Introduction to Python Programming

Introduction: When Programs Need to Think

So far, your Python programs have followed a straightforward path: execute one line after another, from top to bottom. But what if you want your program to behave differently based on certain conditions? What if it needs to decide whether to grant access, apply a discount, or display a specific message? This is where conditional logic comes into play.

Conditional statements are the bedrock of any intelligent, interactive program. They allow your code to evaluate conditions and execute different blocks of code based on whether those conditions are True or False. Think of it like a fork in the road, where your program chooses its path based on a question it asks itself.

In this chapter, UdaanPath will guide you through Python's primary tools for decision-making: the if, elif, and else statements. Mastering these is crucial for building dynamic and responsive applications.

Core Concepts: The Decision-Making Toolkit

if condition:
    # Code to execute if condition is True
    # (This block MUST be indented)
                    

user_age = 20

if user_age >= 18:
    print("You are eligible to vote!")
print("This message always prints.") # This line is outside the if block
                    

You are eligible to vote!
This message always prints.

user_age = 16

if user_age >= 18:
    print("You are eligible to vote!")
print("This message always prints.")
                    

This message always prints.

if condition:
    # Code if condition is True
else:
    # Code if condition is False
                    

username = "admin"
password = "123"

if username == "admin" and password == "secure_pass":
    print("Login successful! Welcome, admin.")
else:
    print("Login failed. Invalid username or password.")

# What if password was "secure_pass"?
username = "admin"
password = "secure_pass"

if username == "admin" and password == "secure_pass":
    print("Login successful! Welcome, admin.")
else:
    print("Login failed. Invalid username or password.")
                    

Login failed. Invalid username or password.
Login successful! Welcome, admin.

if condition1:
    # Code if condition1 is True
elif condition2:
    # Code if condition1 is False AND condition2 is True
elif condition3:
    # Code if condition1 & condition2 are False AND condition3 is True
else:
    # Code if ALL preceding conditions are False
                    

score = 85

if score >= 90:
    grade = "A"
elif score >= 80: # This is checked ONLY if score is NOT >= 90
    grade = "B"
elif score >= 70: # This is checked ONLY if score is NOT >= 80
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F" # This is reached if score is < 60

print(f"With a score of {score}, your UdaanPath grade is: {grade}")

score = 95 # Test with A
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"
print(f"With a score of {score}, your UdaanPath grade is: {grade}")
                    

With a score of 85, your UdaanPath grade is: B
With a score of 95, your UdaanPath grade is: A

is_logged_in = True
user_role = "admin" # Can be "admin", "editor", "viewer"

if is_logged_in:
    print("User is logged in.")
    if user_role == "admin":
        print("Welcome, Administrator! Full access granted.")
    elif user_role == "editor":
        print("Welcome, Editor! You can modify content.")
    else: # user_role is "viewer" or something else
        print("Welcome, Viewer! You have read-only access.")
else:
    print("Please log in to access the system.")

# Test with a different scenario
print("\n--- Another Scenario ---")
is_logged_in = False
if is_logged_in:
    print("User is logged in.")
    if user_role == "admin":
        print("Welcome, Administrator! Full access granted.")
    elif user_role == "editor":
        print("Welcome, Editor! You can modify content.")
    else:
        print("Welcome, Viewer! You have read-only access.")
else:
    print("Please log in to access the system.")
                    

User is logged in.
Welcome, Administrator! Full access granted.

--- Another Scenario ---
Please log in to access the system.

age = 22
has_ticket = True
is_vip = False

# Using 'and'
if age >= 18 and has_ticket:
    print("Welcome to the event!")
else:
    print("Sorry, you cannot enter.")

# Using 'or'
if is_vip or age >= 21:
    print("You can access the VIP lounge.")
else:
    print("VIP lounge access denied.")

# Using 'not'
if not is_vip:
    print("You are a regular guest.")
                    

Welcome to the event!
VIP lounge access denied.
You are a regular guest.

status = "pending"

if status == "pending":
    pass # TODO: Implement notification logic later
elif status == "completed":
    print("Task completed successfully.")
else:
    print("Unknown status.")
                    

$ python -c 'status = "pending"; if status == "pending": pass; elif status == "completed": print("Task completed successfully."); else: print("Unknown status.")'

$ python -c 'status = "completed"; if status == "pending": pass; elif status == "completed": print("Task completed successfully."); else: print("Unknown status.")'
Task completed successfully.

Key Takeaways & Best Practices

• Indentation is paramount: Python uses it to define code blocks. Be consistent (4 spaces recommended).
• Conditions evaluate to Booleans: The expression after `if` or `elif` must result in `True` or `False`.
• Use `elif` for sequential, mutually exclusive checks: This is generally cleaner and more efficient than deeply nested `if` statements for multiple related conditions.
• Order Matters in `elif` chains: Conditions are checked top-to-bottom, and the first `True` block executes, skipping the rest. Structure your conditions carefully (e.g., from most specific to most general).
• Combine with Logical Operators: `and`, `or`, `not` allow you to create powerful and expressive conditions without excessive nesting.
• Consider Edge Cases: When writing conditionals, think about all possible inputs and scenarios, including unexpected ones, to ensure your program behaves as intended.

Interview Tip: You might be asked to differentiate between using `elif` vs. nested `if`s, or to identify issues with incorrect conditional logic flow. Also, be ready to explain Python's reliance on indentation.

Mini-Challenge: UdaanPath Event Ticket Eligibility!

Let's put your conditional logic skills to the test! Create a Python script named `event_eligibility.py` that determines if a user can attend a special UdaanPath workshop.

• Define variables: `user_age` (integer), `has_udaanpath_badge` (boolean), `is_vip_member` (boolean). Initialize them with sample values (e.g., `user_age = 17`, `has_udaanpath_badge = True`, `is_vip_member = False`).
• Implement the following logic using `if-elif-else`:
  • If `user_age` is less than 18, print: "Sorry, you must be 18+ to attend."
  • Else if `is_vip_member` is `True`, print: "Welcome, VIP! You have priority access to the workshop."
  • Else if `has_udaanpath_badge` is `True` and `user_age` is 18 or older, print: "Welcome, UdaanPath Learner! Enjoy the workshop!"
  • Else (if none of the above conditions are met), print: "You are not eligible for this workshop."
• Test your script with at least three different combinations of `user_age`, `has_udaanpath_badge`, and `is_vip_member` to cover different outcomes.

Run your script and observe the decision-making in action!

# Test Case 1: Underage
# user_age = 17
# has_udaanpath_badge = True
# is_vip_member = False

# Test Case 2: VIP Member
# user_age = 25
# has_udaanpath_badge = False
# is_vip_member = True

# Test Case 3: Eligible UdaanPath Learner
# user_age = 20
# has_udaanpath_badge = True
# is_vip_member = False

# Your conditional logic here:
# if ...
#     print(...)
# elif ...
#     print(...)
# ...
                

$ python event_eligibility.py
Sorry, you must be 18+ to attend.
Welcome, VIP! You have priority access to the workshop.
Welcome, UdaanPath Learner! Enjoy the workshop!