5. Conditional Statements

A conditional statement, symbolized by p → q, is an if-then statement in which p is a hypothesis and q is a conclusion. Let’s have a look at some examples to illustrate it.

5.1. if Statement

The most basic conditional statement is the if-statement with an if-clause only. If the condition after the if keyword is true, the statements belonging to the if-block are executed. Otherwise they are ignored.

if 5 < 3:
    # The following statements are NOT executed
    x = 5
    print(x + 3)

print("Hello World")

if True:
    print("True is True, of course!")


If you execute the above example, you’ll get:

Hello World
True is True, of course!

if-statements can be arbitrarily nested as shown in the following example.

x, y = 5, 3

if x == 5:
    if y == 3:
        print("x = 5 and y = 3")
        print("x = 5 but y != 3")
    print("x != 5 and y != 3")

Keep in mind, that this decreases readability, so if you have many levels of nested if-statements, you may want to refactor you code. With the above example in mind, let’s assume you only care about the fact, that x is equal to 5 and y is equal to 3. You could combine it using the logical operator and.

if x == 5 and y == 3:
    print("x = 5 and y = 3")
    print("x != 5 and y != 3")

5.2. else Clause

Optionally, you can append an else-clause to the if-clause, which is only executed if the condition belonging to the if is not True.

if False:
    print("The condition is True!")
    print("Sadly, the condition is False.")


Sadly, the condition is False.

Note: There can only be one else-clause per if-statement.

5.3. elif Statement

Sometimes it’s helpful to check against another condition before you execute a default (else) case. Luckily, Python provides us the elif-clause. The statement belonging to elif is only evaluated if the initial if condition is False.

Note: There can be an arbitrary number of elif-clauses per if-statement. They are evaluated from top-down until the first matches.

x = 5

if x > 2 and x < 7:
    print("x is between 2 and 7 exclusively")
elif x > 2:
    print("x is greater than 6")  # The first condition is already evaluated as False, so if x is greater than 2 it has to be greater than 6 (>= 7)
    print("x is smaller than 3")  # The first two conditions are already evaluated as False, so x has to be smaller than 3 (<= 2)

Note: You can rewrite the first condition to:

if 2 < x < 7:
    print("x is between 2 and 7 exclusively")
That's the beauty of Python!

5.4. Conditional Expression

A conditional expression is comparable to the ternary-operator, which you can find in other languages. In it’s simples form, the Python conditional expression (or conditional operator or ternary operator) looks like this:

<expr1> if <conditional_expr> else <expr2>

conditional_expr is evaluated first. If it evaluates to True, then expr1 is evaluated and the program flow continues. However, if it evaluates to False, then expr2 is evaluated.

>>> 5 if 5 < 3 else 3
>>> "Hello World" if True else False
'Hello World'

You can find more information about Python’s conditional expression in the resources listed below.

5.5. Further Resources