2. Variables, Data Types, and User Input¶

2.1. What Does This Program Do?¶

Remember the four primitive data types we learned in our overview of Python. They are:

• int

• float

• string

• bool

For the following examples, consider the data type of each variable. What will the program output? Why?

2.2. Using Thonny¶

There are two ways to use Thonny: shell mode and program mode. In shell mode, you type Python expressions into the Python shell, and the interpreter immediately shows the result. The example below shows the Python shell at work.

The >>> is called the Python prompt. The interpreter uses the prompt to indicate that it is ready for instructions. We typed 2 + 3, then pressed Enter. The interpreter evaluated our expression and replied 5. On the next line it gave a new prompt indicating that it is ready for more input.

Working directly in the interpreter is convenient for testing short bits of code because you get immediate feedback. Think of it as scratch paper used to help you work out problems.

Alternatively, you can write an entire program by placing lines of Python instructions in a file and then use the interpreter to execute the contents of the file as a whole. Such a file is often referred to as source code. For example, we used Thonny to create a source code file named firstprogram.py with the following contents:

By convention, files that contain Python programs have names that end with .py . Thonny will save your files with a .py extension automatically, and you should be able to open them in Thonny by double clicking them in File Explorer (or Finder).

Check your understanding

2.3. Comments¶

As programs get bigger and more complicated, they get more difficult to read. Formal languages (e.g. programming languages) are dense, and it is often difficult to look at a piece of code and figure out what it is doing, or why. For this reason, it is a good idea to add notes to your programs to explain in natural language what the program is doing. These notes are called comments.

A comment in a computer program is text that is intended only for the human reader - it is completely ignored by the interpreter. In Python, the # token starts a comment. The rest of the line is ignored. Every program you write should start with a comment header, which could look something like this:

Notice that when you run this program, it only prints the phrase Hello, World! None of the comments appear. You’ll also notice that we’ve left a blank line in the program. Blank lines are also ignored by the interpreter, but comments and blank lines can make your programs much easier for humans to parse. Use them liberally!

Check your understanding

2.4. Variables¶

One of the most powerful features of a programming language is the ability to manipulate variables. A variable is a name that refers to a value.

Assignment statements create new variables and also give them values to refer to.

message = "What's up, Doc?"
n = 17
pi = 3.14159

This example makes three assignments. The first assigns the string value "What's up, Doc?" to a new variable named message. The second gives the integer 17 to n, and the third assigns the floating-point number 3.14159 to a variable called pi.

The assignment token, =, should not be confused with equality (we will see later that equality uses the == token). The assignment statement links a name, on the left hand side of the operator, with a value, on the right hand side. This is why you will get an error if you enter:

17 = n

A common way to represent variables on paper is to write the name with an arrow pointing to the variable’s value. This kind of figure, known as a reference diagram, is often called a state snapshot because it shows what state each of the variables is in at a particular instant in time. (Think of it as the variable’s state of mind). This diagram shows the result of executing the assignment statements shown above.

If you ask Python to evaluate a variable, it will produce the value that is currently linked to the variable. In other words, evaluating a variable will give you the value that is referred to by the variable.

In each case the result is the value of the variable.

Variables also have types; again, we can ask the interpreter what they are.

The type of a variable is the type of the object it currently refers to.

We use variables in a program to “remember” things, like the current score at the basketball game. But variables are variable. This means they can change over time, just like the scoreboard at a basketball game. You can assign a value to a variable, and later assign a different value to the same variable.

To see this, read and then run the following program. You’ll notice we change the value of day three times, and on the third assignment we even give it a value that is of a different type.

Check your understanding

2.5. Variable Names and Keywords¶

Variable names can be arbitrarily long. They can contain both letters and digits, but they have to begin with a letter or an underscore. You should use long, descriptive variable names. For example, if you were creating a program to calculate how much gas a car uses, a good variable name might be litres_per_100_kms. A bad variable name in that case would be l. Using a single letter as the variable name usually makes your program harder to understand for other people. Although that can be a bit annoying to type out a long variable name the first time, once you have typed it once in Thonny, you should be able to simply type the first few letters of the variable name, then press Ctrl-Space to have the rest of the variable name auto-complete for you. If that doesn’t work for you, check Thonny’s preferences.

The underscore character ( _) can also appear in a name. It is often used in names with multiple words, such as my_name or price_of_tea_in_china. This is the preferred way to write long variable names in Python, and you should use this style!

There are some situations in which names beginning with an underscore have special meaning, so a safe rule for beginners is to start all names with a letter.

Although it is legal to use uppercase letters, by convention we don’t. If you choose to use uppercase letters, remember that case matters. Bruce and bruce are different variables.

If you give a variable an illegal name, you get a syntax error. In the example below, each of the variable names is illegal.

76trombones = "big parade"
more$ = 1000000
class = "Computer Science 101"

76trombones is illegal because it does not begin with a letter. more$ is illegal because it contains an illegal character, the dollar sign. But what’s wrong with class?

It turns out that class is one of the Python keywords. Keywords define the language’s syntax rules and structure, and they cannot be used as variable names. Python has thirty-something keywords (and every now and again improvements to Python introduce or eliminate one or two):

You might want to keep this list handy. If the interpreter complains about one of your variable names and you don’t know why, see if it is on this list.

Programmers generally choose names for their variables that are meaningful to the human readers of the program — they help the programmer document, or remember, what the variable is used for.

2.6. User Input¶

If you want the user to type something, you can use the input() function. input() will always return a string. You will need to convert it to an int or a float if you are expecting a number.

The following example is not going to work when you try to run it. Can you figure out what is wrong and fix it? Hint: think about data types!

Do not look at this sample solution unless you have spent time attempting to create your own solution!

current_year = input("What is the current year?")
grad_year = input("What year will you graduate from high school?")

# convert user input to be integers, so we can subtract
current_year = int(current_year)
grad_year = int(grad_year)

year_difference = grad_year - current_year

print("You will graduate in", year_difference, "years.")

2.7. Practice Problems¶

Try the following practice problems. Be sure that you know how to do the question with paper/pencil before you attempt to write a solution in Python! You can either work directly in the textbook, or using Thonny. Either way, be sure to save your solution into your Computer Science 20 folder when you finish!

# Area of a Circle Calculator
# Dan Schellenberg
# Oct 18, 2017

pi = 3.14

radius = input("Please enter the radius: ")
radius = float(radius)  #convert input into a number

area = pi*radius**2

print("The area of the circle is", area)

# Area of a Rectangle Calculator
# Dan Schellenberg
# Oct 19, 2017

length = input("Enter the length of the rectangle: ")
width = input("Enter the width of the rectangle: ")

#convert user input into numbers
length = float(length)
width = float(width)

area = length * width
print("The area of the rectangle is", area)

# Gas Mileage Calculator
# Dan Schellenberg
# Oct 19, 2017

kilometers_driven = input("How many kilometers did you drive? ")
litres_used = input("How many litres of gas did it take? ")

#convert user input to numbers
kilometers_driven = float(kilometers_driven)
litres_used = float(litres_used)

gas_usage = litres_used / kilometers_driven * 100

print("Your car is using", gas_usage, "L/100km")