Python supports the usual logical conditions from mathematics:

Equals: a == b
Not Equals: a != b
Less than: a < b
Greater than: a > b

These conditions can be used in several ways, most commonly in “if statements” and loops.

1. if statement

An “if statement” is written by using the if keyword.

a = 33
b = 200
if b > a:
    print("b is greater than a")

In this example, we use the > operator to evaluate if b is greater than a. Since it is, the code block is executed. Remember: Python relies on indentation to define scope. A missing indentation will throw an error.

2. elif statement

The elif keyword is Python’s way of saying “if the previous conditions were not true, then try this condition”.

a = 33
b = 33
if b > a:
    print("b is greater than a")
elif a == b:
    print("a and b are equal")

3. else statement

The else keyword catches anything which isn’t caught by the preceding conditions.

a = 200
b = 33
if b > a:
    print("b is greater than a")
elif a == b:
    print("a and b are equal")
else:
    print("a is greater than b")

You can also have an else without an elif:

if b > a:
    print("b is greater")
else:
    print("a is greater")

4. Short Hand If (Ternary)

If you have only one statement to execute, you can put it on the same line as the if statement.

if a > b: print("a is greater than b")

Short Hand If … Else

If you have only one statement to execute, one for if, and one for else, you can put it all on the same line. This technique is known as Ternary Operators, or Conditional Expressions.

a = 2
b = 33
print("A") if a > b else print("B")

5. Nested if

You can have if statements inside if statements, this is called nested if statements.

x = 41

if x > 10:
    print("Above ten,")
    if x > 20:
        print("and also above 20!")
    else:
        print("but not above 20.")

6. Logical operators in conditions

You can combine conditional statements using logical operators and, or, and not.

And

The and keyword is a logical operator, and is used to combine conditional statements. It returns True only if both conditions are True.

a = 200
b = 33
c = 500
if a > b and c > a:
    print("Both conditions are True")

Or

The or keyword returns True if at least one of the conditions is True.

if a > b or a > c:
    print("At least one of the conditions is True")

Not

The not keyword reverses the result of the conditional statement.

if not a > b:
    print("a is NOT greater than b")

7. Pass statement in if

if statements cannot be empty. If you for some reason have an if statement with no content, put in the pass statement to avoid getting an error. This is useful as a placeholder when you are scaffolding your code.

a = 33
b = 200

if b > a:
    pass # Code to be written later