Conditionals

Conditionals let your program make decisions, running different code depending on whether something is true or false.

Comparison operators

Use these to compare values (they return True or False):

  • == equal to
  • != not equal to
  • > greater than
  • < less than
  • >= greater than or equal to
  • <= less than or equal to

Examples:

print(3 == 3)   # True
print(4 != 5)   # True
print(7 > 10)   # False

Common mistake: Always use == to compare values. = is for assigning a value to a variable.


Basic if / else

Run code only when a condition is true.

age = 18
if age >= 18:
    print("You can vote") # this line ONLY runs if the condition is true
else:
    print("You are too young to vote")

Important: Python uses indentation (spaces) to group the code inside the if or else blocks.


elif (else if)

Check multiple conditions in order.

score = 75
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("Failing")

Python checks conditions from top to bottom.
As soon as one condition is true, that code runs and the rest is skipped.


Logical Operators: and, or, not

Use these to create special conditions for if statements:

  • and: all must be true
  • or: at least one must be true
  • not: reverses True/False
age = 20
has_id = True
if age >= 18 and has_id:
    print("Allowed")

if age < 13 or age > 65:
    print("Special pricing")

if not has_id:
    print("Please show ID")

Truthiness

Values can be treated as True or False in conditions:

  • Numbers: 0 is False, any nonzero is True
  • Empty containers ("", [], {}) are False; non-empty are True
  • None is False
if "":
    print("won't print")
if [1,2]:
    print("list is truthy")

Truthy just means that it evaluates to True

This is useful when checking if something exists or has content:

name = ""
if name:
    print("Name provided")
else:
    print("Name is empty")

Nested Conditionals

You can put if blocks inside other if blocks,

x = 10
if x > 0:
    if x % 2 == 0:
        print("positive even")
    else:
        print("positive odd")

Remember, modulus (%) gives the remainder

Make sure to keep code readable. If nesting gets deep, consider:

  • using elif instead
  • breaking logic into smaller steps

Challenge

Write a program that asks the user for a number and prints whether it's positive, negative, or zero.

Try to use if, elif, and else.

Hint 1: Converting input

Remember to convert the user's input to a number using int() or float() before comparing.

Hint 2: Compare in the right order

Check > 0 first for positive, < 0 for negative, and use else for zero (or check == 0).


Show example solution 
number = float(input("Enter a number: "))
if number > 0:
    print("Positive")
elif number < 0:
    print("Negative")
else:
    print("Zero")

Key Takeaways

  • Use comparison operators (==, !=, >, <, >=, <=) to build conditions.
  • if, elif, and else let your program choose one path of code to run.
  • Combine conditions with and, or, and not to express complex rules.
  • Python treats some values as truthy/falsey — empty containers and 0 are falsey.
  • Keep conditions simple and readable; test edge cases like zero or empty input.