So far we have created code that makes the device do something. This is called output. However, we also need the device to react to things. Such things are called inputs.
It’s easy to remember: output is what the device puts out to the world whereas input is what goes into the device for it to process.
The most obvious means of input on the micro:bit are its two buttons, labelled
B. Somehow, we need MicroPython to react to button presses.
This is remarkably simple:
from microbit import * sleep(10000) display.scroll(str(button_a.get_presses()))
All this script does is sleep for ten thousand milliseconds (i.e. 10 seconds)
and then scrolls the number of times you pressed button
A. That’s it!
While it’s a pretty useless script, it introduces a couple of interesting new ideas:
sleepfunction will make the micro:bit sleep for a certain number of milliseconds. If you want a pause in your program, this is how to do it. A function is just like a method, but it isn’t attached by a dot to an object.
- There is an object called
button_aand it allows you to get the number of times it has been pressed with the
get_presses gives a numeric value and
displays characters, we need to convert the numeric value into a string of
characters. We do this with the
str function (short for “string” ~ it
converts things into strings of characters).
The third line is a bit like an onion. If the parenthesis are the
onion skins then you’ll notice that
button_a.get_presses. Python attempts to work out the
inner-most answer first before starting on the next layer out. This is called
nesting - the coding equivalent of a Russian Matrioshka doll.
Let’s pretend you’ve pressed the button 10 times. Here’s how Python works out what’s happening on the third line:
Python sees the complete line and gets the value of
Now that Python knows how many button presses there have been, it converts the numeric value into a string of characters:
Finally, Python knows what to scroll across the display:
While this might seem like a lot of work, MicroPython makes this happen extraordinarily fast.
Often you need your program to hang around waiting for something to happen. To do this you make it loop around a piece of code that defines how to react to certain expected events such as a button press.
To make loops in Python you use the
while keyword. It checks if something
True. If it is, it runs a block of code called the body of the loop.
If it isn’t, it breaks out of the loop (ignoring the body) and the rest of the
program can continue.
Python makes it easy to define blocks of code. Say I have a to-do list written on a piece of paper. It probably looks something like this:
Shopping Fix broken gutter Mow the lawn
If I wanted to break down my to-do list a bit further, I might write something like this:
Shopping: Eggs Bacon Tomatoes Fix broken gutter: Borrow ladder from next door Find hammer and nails Return ladder Mow the lawn: Check lawn around pond for frogs Check mower fuel level
It’s obvious that the main tasks are broken down into sub-tasks that are
indented underneath the main task to which they are related. So
Tomatoes are obviously related to
Shopping. By indenting
things we make it easy to see, at a glance, how the tasks relate to each other.
This is called nesting. We use nesting to define blocks of code like this:
from microbit import * while running_time() < 10000: display.show(Image.ASLEEP) display.show(Image.SURPRISED)
running_time function returns the number of milliseconds since the
while running_time() < 10000: line checks if the running time is less
than 10000 milliseconds (i.e. 10 seconds). If it is, and this is where we can
see scoping in action, then it’ll display
Image.ASLEEP. Notice how this is
indented underneath the
while statement just like in our to-do list.
Obviously, if the running time is equal to or greater than 10000 milliseconds
then the display will show
Image.SURPRISED. Why? Because the
condition will be False (
running_time is no longer
< 10000). In that
case the loop is finished and the program will continue after the
loop’s block of code. It’ll look like your device is asleep for 10
seconds before waking up with a surprised look on its face.
Handling an Event¶
If we want MicroPython to react to button press events we should put it into
an infinite loop and check if the button
An infinite loop is easy:
while True: # Do stuff
while checks if something is
True to work out if it should
run its block of code. Since
True is obviously
True for all time, you
get an infinite loop!)
Let’s make a very simple cyber-pet. It’s always sad unless you’re pressing
A. If you press button
B it dies. (I realise this isn’t a very
pleasant game, so perhaps you can figure out how to improve it.):
from microbit import * while True: if button_a.is_pressed(): display.show(Image.HAPPY) elif button_b.is_pressed(): break else: display.show(Image.SAD) display.clear()
Can you see how we check what buttons are pressed? We used
elif (short for “else if”) and
else. These are called conditionals
and work like this:
if something is True: # do one thing elif some other thing is True: # do another thing else: # do yet another thing.
This is remarkably similar to English!
is_pressed method only produces two results:
If you’re pressing the button it returns
True, otherwise it returns
False. The code above is saying, in English, “for ever and ever, if
button A is pressed then show a happy face, else if button B is pressed break
out of the loop, otherwise display a sad face.” We break out of the loop (stop
the program running for ever and ever) with the
At the very end, when the cyber-pet is dead, we
clear the display.
Can you think of ways to make this game less tragic? How would you check if
both buttons are pressed? (Hint: Python has
logical operators to help check multiple truth statements (things that