Python Lesson 2 - Conditionals (if, elif, else)

1) Why we need conditionals

Most real programs must make decisions.

Examples:

• If the user enters the correct password, log them in.
• If CPU usage is above 90%, send an alert.
• If a file exists, read it; else create it.

In Python, decisions are written using conditional statements:

• if for the first check
• elif (else-if) for additional checks
• else for the “default” case

2) The bool type and boolean expressions

Python has a boolean type:

• True
• False

A boolean expression is any expression that results in a boolean.

print(10 > 5)     # True
print(10 == 5)    # False
print(10 != 5)    # True

In an if statement, Python evaluates the condition. If it is True, it runs the block.

3) Basic if syntax

age = 20

if age >= 18:
    print("Adult")

Important rules:

1. The condition ends with a colon :
2. The code block is defined by indentation (usually 4 spaces)
3. If the condition is False, the block is skipped

3.1 Indentation matters

This is correct:

x = 5
if x > 0:
    print("positive")
    print("still inside")
print("outside")

This is wrong (indentation error):

# if x > 0:
# print("positive")

4) if + else

Use else when you want a fallback action.

temp = 16

if temp >= 20:
    print("T-shirt")
else:
    print("Jacket")

Only one branch runs:

• If condition True -> if block runs
• If condition False -> else block runs

5) if + elif + else

Use elif when you have multiple cases.

score = 83

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")
else:
    print("F")

How Python executes it:

1. Check the if condition
2. If False, check the first elif
3. Keep checking elif blocks in order
4. If none match, run else (if present)

Important notes:

• You can have zero or more elif
• else is optional
• Only the first matching branch runs

6) Comparison operators

These return True or False:

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

Examples:

x = 10

print(x == 10)  # True
print(x != 10)  # False
print(x < 3)    # False
print(x >= 8)   # True

6.1 The biggest beginner mistake: = vs ==

• = assigns a value
• == compares two values

Wrong:

# if x = 10:
#     print("...")

Correct:

if x == 10:
    print("x is 10")

7) Boolean operators: and, or, not

7.1 and

and is True only if both sides are True.

age = 22
has_id = True

if age >= 18 and has_id:
    print("Enter")
else:
    print("No entry")

7.2 or

or is True if at least one side is True.

is_admin = False
is_manager = True

if is_admin or is_manager:
    print("Access granted")

7.3 not

not reverses the boolean.

logged_in = False

if not logged_in:
    print("Please login")

7.4 Grouping conditions with parentheses

Use parentheses to make your logic clear.

age = 19
has_ticket = True
is_vip = False

if (age >= 18 and has_ticket) or is_vip:
    print("Allowed")

8) Truthy and falsy (very important)

In Python, not only True and False exist. Many values are treated as boolean in conditions.

8.1 Falsy values

These are considered False:

• False
• None
• 0 and 0.0
• "" (empty string)
• [] (empty list)
• {} (empty dict)
• set() (empty set)

Everything else is considered truthy.

name = ""

if name:
    print("Name exists")
else:
    print("Name is empty")

8.2 Why this matters

Instead of writing:

# if len(items) > 0:
#     ...

You can write:

items = [1, 2, 3]
if items:
    print("Not empty")

9) Nested conditionals

You can put an if inside another if:

username = "admin"
password = "1234"

if username == "admin":
    if password == "1234":
        print("Welcome admin")
    else:
        print("Wrong password")
else:
    print("Unknown user")

Nesting is sometimes necessary, but too much nesting can make code hard to read.

Tip:

• If nesting becomes deep, consider rewriting the logic with and/or, or using functions and early returns.

10) Common patterns

10.1 Input validation (don’t crash on bad input)

input() returns a string. If you do int(input()) and the user types letters, your program crashes.

A safe pattern:

text = input("Enter age: ")

if text.isdigit():
    age = int(text)
    print(f"Your age is {age}")
else:
    print("Invalid age. Please enter numbers only.")

10.2 Ranges (important ordering)

Always go from most specific / highest range to lowest range.

Correct:

score = 95

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
else:
    print("Below 80")

Incorrect ordering:

# score = 95
# if score >= 80:
#     print("B")
# elif score >= 90:
#     print("A")

10.3 Simple menu program

print("1) Show profile")
print("2) Settings")
print("3) Exit")

choice = input("Choose: ")

if choice == "1":
    print("Profile")
elif choice == "2":
    print("Settings")
elif choice == "3":
    print("Bye")
else:
    print("Invalid option")

11) Common mistakes (and fixes)

11.1 Missing :

Wrong:

# if x > 0
#     print("...")

Correct:

if x > 0:
    print("...")

11.2 Comparing numbers as strings

If you forget to convert input to int, comparisons become wrong.

age = input("Enter age: ")

# age is a string
# "20" > "100" is True (because it compares text)

Fix:

age = int(input("Enter age: "))
if age > 100:
    print("Very old")

11.3 Using and / or incorrectly

Wrong:

# if x == 1 or 2:
#     print("x is 1 or 2")

Correct:

if x == 1 or x == 2:
    print("x is 1 or 2")

# or using membership
if x in (1, 2):
    print("x is 1 or 2")

–