In the previous lesson the students learned how to light up the SenseHAT. In this lesson they will learn how to customize SenseHAT output by passing parameters to functions. They will first encounter complex data structures - lists. They will also learn what some of the common mistakes are and how to debug them.
Functions, Parameters, Input, Computation, Output, Debugging
Raspberry Pi 3 with SenseHAT
This part builds on what students wrote in the last session, in which
they wrote their first program to use the sense.show_message()
function to output a scrolling message of their choice to SenseHAT. In
this lesson we will first answer this burning question: how do we change
the output?
In the last lesson we learned that by passing different strings to the
sense.show_message() function, we can change the output message that
we show on the SenseHAT. We also learned that functions are small,
pre-written programs that accept different parameters to do
different things. It turns out that we can also adjust many other things
by adding more parameters. When there are two or more parameters, we
have a slightly different way of passing them to a function. Here is
how:
function(parameter1,parameter2,parameter3,…)
Parameters are separated by commas. Each parameter has a name. For
example, in order to change the speed at which the text scrolls, we can
add this parameter (write it down on the whiteboard)
scroll_speed=0.05. scroll_speed is the name of the parameter, and
like how we assign values to variables, we also use the equal sign =
to assign values to parameters. Now you can change the value 0.05 to
see how the scroll speed of the text changes.
We usually omit the name of the first parameter.
Let’s go on to passing the function more parameters so we can also customize the colors of the SenseHAT output. But before that, think of this question: How does the computer know different colors?
Computers are not like us. It cannot automatically translate our
language to what it understands. However, it can understand numbers.
Therefore, computer programmers developed a system called RGB for
computers to understand and show us different colors. In the RGB
system, R stands for red, G stands for green, and B for
blue. For each color, we have 0 to 255, a total of 256
levels of intensity. By mixing different color intensities, we get
different colors. It is like using a color palette.
Feel free to use this Online Color Palette to get your own colors. Make sure you write down the colors and the RGB values that you find desirable.
Now, in order for Python to understand the colors you have picked, we
need to pass these three values to Python. However, it also needs to
understand that these three values need to be understood as a whole. We
use lists to do this job. Lists the easiest way in Python to put
things together as a whole. To use lists, you put comma-separated values
(or variables) into a pair of square brackets. For example, this list
means red: [255, 0, 0].
We can now pass these lists to two new parameters text_colour= and
back_colour= to change the text color and background color
respectively. Note the spelling of colour. Raspberry Pi and SenseHAT
are developed by the British so you will see British spelling of words a
lot.
sense.show_message("Hello Python", scroll_speed = 0.05,
text_colour = [255, 0, 0], back_colour = [0, 0, 255])
Notice that sometimes code can be too long to be shown on one line. You
can break the code into different lines by inserting line breaks
(ENTER key) into the brackets of functions right after the commas.
Often the code simply does not work, which can be really frustrating. Here we are going to break our code and see how Python behaves. Sometimes Python will show us exactly where things go wrong, and why things go wrong.
Misspelling sense_hat package name:
from sensehat import SenseHat
sense = SenseHat()
sense.show_message("Hello Python!")
Traceback (most recent call last):
File "/home/pi/Desktop/sense.py", line 1, in <module>
from sensehat import SenseHat
ImportError: No module named 'sensehat'
Misspelling SenseHat class name:
from sense_hat import Sensehat
sense = SenseHat()
sense.show_message("Hello Python!")
Traceback (most recent call last):
File "/home/pi/Desktop/sense.py", line 1, in <module>
from sense_hat import Sensehat
ImportError: cannot import name 'Sensehat'
Missing ()
from sense_hat import SenseHat
sense = SenseHat
sense.show_message("Hello Python!")
Traceback (most recent call last):
File "/home/pi/Desktop/sense.py", line 4, in <module>
sense.show_message("Hello Python")
TypeError: show_message() missing 1 required positional argument: 'text_string'
Misspelling SenseHat() object
from sense_hat import SenseHat
sense = Sensehat()
sense.show_message("Hello Python!")
Traceback (most recent call last):
File "/home/pi/Desktop/sense.py", line 2, in <module>
sense = Sensehat()
NameError: name 'Sensehat' is not defined
Misspelling sense.show_message() function name
from sense_hat import SenseHat
sense = Sensehat()
sense.show_Message("Hello Python!")
Traceback (most recent call last):
File "/home/pi/Desktop/sense.py", line 4, in <module>
sense.show_Message("Hello Python")
AttributeError: 'SenseHat' object has no attribute 'show_Message'
Missing quotation marks around the string
from sense_hat import SenseHat
sense = Sensehat()
sense.show_message(Hello Python)
File "/home/pi/Desktop/sense.py", line 4
sense.show_message(Hello Python)
^
SyntaxError: invalid syntax