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Suppose we are trying to write a division function that accepts only integers. We have written out our division algorithm, but regardless of what it is, we know it cannot be executed when the divisor is 0. Thus, we should execute the algorithm only when the divisor is not 0.

In order to write our code so that it only executes when the divisor is non-zero, we need to use something called conditionals. These are statements that are checked for being True before executing a given block of code. Here is a basic example:

x = 2

if x == 2:
    print("Hello!")

In this case, we will output:

Hello!

To create a basic conditional, we write the keyword if followed by the condition that we want to check. In this case, we want to check if is x is equal to 2, as denoted by the operator ==. We then terminate the line with a colon.

Afterwards, all code that we want to exclusively execute given that the condition is True is written underneath, with each line tabbed in at least once. In this case, it is to print the phrase, "Hello!". Suppose we had the following code instead:

x = 3

if x == 2:
    print("Hello!")

In this case, nothing would have been printed out because x == 2 is now False.

Suppose that we want to execute another block exclusively if the condition we are checking is in fact False, we can use another keyword, else:

x = 3

if x == 2:
    print("Hello!")
else:
    print("Hi!")

In this case, we will output:

Hi!

As with the if statement, any code exclusively executed for else must be tabbed in at least once. Notice that we print out "Hi!" because the if condition is False, so we execute the code under the else block.

What if we want to check more than two conditions? We can use another keyword elif:

x = 3

if x == 2:
    print("Hello!")
elif x == 3:
    print("Hey!")
else:
    print("Hi!")

In this case, we will output:

Hey!

In this case, Python checks if x == 2. That statement is False, so we move on to the elif statement. This statement is True, so we execute the code under that block (tabbed in as with if and else). Note that the else block is only executed when all other statements (if and elif) are False.

It should also be noted that the else block is entirely optional. Sometimes the only thing we want to do exclusively if all the other statements are False is nothing. :)

We can have as many conditions as we like by including more elif statements:

x = 3

if x == 2:
    print("Hello!")
elif x - 1 == 2:
    print("Howdy!")
elif x == 3:
    print("Hey!")
else:
    print("Hi!")

In this case, we will output:

Howdy!

Several observations can be made here:

  • All the elif conditions must be placed in-between the if and else conditions.

  • Python checks conditional statements in order from top to bottom. Once one of the statements is True, Python will not check any of the other conditionals. In this case, the first conditional that is True is x - 1 == 2, which is why we print "Howdy!" and not "Hey!", even though that conditional is also True.

  • By “not check any of the other conditionals,” we mean any subsequent elif conditionals or the else block. If we write another if conditional later on, Python will resume checking conditionals:

    x = 3

if x == 2:
print("Hello!")
elif x - 1 == 2:
print("Howdy!")
elif x == 3:
print("Hey!")
else:
print("Hi!")

if x == 3:
print("Done!")

In this case, we will output:

Howdy!
Done!

Conditionals open up a whole new world of possibility for our code, especially since you can write as many elif statements as you like. That being said, it also introduces a lot of complexity in our writing, so we’ll stop for now and practice what we have learned:

  • Create a variable x. Then write code such that we print "Success" only when x is equal to "foo". (solution)

  • Add two elif statements so that we print "Okay" if x is equal to "car", and we print "Nice!" if the length of x is equal to 3. Note that the "car" conditional should come first. (solution)

  • Add an else conditional so that we print "Oops!" if all of the other conditionals from above failed. (solution)

  • Add a new if conditional subsequent to that first if-else code block that prints "Cow" if the length of x is equal 5. (solution)

  • Change your code so that x is initialized to Mouse. What will be output? (solution)

This is not an easy concept to master initially, so definitely practicing (multiple times) the syntax that has been taught here will be extremely important in order to become comfortable with this new functionality of ours.

When you’re feeling comfortable, feel free to move on to for loops!