Variables in Python

Video Tutorial

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Text Tutorial

A variable is a “bucket” that we can use to store data. In Python, we can save data like numbers, words, letters, True/False, or more types of values inside of variables. Much like in math, we also use variables in programming.

# In math:
x = 5
y = 2
z = x + y

# In Programming:
distance = 12
speed = 95.5
name = "rover_1"
is_driver_controlled = True

The main difference between variables in math, and in programming, are that:

1. We use much more descriptive names in programming. “x” becomes “distance”. It’s a way that we can abstract away complexity in our code through proper naming.
   • “z = y * x” is much harder to understand than “distance = speed * time”
2. We can store more than numbers in variables in programming:
   • Numbers (0, 4, 123): We call these variables “integers” or “Int”
   • Decimal Numbers (0.43, 8.8): We call these variables “floats” as they are floating point numbers
   • Words & Letters *(“hello:, “J”, “rover_1”): We call these variables “strings” or “str” as they are strings of characters. Note: all strings are surrounded by “quotation marks”
   • True/False: We call these variables “Booleans” or “Bool”. They only store True or False values. Useful for storing states like “light_on” or “motors_running”.
3. the variables above are assignment statements. We are “assigning” values to these variables using a single = sign. These are the main 4 types of variables that we’ll use in this course. More exist, but this will cover the basics.

Variable Practice 1

1. Plug in your metro M4 board to your computer, open code.py, and open the serial port.
2. create 4 different variables, one of each type. For example:

   power = 5
   light_intensity = 84.8
   rover_name = "Jenkins"
   has_rc_control = True
   

3. After creating 4 variables, print each variable and observe the output. Notice, we do not put variable names “in between quotes” as only string variable values are in between “quotes”.

print(power)
print(light_intensity)
print(rover_name)
print(has_rc_control)

print("power")
print("light_intensity")
print("rover_name")
print("has_rc_control")

1. Observe the differences between these 2 sets of print statements.
   • The first set printed the values of the variables
   • The second set printed 4 different “string variables”, rather than their variable values, because all 3 were surrounded by “quotation marks”

Naming Variables

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See below for legal vs “illegal” variable names in Python image credit

• Good Variable Names: follow the snake_case_convention, are easy to read, and clearly indicate what they represent.
• Not So Good Variable Names: These are not incorrect but do not follow the Python convention of snake case, making them less ideal. They might still convey the meaning but are harder to read at a glance.
• Bad Variable Names: These include characters or formats that are invalid in Python (e.g., starting with a number or including special characters), making them unusable or hard to understand.

Another convention in naming variables in python is that all names are in lower case. If we have to have multiple words, we_separate_them_with_an_underscore. This is called “snake_case”, a well fitting name for Python.

Variable Practice 2

1. Observe what happens to the following code in your serial output:

   distance = 50
   print(distance)
   distance = 12
   print(distance)
   

2. This is an example of sequencing. All code runs sequentially, Top down. We can re-assign variables new values later on in code.
3. Try it for yourself.

Variable Practice 3

Just like in math, we can also use arithmatic operators on variables.

+ add
- subtract
* multiplication
/ division
// integer division (no decimals)
% Modulus (remainder division)

Try this:

time = 10
speed = 40
distance = time * speed
print(distance)

Copy and paste the following code inside code.py. Then, follow the #code comments to practice arithmatic operators in Python:

# Note: any line that starts with a "#" hashtag, is a special line in Python
# it is a "code comment", something that is useful for you, but the computer ignores

# don't change these variables
wallet = 25
bank_account = 50
kaas_broodje = 2.25


# assign total money to the sum of wallet and bank_account
total_money = 

# You now purchase a kaas broodje from your wallet. subtract this value from total_money and form your wallet. 
total_money = total_money - 
wallet = wallet - 


#print all 3 values. Don't change these print statements. They'll print first the string, then the value of the variable listed after the comma
print("Total sum in wallet:", wallet)
print("Total sum in Bank:", bank_account)
print("Total money all together:", total_money)

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Changing & Updating Variables

Changing variables is critical in computer programs, and one of the most important things we can do. Imagine we have a distance sensor, that only reads the distance to an object once. Not a very useful design! We always need to be constantly re-writing the value of distance in order to update our code. Let’s practice

Task 1: Using and Modifying Variables

Here’s a series of calculations using variables. Your task is to determine the final values of boxes and items.

boxes = 3 
items = boxes * 5  # items = ?
items = items - 4  # items = ?
boxes += 2         # boxes = ?
boxes *= 2         # boxes = ?

Click to reveal a hint

# Use arithmetic operators for changing values and shorthand for assignments such as += and -=.

Click to reveal the expected final values

boxes = 10
items = 11

Task 2: Calculate Total Price

Calculate the total price. Copy and paste the following code in, and modify this code.

# Variables for item prices
item1 = 10
item2 = 20
tax = 0.08
# Don't change the above code ^^^

# Assign total_price to the sum of item1 and item2
total_price = 

# Assign tax_amount to the total_price multiplied by tax
tax_amount = 

# Assign final_price to the total_price plus the tax_amount
final_price = 

# Here we're printing the final values, don't change the code below!
print("Total Price:", total_price)
print("Tax Amount:", tax_amount)
print("Final Price:", final_price)

Click to reveal a hint

# Use basic arithmetic to calculate totals and percentages.

Click to reveal the expected output if completed correctly

Total Price: 30
Tax Amount: 2.4
Final Price: 32.4

Task 3: Calculate Area and Perimeter

again, copy and paste the following code into your code.py file.

# Dimensions of a rectangle
length = 15
width = 10
# Don't change the above code ^^^

# Assign area to the product of length and width
area = 

# Assign perimeter to the sum of all sides (2 * length + 2 * width)
perimeter = 

# Here we're printing the final values, don't change the code below!
print("Area:", area)
print("Perimeter:", perimeter)

Click to reveal a hint

# For area, use multiplication, and for perimeter, account for both pairs of sides.

Click to reveal the expected output if completed correctly

Area: 150
Perimeter: 50