Until now, our programs have always executed in the same way.

For example,

print("Welcome!")

will always print the same message, no matter what happens.

But programs often need to make decisions.

For example:

  • if a user is an adult - show one message;
  • otherwise - show another one.

For this purpose, Python provides the if statement.

The simplest condition

For example,

age = 20

if age >= 18:
    print("Access granted")

This code reads almost like a normal sentence:

If the age is greater than or equal to 18, print the message.

Since the age is 20, the program prints:

Access granted

What if the condition is false?

Let’s change the age.

age = 15

if age >= 18:
    print("Access granted")

Now the program prints nothing.

The condition is false, so Python simply skips this block of code.

What does the colon mean?

Pay attention to this line:

if age >= 18:

There is a colon after the condition.

It tells Python:

A block of code that should run only if this condition is true starts here.

Why are the lines indented?

Look at the example again:

if age >= 18:
    print("Access granted")

The print() line starts slightly to the right.

This is called indentation.

In Python, indentation has special meaning.

Everything with the same indentation level after an if statement belongs to that condition.

For example:

if age >= 18:
    print("Access granted")
    print("Welcome!")

Both lines will execute only when the condition is true.

Executing a different action

Very often, we need to do something in the opposite case.

For this, Python provides else.

age = 15

if age >= 18:
    print("Access granted")
else:
    print("Access denied")

Now the program always prints one of the two messages.

Multiple options

Sometimes there are more than two possible outcomes.

For example:

score = 85

if score >= 90:
    print("Excellent")
elif score >= 75:
    print("Good")
else:
    print("Try again")

Python checks conditions from top to bottom.

As soon as one condition is true, the remaining conditions are skipped.

What is a boolean value?

Any condition in Python can be either true or false.

For example:

age >= 18

If the age is 20, the result is:

True

If the age is 15:

False

These values have a special data type called bool.

For now, it is enough to remember that every condition eventually becomes either True or False.

What we learned today

Today we learned how to make programs that can make decisions.

  • if executes code only when a condition is true.
  • else provides an alternative option.
  • elif allows checking additional conditions.
  • Indentation defines which lines belong to a condition.
  • Every condition in Python evaluates to either True or False.

Try it yourself

Write a program that asks the user for their age.

If the age is greater than or equal to 18, print:

Welcome!

Otherwise, print:

Access denied.

After that, try adding another option.

For example:

younger than 14; between 14 and 17; 18 or older. What comes next?

Now our program can choose between several options.

But sometimes we need to perform the same action many times.

For example:

  • print numbers from 1 to 10;
  • ask for a password until it is correct;
  • keep running a program until the user gives a command.

For this purpose, Python provides loops.

In the next article, we will learn about the while loop.