Conditionals

Conditionals allow you to execute instructions only if a certain condition is true.

if

Its simplest form is:

🐍 Python Script

    if condition:
        instructions

where the condition can be for example a comparison between two values.

Indentation

The block of instructions is moved to the right by 4 spaces, or one tab. That's what we call indentation and we use it to identify the instructions that need to be executed if the condition is true.

elif

Sometimes you want to test if a condition is true and if not, you want to test another condition. And if the other condition is not true either, you may want to test another condition, etc. In order to achieve this, you can add as many elif as you want:

🐍 Python Script

    if condition:
        instructions
    elif condition:
        instructions
    elif condition:
        instructions
    ...

else

And finally, if you want to select all the cases where no conditions written before are true, you can use the keyword else. You can only add one else at the very end of your conditional statement. It does not need any condition since it will use the opposite of the conditions written before.

🐍 Python Script

    if condition:
        instructions
    elif condition:
        instructions
    elif condition:
        instructions
    ...
    else:
        instructions

Examples

Simple ExampleComplex Example

🐍 Python Script

age = int(input("Give me your age: "))
if age >= 18:
    print ("You are an adult.") 

This code will execute the instruction to print the message only if the value saved in the variable age respects the condition >=18.

🐍 Python Script

number = int(input("Give me a number: "))
if number < 0:
    print("The number is negative.")
elif number > 5:
    print("The number is higher than 5.")
elif number == 3:
    print("The number is equal to 3.")
else:
    print("The number is not negative, not higher than 5 and not equal to 3.")

This code will execute the right instructions based on the value saved in the variable number.

If the value is negative, it will say that is negative. Else if the value is higher than 5, it will say that the number is higher than 5. Else if the value is equal to 3, it will say that it is equal to 3. If none of those conditions are met, it will execute the instruction after the else.

Indentation

As explained above, the indentation (moving your instruction 4 spaces to the right) is very important in Python. Let's compare those two script:

1. 🐍 Python Script

   x = int(input("Give me a number: "))
   if x > 0:
       print("The value is striclty positive.")
   print("Goodbye")
   

2. 🐍 Python Script

   x = int(input("Give me a number: "))
   if x > 0:
       print("The value is striclty positive.")
       print("Goodbye")
   

When you execute the first one, Goodbyewill be printed each time, no matter the value of x.

However, the second one will print Goodbye only if x is higher than 0. The instruction print("Goodbye") is indented, so it is part of the if and executed only if the condition is true.