Basics of Python

Recap / What is Python?

As we were setting up Python last time, we ran some sample code and just checked that everything worked (If it didn’t, make sure to go back through the previous page).

Today let’s try to explain what is going on in this example and understand Python’s basic concepts, giving you the skills to write similar programs yourself.

First, a bit about Python. Python code is run line by line, meaning that in the code we viewed previously, line 1 would run, then line 2, and line 3, and so on. Much like the steps to a cooking recipe, a program often has a rather obvious flow. You complete one step and move on to the next.

Armed with this information, let’s try to unpack what Python is doing in the code from last time:

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my_number = 3 + 4
my_name = "Mr. Robot"
message = "You've done everything correctly!"

print("Hello from test.py!")
print(message)
print(f"Hi {my_name}. Do you like the number {my_number}?")

Line 1 my_number = 3 + 4

We create a variable. A recipe might ask to remember something for later use (Remember how many minutes you left the cake in the oven, this will affect the resting time). Similarly, we can tell Python to remember certain data through the use of variables. A variable has a name (in this case, my_number) which we use to look at or change what Python remembers. Later on in the code, if we use this name, Python knows to replace it with the value we remembered (So my_number + 1 is 8). To remember some value, we write variable_name = variable_value. In this case, we store the number 7 into my_number.

Strings

Before we continue, let’s talk a tiny bit about data. Python, and many other programming languages, can answer simple questions, like:

• What’s 3 plus 4?
• Is there an ‘a’ in the word ‘giraffe’?

But to do so, we need to represent numbers and words in Python somehow. For numbers, this is easy; we can just use the numbers themselves, as you saw above with my_number = 3 + 4. For words/messages, we need some way to tell Python that this text is not code and is just a message. We do this by surrounding the message with quotation marks ".

So while Hey python, do this! is code, "Hey python, do this!" is a bit of data, called a string.

Lines 2 and 3 my_name = "Mr. Robot", message = "You've done everything correctly!"

On line 2, we create another variable. However, in this example, the value we create is not a number; it’s a string!

On line 3, we do the same thing.

Lines 5 and 6 print("Hello from test.py!"), print(message)

On line 5, we use the function print. print allows us to print data to the screen. Any data between the () brackets gets printed to console, so print(my_number) would print 7. If you want to print multiple bits of data, they need to be separated by commas (try print(my_number, "test")).

Line 5 has just a string in the brackets, so that string gets printed to the console (Without the quotation marks!).

Line 6 is similar, except it uses a variable. Python then replaces that variable with the value we are remembering, and prints it!

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Hello World!
30 + 3 = 33

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my_message = Hello World!
my_number = 33

print(my_message)
print("30 + 3 = my_number")

Line 7 print(f"Hi {my_name}. Do you like the number {my_number}?")

Before the quotation marks, we have an f on line 7. Surely this is a typo! But actually, this f is vital. It stands for format and allows us to insert data into our string! Anywhere in the string, if some text is surrounded by curly brackets ({}), Python will assume this is code and run it. So we can write print(f"5 + 4 = {5+4}") to get ‘5 + 4 = 9’!

Because we have {my_number} and {my_name} on line 7, these get replaced with 7 and Mr. Robot.

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Hello from test.py!
You've done everything correctly!
Hi Mr. Robot. Do you like the number 7?

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Hello from test.py!
You've done everything INcorrectly >:)
Hi 24. Do you like the number seven?

Writing your own stuff

Python can do a lot more with variables and data than print it to a console, however. What makes programming interesting is that we can modify these values to do more exciting things. For example, we can do simple maths with numbers:

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# Create some variables
x = 2
y = 3
print(x + y)
# The * is for multiplication.
# You can write it with Shift and 8 on the keyboard
# The # is written with Shift and 3 on the keyboard
print(x * y)
print(x / y)
# A late entrant! Variables from variables!?
z = x + y
print(z / 3)

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### Output ###
>>> 5
>>> 6
>>> 0.66666667
>>> 1.66666667

Hopefully, you can see by now that variables are quite versatile. We can use them to store values. We can even have variables based on other variables (z = x + y). They are limited in one way though - their values don’t change unless you change them yourself.

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x = 2
y = 3
z = x + y
print(z)
# Here I'll update y. It won't affect the value of z however.
y = 5
# So x+y = 7 now
print(z, x+y)

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### Output ###
>>> 5
>>> 5 7

This is because of the way variables are created. When you write something like z = x + y, The following happens:

• First, Python inspects the right-hand side of the = sign. This means that the x becomes 2, the y becomes 3 because of the data these variables contain, and so x + y becomes 5.
• The right-hand side value is then stored in the left-hand side variable. So 5 is stored in z.

Additionally, the left-hand side can only be one variable name; you can’t have x=3, y=2 and then set x+y=z. So the code won’t work unless you move it around so that z is by itself on the left-hand side.

This can seem restrictive, but there’s more fun stuff we can do because of this restriction:

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variable = 1
variable = variable + 1
print(variable)
variable = variable * variable * variable
print(variable)

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### Output ###
>>> 2
>>> 8

Let’s see why this works by analysing the second line, variable = variable + 1:

• First, Python inspects the right-hand side of the = sign. This means that variable becomes 1, so variable + 1 becomes 2.
• 2 is stored in variable (from the left-hand side).

So a variable can change itself!

Controlling Motors

Now that we’ve got some of the basics down pat, we can start controlling the robots.

Next, we’ll start writing code to control the motors. Here’s a simple script that will turn one motor forward for 1 second, and then reverse the same motor for 1 second.

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# Include LargeMotor functionality.
from ev3dev2.motor import LargeMotor, OUTPUT_A, OUTPUT_B

# Connect to the large motor on port outA
first_motor = LargeMotor(OUTPUT_A)

# Go forwards at full speed
first_motor.on_for_seconds(100, 1)
# Go backwards at full speed
first_motor.on_for_seconds(-100, 1)

The first line imports functions that we can use in our code. Just like print is a keyword we can use to run some command (sending data to the screen), importing allows us to add more keywords that reference other commands (Like connecting to motors).

To send commands to the LargeMotor, we have to connect to the device, and then store this connection in a variable, as line 5 does. OUTPUT_A corresponds to the port outA we selected before (We can use the keyword OUTPUT_A here because we imported it on line 2).

After this, we can send commands to the motor through the variable we created. For example, my_motor.on_for_seconds(x, y) then turns the motor on for y seconds at x speed (x can be anything from -100 to 100, and y must be positive). If you start the simulator, this code should move the robot forwards then backwards.

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from ev3dev2.motor import LargeMotor, OUTPUT_A, OUTPUT_B

first_motor = LargeMotor(OUTPUT_A)
second_motor = LargeMotor(OUTPUT_B)

# This first command won't block.
first_motor.on_for_seconds(100, 1, block=False)
# This second command will block.
second_motor.on_for_seconds(-100, 1)

Getting user input

Another thing that Python can do is ask for data while we are running the code. We do this with the input function.

Try running the following code:

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name = input("What's your name?")
print(f"Hello {name}!")

When running this code in the simulator, a box should appear below the console. Type your name and then press the enter key. You should then get the personalised message printed with your name!

The data in the () brackets of input will first be printed to console as a prompt. Then, once you enter your name (or any other data) and press enter, the string (remember: message) of that data is stored in the variable name.

So if you entered Chris as your name, the code would do the same thing as

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name = "Chris"
print(f"Hello {name}!")

However, because this input is always a string, we need to change this slightly to supply our program with numbers. Python has a function int, which converts any string into an Integer (Whole number). So int("4") is the number 4 in Python.

Using this int function, we can convert our input into a number:

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my_number = int(input("How many programs have you written?"))
print(f"I've written {my_number + 1} :p")

Checkpoint Task

Now that we can control motors, print information and store information in variables, let’s combine this into one big project!