#64
Seven-Segment Display Module

A seven-segment display is a type of LCD component used to display numbers in pocket calculators, microwave ovens, and other small electronic devices. Through different combinations of seven line-shaped segments in an LCD, a seven-segment display can represent the digits 0 through 9. They look like this:

   __         __    __          __    __   __    __    __
  |  |    |   __|   __|  |__|  |__   |__     |  |__|  |__|
  |__|    |  |__    __|     |   __|  |__|    |  |__|   __|

The benefit of this program is that other programs can import it as a module. Project 14, “Countdown,” and Project 19, “Digital Clock,” import the sevseg.py file so they can use its getSevSegStr() function. You can find more information about seven-segment displays and other variations at https://en.wikipedia.org/wiki/Seven-segment_display.

The Program in Action

Even though it’s a module, sevseg.py outputs a sample demo of the digits it produces when you run the program directly. The output will look like this:

This module is meant to be imported rather than run.
For example, this code:
    import sevseg
    myNumber = sevseg.getSevSegStr(42, 3)
    print(myNumber)

Will print 42, zero-padded to three digits:
 __        __
|  | |__|  __|
|__|    | |__

How It Works

The getSevSegStr() function first creates a list of three strings. The strings represent the top, middle, and bottom row of the numeric digits. Lines 27 to 75 have a long list of if-elif statements for each digit (and the decimal point and minus sign) that concatenates the rows of each digit to these strings. These three strings are joined together with newlines on line 84 so that the function returns a single multiline string suitable to pass to print().


  1. """Sevseg, by Al Sweigart [email protected]
  2. A seven-segment number display module, used by the Countdown and Digital
  3. Clock programs.
  4. More info at https://en.wikipedia.org/wiki/Seven-segment_display
  5. This code is available at https://nostarch.com/big-book-small-python-programming
  6. Tags: short, module"""
  7. 
  8. """A labeled seven-segment display, with each segment labeled A to G:
  9.  __A__
 10. |     |    Each digit in a seven-segment display:
 11. F     B     __       __   __        __   __  __   __   __
 12. |__G__|    |  |   |  __|  __| |__| |__  |__    | |__| |__|
 13. |     |    |__|   | |__   __|    |  __| |__|   | |__|  __|
 14. E     C
 15. |__D__|"""
 16. 
 17. 
 18. def getSevSegStr(number, minWidth=0):
 19.     """Return a seven-segment display string of number. The returned
 20.     string will be padded with zeros if it is smaller than minWidth."""
 21. 
 22.     # Convert number to string in case it's an int or float:
 23.     number = str(number).zfill(minWidth)
 24. 
 25.     rows = ['', '', '']
 26.     for i, numeral in enumerate(number):
 27.         if numeral == '.':  # Render the decimal point.
 28.             rows[0] += ' '
 29.             rows[1] += ' '
 30.             rows[2] += '.'
 31.             continue  # Skip the space in between digits.
 32.         elif numeral == '-':  # Render the negative sign:
 33.             rows[0] += '    '
 34.             rows[1] += ' __ '
 35.             rows[2] += '    '
 36.         elif numeral == '0':  # Render the 0.
 37.             rows[0] += ' __ '
 38.             rows[1] += '|  |'
 39.             rows[2] += '|__|'
 40.         elif numeral == '1':  # Render the 1.
 41.             rows[0] += '    '
 42.             rows[1] += '   |'
 43.             rows[2] += '   |'
 44.         elif numeral == '2':  # Render the 2.
 45.             rows[0] += ' __ '
 46.             rows[1] += ' __|'
 47.             rows[2] += '|__ '
 48.         elif numeral == '3':  # Render the 3.
 49.             rows[0] += ' __ '
 50.             rows[1] += ' __|'
 51.             rows[2] += ' __|'
 52.         elif numeral == '4':  # Render the 4.
 53.             rows[0] += '    '
 54.             rows[1] += '|__|'
 55.             rows[2] += '   |'
 56.         elif numeral == '5':  # Render the 5.
 57.             rows[0] += ' __ '
 58.             rows[1] += '|__ '
 59.             rows[2] += ' __|'
 60.         elif numeral == '6':  # Render the 6.
 61.             rows[0] += ' __ '
 62.             rows[1] += '|__ '
 63.             rows[2] += '|__|'
 64.         elif numeral == '7':  # Render the 7.
 65.             rows[0] += ' __ '
 66.             rows[1] += '   |'
 67.             rows[2] += '   |'
 68.         elif numeral == '8':  # Render the 8.
 69.             rows[0] += ' __ '
 70.             rows[1] += '|__|'
 71.             rows[2] += '|__|'
 72.         elif numeral == '9':  # Render the 9.
 73.             rows[0] += ' __ '
 74.             rows[1] += '|__|'
 75.             rows[2] += ' __|'
 76. 
 77.         # Add a space (for the space in between numerals) if this
 78.         # isn't the last numeral and the decimal point isn't next:
 79.         if i != len(number) - 1 and number[i + 1] != '.':
 80.             rows[0] += ' '
 81.             rows[1] += ' '
 82.             rows[2] += ' '
 83. 
 84.     return '\n'.join(rows)
 85. 
 86. 
 87. # If this program isn't being imported, display the numbers 00 to 99.
 88. if __name__ == '__main__':
 89.     print('This module is meant to be imported rather than run.')
 90.     print('For example, this code:')
 91.     print('    import sevseg')
 92.     print('    myNumber = sevseg.getSevSegStr(42, 3)')
 93.     print('    print(myNumber)')
 94.     print()
 95.     print('...will print 42, zero-padded to three digits:')
 96.     print(' __        __ ')
 97.     print('|  | |__|  __|')
 98.     print('|__|    | |__ ')

After entering the source code and running it a few times, try making experimental changes to it. On your own, you can also try to figure out how to do the following:

Create new fonts for numbers, such as using five rows and the block character string returned by chr(9608).
Look at the Wikipedia article for seven-segment displays to find out how to display letters and then add those to sevseg.py.
Learn about sixteen-segment displays from https://en.wikipedia.org/wiki/Sixteen-segment_display and create a sixteenseg.py module to generate numbers in that style.

Exploring the Program

Try to find the answers to the following questions. Experiment with some modifications to the code and rerun the program to see what effect the changes have.

What happens if you change the single-space strings on lines 80, 81, and 82 to empty strings?
What happens if you change the minWidth=0 default argument on line 18 to minWidth=8?