LEARN COMPLETE PYTHON IN 24 HOURS

5. Control Flow (Decision Making)

Control flow lets your program make decisions — like "if it's raining, take umbrella" or "if age > 18, show adult content". Python uses if, elif, else for this.

5.1 if, elif, else Statements

Syntax (basic structure):

Python

if condition: # code runs if condition is True elif another_condition: # optional, can have many # code runs if previous was False and this is True else: # optional # code runs if all above are False

Important Rules:

• Colon : after condition

• Indentation (usually 4 spaces) is mandatory — Python uses it to know which code belongs to the block

• Conditions must evaluate to True or False

Example 1 – Simple if

Python

age = 20 if age >= 18: print("You are an adult. You can vote!")

Output (if age = 20):

text

You are an adult. You can vote!

Example 2 – if + else

Python

temperature = 15 if temperature > 30: print("It's very hot! Wear light clothes.") else: print("It's cool. Enjoy the weather!")

Output (temperature = 15):

text

It's cool. Enjoy the weather!

Example 3 – if + elif + else (most common)

Python

marks = 85 if marks >= 90: print("Grade: A+") elif marks >= 80: print("Grade: A") elif marks >= 70: print("Grade: B") elif marks >= 60: print("Grade: C") else: print("Grade: Fail")

Output (marks = 85):

text

Grade: A

Real-life Example – Traffic Light

Python

light = "red" if light == "green": print("Go!") elif light == "yellow": print("Slow down!") elif light == "red": print("Stop!") else: print("Invalid light color")

Output:

text

Stop!

Tip: You can have as many elif as you want, but else comes only at the end (and is optional).

5.2 Nested if-else

You can put if inside another if — called nested if.

Syntax:

Python

if condition1: if condition2: # both true else: # condition1 true, condition2 false else: # condition1 false

Example – Check eligibility for driving license

Python

age = 17 has_learner = True if age >= 18: if has_learner: print("You can apply for permanent license!") else: print("You need learner's license first.") else: print("You are too young to drive.")

Output (age = 17):

text

You are too young to drive.

Another Example – Login System (nested)

Python

username = "anshuman" password = "python123" if username == "anshuman": if password == "python123": print("Login successful! Welcome back.") else: print("Incorrect password.") else: print("User not found.")

Tip: Avoid too many levels of nesting (max 2–3) — code becomes hard to read. Use logical operators (and, or) instead when possible.

Better version using and:

Python

if username == "anshuman" and password == "python123": print("Login successful!") else: print("Invalid credentials.")

5.3 One-line if / Ternary Operator

Short way to write simple if-else in one line.

Syntax:

Python

value_if_true if condition else value_if_false

Examples:

1. Basic

Python

age = 20 status = "Adult" if age >= 18 else "Minor" print(status) # Adult

2. With print

Python

marks = 75 print("Pass" if marks >= 40 else "Fail") # Pass

3. Assign value

Python

temperature = 35 message = "Hot day" if temperature > 30 else "Normal day" print(message) # Hot day

4. Nested ternary (possible but avoid deep nesting)

Python

score = 92 grade = "A" if score >= 90 else "B" if score >= 80 else "C" print(grade) # A

When to use: For very short, simple decisions. When NOT to use: For complex logic — use normal if-else for readability.

5.4 pass Statement

pass is a placeholder — does nothing. Used when Python expects a block of code but you don't want to write anything yet.

Common Uses:

• Empty if/else blocks during development

• Empty functions/classes (later topics)

• Ignore certain conditions temporarily

Example:

Python

age = 15 if age >= 18: print("Adult section") else: pass # do nothing for minors right now (maybe add later)

Another Example – Future function

Python

def calculate_tax(income): if income > 1000000: # complex logic later pass else: print("No tax")

Without pass → IndentationError!

Tip: Use pass as a temporary "TODO" marker. Replace it later with real code.

Mini Project – Simple Decision Maker

Python

print("=== Weather Advisor 2026 ===") temp = int(input("Enter current temperature (°C): ")) if temp >= 35: advice = "Very hot! Stay indoors, drink water." elif temp >= 25: advice = "Warm day. Wear light clothes." elif temp >= 15: advice = "Pleasant weather. Enjoy a walk!" else: advice = "Cold! Wear jacket and stay warm." print(f"Advice: {advice}")

Sample Output:

text

=== Weather Advisor 2026 === Enter current temperature (°C): 28 Advice: Warm day. Wear light clothes.

This completes the full Control Flow (Decision Making) section — now your programs can make smart decisions!

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