3. Organising with selection and definitions

3.1. Definitions

Definitions help break up lengthy code into smaller well defined blocks.

Definitions use the same naming conventions as for variables. In general, use lowercase letters.

Rules for Python variable names:

A variable name must start with a letter or the underscore character

A variable name cannot start with a number

A variable name can only contain alpha-numeric characters (A-z, 0-9) and underscores ( _ )

Variable names are case-sensitive (age, Age and AGE are three different variables)

Definitions start with the def keyword.
All the code for a definition is indented.
Below is an example of a definition that scrolls some text and displays an image.
The definition is run by calling it via doA().

from microbit import *

def doA():
    display.scroll('A')
    display.show(Image.HAPPY)

doA()

3.2. Organising blocks into definitions

Compare the two code examples below.
The first example does not have any definitions.
The second example takes each block of code in the various branches of the if block and reallocates them to separate definitions.
This makes the main block of code in the while True loop more compact and more readable.
Each definition block can be easily edited and changed.
Reorganizing the code is called refactoring.

Basic unorganised code:

from microbit import *

while True:
    if button_a.is_pressed():
        display.scroll('A')
        display.show(Image.HAPPY)
    elif button_b.is_pressed():
        display.scroll('B')
        display.show(Image.SAD)
    else:
        display.scroll('')
    sleep(1000)

Re-organised code using definitions:

from microbit import *


def doA():
    display.scroll('A')
    display.show(Image.HAPPY)


def doB():
    display.scroll('B')
    display.show(Image.SAD)


def doC():
    display.scroll('C')


while True:
    if button_a.is_pressed():
        doA()
    elif button_b.is_pressed():
        doB()
    else:
        doC()
    sleep(1000)

Side by side comparison:

Tasks

Reorganise the code below to follow the structure of the examples above.

from microbit import *

while True:
    if button_a.is_pressed():
        for char in 'go team':
            display.scroll(char, delay=80)
    elif button_b.is_pressed():
        for sport in ['swimming', 'rowing', 'canoeing']:
            display.scroll(sport, delay=80)
    else:
        display.clear()

Reorganise the code below to follow the structure of the examples above.

from microbit import *

while True:
    if button_a.is_pressed():
        for num in range(1, 10, 2):
            display.scroll(num, delay=80)
    elif button_b.is_pressed():
        for num in range(8, -1, -2):
            display.scroll(num, delay=80)
    else:
        display.clear()