Flow of Control & Errors

Introduction

Every program you've written so far ran one line after another, top to bottom, with no choices along the way. Real programs need to make decisions — this chapter starts that, beginning with how Python marks out which lines belong together.

Concept

Control flow is the order in which a program's statements actually execute. Python has three basic types:

• Sequential — statements run one after another, top to bottom (everything you've written until now).
• Selection — the program chooses between different paths based on a condition (covered in this chapter).
• Iteration — a block of statements repeats (covered in the next chapter).

Indentation

Python uses indentation — leading whitespace — to mark which lines belong to the same block, instead of curly braces {} like many other languages. Every statement inside the same block must be indented by the exact same amount.

Worked Example

marks = 75
if marks >= 40:
    print("You have passed.")
    print("Well done.")
print("Result processing complete.")

The two print() lines directly under if marks >= 40: are indented identically, so Python treats them as one block belonging to that if. The final print() is back at the original indentation level, so it is not part of the if block — it runs regardless of whether marks >= 40 is True or False.

Warning: Mixing tabs and spaces, or indenting two lines of the same block by different amounts, raises an IndentationError — Python treats consistent indentation as part of the code's actual meaning, not just a style choice.

Explanation

This is the single biggest syntax difference a student coming from a language that uses { } needs to unlearn: in Python, indentation is the block structure. There is no closing symbol to mark where a block ends — it ends the moment a line returns to a shallower indentation level.

Summary

Control flow is the order statements execute in: sequential (top to bottom), selection (choosing a path), and iteration (repeating a block). Python marks out blocks using consistent indentation instead of braces — every statement in the same block must be indented by exactly the same amount.

Introduction

The simplest kind of selection is a single yes/no check: run a block of code only if a condition is true, otherwise skip it entirely. That's exactly what Python's if statement does.

Concept

if condition:
    statement(s)

condition is any expression that evaluates to True or False. If it's True, the indented block runs. If it's False, the block is skipped completely, and the program continues with whatever comes after it.

Worked Example

temperature = 40

if temperature > 35:
    print("Heatwave warning issued.")

print("Weather check complete.")

temperature > 35 evaluates to True (40 > 35), so the warning prints. Then "Weather check complete." prints regardless, because it's outside the if block. If temperature were 30 instead, the warning would be skipped entirely, and only "Weather check complete." would print.

Note: An if with no matching else simply does nothing when its condition is False — it does not raise an error, and it does not stop the program.

Explanation

An if block is only ever entered when its condition is exactly True — not "truthy-looking," not a near match. temperature > 35 must genuinely evaluate to the Boolean True for the block to run, which is why writing the condition correctly (using the right relational or logical operator from the last chapter) matters as much as writing the block itself.

Summary

if condition: runs its indented block only when condition evaluates to True; when it's False, the block is skipped entirely and the program continues after it, with no error raised either way.

# Program to demonstrate the simple if statement
score = 94

if score >= 90:
    print("Excellent work!")
    print("You qualify for the KV Coders Scholarship.")
    print("Please collect your certificate.")

print("Program execution complete.")

Introduction

A plain if only handles one path — do something, or do nothing. Most real decisions need a second path for when the condition is false, which is what else adds.

Concept

if condition:
    statement(s) run when condition is True
else:
    statement(s) run when condition is False

Exactly one of the two blocks runs, every time — never both, and never neither.

Worked Example

marks = 32

if marks >= 40:
    print("Result: PASS")
else:
    print("Result: FAIL")

marks >= 40 is False (32 is less than 40), so the if block is skipped and the else block runs instead, printing Result: FAIL. If marks were 55, the if block would run and print Result: PASS, and the else block would be skipped.

Note: else never has its own condition — it simply means "whenever the if above it was False." Writing else if is not valid Python syntax at all (the correct way to check another condition is covered in the next topic).

Explanation

Because exactly one branch always runs, if-else is the right structure whenever a decision genuinely has two outcomes and every case must be handled — unlike a plain if, which is only appropriate when doing nothing is itself an acceptable outcome.

Summary

if-else runs exactly one of two blocks: the if block when its condition is True, or the else block when it's False — one of the two always executes, and else never takes its own condition.

# Program to check if a number is Even or Odd
num = 15

if num % 2 == 0:
    print(num, "is an Even number.")
    print("It is divisible by 2.")
else:
    print(num, "is an Odd number.")
    print("It leaves a remainder when divided by 2.")

print("Calculation finished.")

Introduction

if-else only chooses between two outcomes. Many real decisions — like assigning a grade from a mark — have more than two possible outcomes. The if-elif-else ladder handles exactly that.

Concept

if condition1:
    statement(s)
elif condition2:
    statement(s)
elif condition3:
    statement(s)
else:
    statement(s)

Python checks each condition top to bottom and runs the block belonging to the first one that is True — every condition below it is skipped, even if it would also have been True. If none of the conditions are True, the final else block runs. elif (short for "else if") can repeat as many times as needed; else is optional, and always goes last.

Worked Example

marks = 78

if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
elif marks >= 60:
    print("Grade: C")
else:
    print("Grade: D")

Python checks marks >= 90 first — False (78 is less than 90). It then checks marks >= 75 — True (78 is at least 75) — so Grade: B prints, and every condition below it (marks >= 60, and the else) is skipped entirely, even though marks >= 60 would also have been True.

Warning: Ordering matters. If the conditions above were written from lowest to highest instead (marks >= 60 checked first), a mark of 95 would incorrectly print Grade: C, because the first matching condition wins and the ladder never continues checking after that. Write ladders like this from the highest boundary down.

Explanation

"First match wins, then stop" is the one rule this entire structure depends on — it's exactly why the boundary marks in a grading ladder must be ordered from strictest to loosest, top to bottom, and not the other way around.

Summary

if-elif-else checks each condition in order and runs only the block belonging to the first one that's True, skipping every condition after it — a ladder handling boundary-based decisions (like grading) must be ordered from the strictest condition to the loosest, top to bottom.

# Program to calculate Grade based on Marks
marks = 65

if marks >= 90:
    print("Grade: A+ (Outstanding)")
elif marks >= 80:
    print("Grade: A (Excellent)")
elif marks >= 70:
    print("Grade: B (Good)")
elif marks >= 33:
    print("Grade: C (Pass)")
else:
    print("Grade: F (Fail/Re-appear)")

print("Evaluation Complete.")

Introduction

Sometimes a second decision only makes sense once a first condition is already known to be true — like checking a specific eligibility rule, but only for students who already passed. Placing an if inside another if handles exactly that.

Concept

A nested if is an if (or if-else) statement placed inside the block of another if (or else). The inner statement only runs at all if the outer condition was True first. Each nested level needs its own, further level of indentation.

Worked Example

marks = 85
attendance = 78

if marks >= 40:
    print("Result: PASS")
    if attendance >= 75:
        print("Eligible for merit certificate.")
    else:
        print("Not eligible for merit certificate (attendance below 75%).")
else:
    print("Result: FAIL")

marks >= 40 is True, so Result: PASS prints, and Python enters that block — which is where the nested if attendance >= 75: lives. attendance >= 75 is also True (78 is at least 75), so Eligible for merit certificate. prints too. If marks were below 40 instead, the entire nested if-else would never even be checked — the outer else would run, printing only Result: FAIL.

Note: The nested if-else is indented one further level than the outer if's own block — its if/else keywords must line up with each other, and everything inside them is indented one level deeper still.

Explanation

Nesting exists specifically for decisions that are only meaningful given an outer condition — checking attendance for a merit certificate is pointless for a student who hasn't even passed, which is exactly why that check belongs inside the PASS branch rather than as a separate, independent if.

Summary

A nested if is an if/if-else placed inside another if's block, and it only runs when the outer condition was already True — each nested level requires its own further indentation, keeping the block structure that decides which lines depend on which condition unambiguous.

# Change this value to test different logic paths
num = 14 

if num >= 0:
    print("The number is Positive.")
    
    # Nested check: only happens if num is positive
    if num % 2 == 0:
        print("Specifically, it is an Even number.")
    else:
        print("Specifically, it is an Odd number.")
        
else:
    print("The number is Negative.")