🔁 Python if...else Statement – Binary Control Flow Explained

🧲 Introduction – Why if...else Matters

Real-world programs often need to make binary decisions—if something is true, do one thing; otherwise, do another. Python’s if...else structure helps you implement this logic efficiently.

For example:

• ✅ If a user is logged in, show the dashboard.
• ❌ Else, redirect to the login page.

Let’s explore how to write and use if...else statements in Python with real-world clarity.

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🔑 Syntax of if...else

if condition:
    # code block if condition is True
else:
    # code block if condition is False

• The if evaluates a condition.
• If it’s True, the indented block under if runs.
• Otherwise, the else block runs.

💡 Note: Use proper indentation—Python relies on it!

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🧪 Example: Even or Odd Number Checker

num = int(input("Enter a number: "))

if num % 2 == 0:
    print("The number is even.")
else:
    print("The number is odd.")

✅ Output (if num = 7)

The number is odd.

✅ Explanation:

• The condition num % 2 == 0 checks for evenness.
• If True, it prints “even”, otherwise “odd”.

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🧮 Example: Simple Login Check

username = input("Enter username: ")

if username == "admin":
    print("Welcome, admin!")
else:
    print("Access denied.")

✅ Output:

Access denied.

📘 Best Practice: Use == for comparison, not = (which is assignment).

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🧱 Nested if...else Example

age = int(input("Enter your age: "))

if age >= 18:
    if age >= 65:
        print("You are a senior citizen.")
    else:
        print("You are an adult.")
else:
    print("You are a minor.")

✅ Output (if age = 70):

You are a senior citizen.

💡 Tip: Avoid excessive nesting for readability. Use elif if appropriate.

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⚠️ Common Errors and Fixes

❌ Error	⚠️ Reason	✅ Fix
if x = 5:	Uses = instead of ==	if x == 5:
Missing colon	SyntaxError	Add : after condition
Incorrect indentation	IndentationError	Indent blocks consistently
Unreachable else block	Condition is always True or False	Re-check logic

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📊 Comparison Table: if vs if...else

Feature	if Only	if...else
Condition True	Executes code block	Executes if block
Condition False	Skips the block (does nothing)	Executes else block
Binary Logic	No	Yes
Suitable For	Optional execution	Yes/No decisions

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📌 Summary – Python if...else Statement

• ✅ Purpose: To control the flow of a program by executing one block of code when a condition is True, and another when it’s False.
• 🔠 Syntax:

if condition:
    # run if true
else:
    # run if false

🔍 Key Takeaways

• if...else enables binary decisions in your Python program.
• The else block runs only if the if condition fails.
• Python depends on indentation—not braces {}—to define blocks.
• Use with logical operators like and, or, not for compound conditions.

⚙️ Real-World Relevance

• Used in authentication, form validations, workflow branching, and more.
• Foundation for more advanced decision-making tools like elif and match-case.

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❓ FAQ – Python if...else Statement

❓ What is the purpose of else in Python?

else provides an alternate code path when the if condition evaluates to False.

❓ Can I use else without if?

No. else must always follow an if (or elif) statement.

❓ Can I put if...else on one line?

Yes, but it should only be used for simple expressions:

print("Even") if num % 2 == 0 else print("Odd")

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