | curses.A_BOLD)
   if "" != window3Caption:
    # The "Y coordinate" right here is the underside of the *window* and never the display screen.
    window3Obj.addstr(window3[3] - 2, 0, window3Caption, 
     curses.color_pair(3) | curses.A_BOLD)
   window3Obj.refresh()

   # Obligatory so we are able to "pause" on the window output earlier than quitting.
   window3Obj.getch()

   # Debugging output.
   #stdscr.addstr(0, 0, "Chosen format is [" + chosenLayout + "]")
   #stdscr.addstr(1, 10, "Window 1 params are [" + str (window1)+ "]")
   #stdscr.addstr(2, 10, "Window 2 params are [" + str(window2) + "]")
   #stdscr.addstr(3, 10, "Window 3 params are [" + str(window3)+ "]")
   #stdscr.addstr(4, 10, "Colours are [" + str(colors) + "]")
   #stdscr.addstr(5, 0, "Press a key to proceed.")
   #stdscr.refresh()
   #stdscr.getch()
  besides Exception as err:
   caughtExceptions = str(err)

  # Finish of Program...
  # Flip off cbreak mode...
  curses.nocbreak()

  # Flip echo again on.
  curses.echo()

  # Restore cursor blinking.
  curses.curs_set(True)

  # Flip off the keypad...
  # stdscr.keypad(False)

  # Restore Terminal to authentic state.
  curses.endwin()

  # Show Errors if any occurred:
  if "" != caughtExceptions:
   print ("Received error(s) [" + caughtExceptions + "]")
  return 0

if __name__ == "__main__":
  important(sys.argv[1:])

The colour constants, together with the formatting constants for the bolded textual content, are all outlined at curses — Terminal dealing with for character-cell shows — Python 3.10.5 documentation.

There are 4 attainable forms of outputs to this program, as indicated by the layouts listing on the prime of the code:

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The Window Object

Every window within the output above is represented by a definite instantiation of the curses window object. The window object is returned by every name to curses.newwin(…) operate. Be aware that, whereas every of the three home windows above is instantiated through the curses.newwin(…) operate, this operate by itself doesn’t initialize the ncurses module (nor do they de-initialize the identical for return to the immediate). That also must be finished with curses.initscr(), regardless that the stdscr window object returned by this name isn’t going for use on this code.

The curses.newwin(…) operate has two overloads, the second of which is getting used because it permits for the location of the top-left nook anyplace on the display screen, along with specifying the dimensions of the window:

curses.newwin(number-of-lines, number-of-columns, starting-column-position, starting-row-position)

All 4 parameters are non-negative integers.

The primary two values number-of-lines and number-of-columns are calculated as “random” integers that vary in between and the peak and width of the terminal window, respectively.

On this explicit instance, no matter what the worth of one of many randomly chosen layouts is, the primary window is at all times the one which occupies the top-left nook of the terminal. The opposite two home windows’ sizes and positions are calculated relative to the primary window.

Textual content Inside Home windows

Any textual content that’s positioned inside a window created utilizing the curses.newwin(…) operate should absolutely match throughout the window. Any textual content that ends exterior of the bounds of the window will trigger an exception to be raised. The code above creates a caption that’s positioned in no matter window finally ends up occupying the bottom-left nook of the display screen. Relying on what width is calculated for the dimensions of this window, it’s attainable that the size of the caption might exceed the width of the window. If this isn’t accounted for, an exception will probably be raised.

Within the above code instance, it may be seen that the caption isn’t on the bottom-most line. It’s because this explicit implementation of curses will wrap the textual content to the subsequent line ought to it overshoot the sting of the window, as proven beneath:

Python curses tutorial

For this explicit instance, the terminal window was shrunk down considerably.

Nonetheless, if there isn’t a additional line to which the rest of the textual content could be wrapped, an exception will probably be raised. Be aware: this line-wrapping habits is probably not constant throughout all implementations of ncurses. In a manufacturing atmosphere, additional code ought to be used to separate the road into separate calls to window.addstr(…).

Shade Pairs

The Python curses module offers with colours in pairs. Every Python color_pair object incorporates a foreground textual content coloration and a background coloration. The rationale for it’s because for any textual content that’s drawn to the display screen, a foreground and background coloration have to be specified, and for the sake of a “good look,” the background coloration of any textual content drawn or positioned in a window ought to match the background coloration of the window.

It might be famous that the examples earlier to this itemizing presumed that the foreground textual content coloration was white and the background coloration was black. Relying on the terminal, and the way meticulous a programmer could also be, this is probably not a smart presumption.

Closing Ideas on Drawing Textual content with Python curses

That’s it for half two of this three-part programming tutorial collection discussing learn how to work with the Python curses library to attract textual content in Linux. We’ll wrap up the ultimate half on this collection in our closing piece. Test again right here for the hyperlink as soon as it’s revealed!



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