Python Programming: Variables

Lesson Objectives

After this lesson, you will be able to…

Create and re-assign numerical and string variables.
Use numerical operators.
Print complex variable structures.

What’s a Variable?

Turn to the person next to you, and together come up with as many definitions for the word “variable” as you can.

Consider contexts such as mathematics, the sciences, weather, etc.
No cheating! Phones off and laptops closed.

Variable

Variables:

Are boxes that can hold all kinds of information for you.
Make it easier to store and re-use values.
Are the most basic piece of code.

To use a variable, we simply announce that we want to use it (we declare it).

# I've eaten 3 cupcakes
cupcakes_ive_eaten = 3
print(cupcakes_ive_eaten)
# Prints 3

Teaching Tips:

Bring up a repl.it (they haven’t seen an interpreter yet!) and run this. It’s already written in here.
Point out the print statement and comments, as review of the prework.
For variation, assign simple tasks like printing negatives and decimals.

Talking Points:

While variables can really be anything, we’ll keep it simple to start by looking at numerical variables.
Go over the syntax!
“Here, we create a variable called cupcakes_ive_eaten and have it hold the number 3. This is also called assignment. If we print out the contents of cupcakes_ive_eaten, Python will tell us that cupcakes_ive_eaten currently holds 3. When you create a variable, it’s called declaring a variable. The syntax for declaring a variable is what you want the variable to be named, followed by an equal sign, and finally what you want the variable to hold. Notice the space before and after the equal sign! Also, note that in Python, the equal sign doesn’t evaluate things the way it does in math. Instead, it assigns them values. Here, we’re using the = to tell Python that the cupcakes_ive_eaten label is going to hold the value 3.”
“See that underscore? We’ll look at variable naming in a second!”

Naming Conventions: Mistakes and Syntax

Some common naming mistakes:

Not using meaningful names. delicious = 3 doesn’t mean anything - cupcakes_ive_eaten = 3 does!
Case sensitivity (CUPCAKES_IVE_EATEN and cupcakes_ive_eaten are not the same!)
No spaces or punctuation (“cupcakes i’ve eaten” isn’t allowed)
- This is invalid syntax
- Use snake_case: lowercase_letters_with_underscores (it’s in the official Python style guide)

Teaching Tips:

Bring up a replit - this one already has the code. As you go through, demo each of these and show it breaking.
Remind the students that if they make a typo, it will create a new variable!

Talking Points:

“You can name a variable anything you’d like, but you probably shouldn’t. If I called my variable”delicious“, would you have any idea that the number it held was the number of cupcakes I’ve eaten? Probably not. In Python, we try to name variables as closely as we can to what it actually is. Technically, variable names can be almost anything, so it’s up to us to be organized and thoughtful when we pick them!”
“Any name is allowed - Python doesn’t enforce helpfulness. Try to always have meaningful variable names so that later, you know what that variable holds!”
"Variables are case sensitive.
“When you want to use a variable name consisting of several words, you will get an error if you have spaces between the words. Try creating a variable called my number and you’ll get an error; specifically, one that says”Invalid Syntax."
“Syntax is the set of rules surrounding how a language looks. In the English language, we call syntax”grammar." For example, sentences always start with a capital letter and end with something such as a period, exclamation point, or question mark. Other languages have syntax, too, including programming languages like Python."
“Here, Python is telling us that putting a space between two words in a variable name is invalid syntax. It’s not proper grammar and it will not run.”

Discussion: Changing Values

What if, later, you eat more cupcakes? Now, this is wrong.

cupcakes_ive_eaten = 3

What do you think we need to do?

Discussion: Reassigning Variables

In the example below, what do you think the output of the code is?

cupcakes_ive_eaten = 3
print(cupcakes_ive_eaten)
cupcakes_ive_eaten = 4
print(cupcakes_ive_eaten)

Teaching Tips:

Bring up a repl.it and show this. Here is one that has this code.
Make it clear that only the last value is saved!

Talking Points:

“We can always change what a variable is holding; it’s just a container. Initially setting a variable to a value is known as assigning the variable. In our example above, we declared the cupcakes_ive_eaten variable and assigned it the value of 3.”
“If we eat more cupcakes, we can reassign the variable to a different value - here, 4.”
“We can reassign our variable as many times as we want. However, only the most recent value of a variable will be retained. Once a variable is reassigned, its original value is lost forever. If you think of a box, reassigning a variable is like throwing away what’s in the box and replacing it with something new.”

Quick Review

A variable is a box that holds a value.
It can be declared, called, and changed within your program.
When declaring variables, syntax and naming conventions matter!
Variables can be reassigned as often as you like, but only the most recent declaration counts.

UP NEXT: Math!

Mathematical Operators

Math works on numerical variables, too!

The +, -, * (multiply), and / (divide) operators work just like they do with regular math.

cupcakes_ive_eaten = 6 + 3
print(cupcakes_ive_eaten)
# Prints 9

cupcakes_ive_eaten = 6 - 3
print(cupcakes_ive_eaten)
# Prints 3

cupcakes_ive_eaten = 6 * 3
print(cupcakes_ive_eaten)
# Prints 18

cupcakes_ive_eaten = 6 / 3
print(cupcakes_ive_eaten)
# Prints 2

Even More Mathematical Operators

Beyond the +, -, * (multiply), and / (divide) operators, we have modulus and exponents.

making_exponents = 10 ** 2
print(making_exponents)
# Prints 100

more_exponents = 10 ** 3
print(more_exponents)
# Prints 1,000

making_modulus = 10 % 3
print(making_modulus)
# Prints 1

more_modulus = 6 % 2
print(more_modulus)
# Prints 0

Math On The Same Variable

You can reassign a variable using that very same variable - or other variables!

cupcakes_ive_eaten = 3
cupcakes_ive_eaten = cupcakes_ive_eaten + 1
print(cupcakes_ive_eaten)
# Prints 4.
cupcakes_left_in_box = 6
cupcakes_left_in_box = cupcakes_left_in_box - 1 print(cupcakes_ive_eaten)
# Prints 5.
cupcakes_left_in_box = cupcakes_left_in_box - cupcakes_ive_eaten print(cupcakes_ive_eaten)
# Prints 1.

Talking Points:

“You may have a scenario where you want to reassign a variable to itself. We do this so we don’t have to create extra variables. In our cupcakes example, we don’t want to have to use a whole new variable just to add a cupcake.”
“Note that this works because the right side of the equals sign is evaluated first. Thus, the 1 + 3 in the second line is happening before the reassignment. The right side becomes 4, then that value is set to cupcakes_ive_eaten.”
“A great thing about variables is that, because they store a value, you can use them later - even to create new variables. For example, if we have a variable declared cupcakes_in_a_box = 3, and we know that there are 5 cupcakes in a box, we can multiply them to determine the number of cupcakes I will eat. Here’s an example:”

Partner Exercise: Mathematical Operators

Pair up and choose roles:

Driver
Navigator

Try to code the below:

10 MINUTES

Teaching Tips

Facilitate a pair programming activity:
- Ask students to pair up.
- One student (the “driver”) should type the code on the slide into repl.it and run it, making sure it works.
- Then, working together, the two students should complete the code challenges in the comments.
Once students have all finished, show the answer and recap how it works.
Whenever possible, have students experiment beyond the prompts on the slide.
If students grasp the mathematical operators easily, encourage more difficult exercises in the embedded editor - using more than one variable or combining different math symbols. Give few different example exponents to try, like 5**6 or 10%2. Have them guess the answers before running it.

Talking Points - Let’s give it a shot! In a second, I’m going to ask you to pair up and program together in repl.it - In pair programming, one person serves as the driver – their hands are on the keyboard – and one person serves as the navigator – they provide guidance as the code is written. - When you pair up, decide who will be the driver and who will be the navigator. Then, follow the directions in the comments. - Raise your hand when you’ve managed to complete each step, and I’ll come around and provide feedback.

Repl.it Note: The code is all comments:

Make a variable to hold the number of guitars you own (3).
Make a variable to hold the number of guitars Nikhil owns (1).
You give 2 of your guitars to Nikhil, so subtract 2 from you and add 2 to Nikhil.

Reassignment Shorthand

This is okay:

my_num = 9
my_num = my_num + 7
# my_num is now 16

But this is better:

my_num = 9
my_num += 7 # += is short for theSameVariable = theSameVariable + 7
# my_num is now 16

This works with +=, -=, *=, /= - any math operations.

Partner Exercise: Numerical Reassignment

Get with the same partner, but switch driver and navigator roles.

In the environment below, follow the prompts:

10 MINUTES

Teaching Tips

Facilitate a pair programming activity:
- Ask students to pair up with the same partner, but switch roles.
- One student (the “driver”) should type the code on the slide into repl.it and run it, making sure it works.
- Then, working together, the two students should complete the code challenges in the comments.
Once students have all finished, show the answer and recap how it works.
After showing the answer, continue showing experimentations. Spend the majority of the time on the interactive slide - get students doing different shorthands (+=, -=). These are built into the slide in comments. Encourage experimentation for advanced students.

Talking Points - Let’s give it a shot! In a second, I’m going to ask you to pair up and program together in repl.it - Remember, in pair programming, one person serves as the driver – their hands are on the keyboard – and one person serves as the navigator – they provide guidance as the code is written. - Follow the directions in the comments. - Raise your hand when you’ve managed to complete each step, and I’ll come around and provide feedback.

Repl.it Note: The code is all comments:

Declare two variables num1 and num2 and assign them to any numbers you’d like.
Set num1 to the result of subtracting num1 from the num2.
Create a new variable num3 that will help us tell if num2 is even or odd.
Using shorthand, add 5 to num1.
Print out num1, num2, and num3

Important Aside: Even or Odd?

Is 6 even or odd?

Is 7 even or odd?

How do you think a computer knows?

Modulus operator shows the remainder of a division problem.

Modding by 2 only gives a 0 or a 1.

4 % 2:
- 4 % 2 = 0. Even!
5 % 2:
- 5 % 2 = 1. Odd!

Quick Review

A variable is a value that can be defined, declared, called and changed within your program.
- my_number = 5
Naming:
- Variable names are case sensitive.
- Use snake_case!
Variables can be reassigned as often as you like, but only the most recent declaration counts.
Python can do math using operators, such as +, -, *, and /
- You can shorthand the math assignments: my_num += 7

Taking a Breather

That was a lot of math!

When it comes down to it, computers operate with a simple, straightforward logic.

Let’s switch gears. Up next: Strings!

Introducing Strings

A character is:

Anything on your keyboard , such as a letter or a number.
“Apple” is five characters: a, p, p, l, e.
Spaces count! (they’re on the keyboard!)

A string is:

A complete list of characters.
“Apple”
“Chocolate Cupcake”
This entire sentence: “Hello, you are 1 of a kind!”

How Do I Create Strings in Python?

You tell Python that your variable will hold a string using quotation marks.

box_contents = "cupcakes" # This is a string
print(box_contents) # It's a normal variable - we can print it.
best_snack = "Frosted Cupcakes" # This is a string.
cupcakes_ive_eaten = 5 # No quotes - this is a number.
cupcakes_ive_eaten_as_string = "5" # Because this is in quotes, this is a string.

We Do: Declaring Strings

A “We Do” means let’s practice together. Follow along!

We’ll declare a variable called name and assign it the value Marty
We’ll declare a variable called car and assign it the value Delorean
We’ll declare a variable called speed and assign it the string value "88"
We’ll print out these variables
We’ll add 4 to speed- what happens?

We Do: Declaring Strings

Talking Points:

“Let’s declare some variables in and print them. You’ll be doing a lot of programming, and if you practice typing variable declarations out, you’ll get faster every time.”

After you run the very last one…

“It’s important to note that you cannot do math on strings! Even if what’s inside the quotations is a number, Python will interpret it as a word.”
“Can you predict what the code below will print?” <wait for them to discuss, then run it.>
“In our last example, instead of printing 92 like you might expect, Python will throw an error. It sees speed as a string, not a number, so it doesn’t know how to do addition on it.”
“Make sure that when you want a numerical value you do not use quotes!”

Teaching Tips:

Do this with the students! But make sure they do it, too, so they get the typing practice.
Go over the phrase “numerical variable” versus “string variable”
Give them a second to guess the math failure before doing it.

String Concatenation

+ on:

Numerical variables adds (5 + 5 = 10).
String variables concatenate ("Doc" + "Brown" = "DocBrown").
- Pssst: Pronunciation tip: con-CAT-en-ATE
Numerical strings concatenate to new strings! ("5" +“4”=“54”`)

first_name = "Doc"
last_name = "Brown"
full_name = first_name + last_name
print full_name
# Prints "DocBrown".

We Do: Spaces in Concatenation

It’s another “We Do.” Let’s do this together - follow along!

To begin: sentence = name + "is driving his" + car + speed

We expect the sentence to be Marty is driving his Delorean 88mph. Is that what we got?

Teaching Tips:

Do this on the slide, but make sure students follow along.
Explain it as you go. After students see the lack of space, show them adding in the spaces yourself: sentence = name + " is driving his " + car + " " + speed. Then, show commas: print(name, "is driving his", car, speed)
Stress that commas only work for printing, not for concatenation.

Talking Points:

Python put the strings together, but do you notice anything wrong? There is no space between the words! This is because we didn’t add the spaces in. It’s just one of many reasons why we have to carefully watch our spacing and grammar!"
“Since a space is a character - it’s on the keyboard - we can make it a string. Therefore, we can add it into our concatenation. By default, concatenation doesn’t have spaces - you’ll always have to add them yourself.”
“You can also print directly; you don’t necessarily need an extra variable. To print strings next to each other, you separate them with a comma. Then, Python will add the space for you. This isn’t concatenating variables, but it’s useful to know!” Change code to: sentence = name + " is driving his " + car + " " + speed
“When printing, commas also create spaces.” Change code to: print(name, "is driving his", car, speed)

Repl.it Note: The code is:

name = "Marty"
car = "Delorean"
speed = "88mph"

Strings and Printing: Review

Strings are made with quotes:

name = "Marty"
car = "Delorean"
speed = "88"

String Concatenation - we need to add the spaces!

sentence = name + " is driving his " + car + " " + speed
string_numbers = "88" + "51"
# string_numbers = 8851

To easily create spaces while printing:

print(name, "is driving his", car, speed)

Discussion: Some Common Mistakes: 1

Do you think this will run? If yes, what does it print?

my_num
print(my_num)

Discussion: Some Common Mistakes: 2

How about this? Does it run? If so, what does it print?

my_num = 5
print()

Discussion: Some Common Mistakes: 3

How about this? Does it run? If so, what does it print?

my_num = 5
my_string = "Hello"
print(my_num + my_string)

Discussion: Some Common Mistakes: 4

One last question. What does this do?

my_num1 = "10"
my_num2 = "20"
print(my_num1 + my_num2)

Q&A and Summary

We learned a lot today!

We created, used, and re-assigned number and string variables.
We used the numerical operators + - / * // %
We did some complex stuff with the print function!

Congrats! You’ve finished your first programming lesson!

Python Programming: Variables

Lesson Objectives

What’s a Variable?

Variable

Naming Conventions: Mistakes and Syntax

Discussion: Changing Values

Discussion: Reassigning Variables

Quick Review

Mathematical Operators

Even More Mathematical Operators

Math On The Same Variable

Partner Exercise: Mathematical Operators

Reassignment Shorthand

Partner Exercise: Numerical Reassignment

Important Aside: Even or Odd?

Quick Review

Taking a Breather

Introducing Strings

How Do I Create Strings in Python?

We Do: Declaring Strings

We Do: Declaring Strings

String Concatenation

We Do: Spaces in Concatenation

Strings and Printing: Review

Discussion: Some Common Mistakes: 1

Discussion: Some Common Mistakes: 2

Discussion: Some Common Mistakes: 3

Discussion: Some Common Mistakes: 4

Q&A and Summary

Additional Resources