try and except Basics — Syntax for Handling Exceptions

Definition: Exception handling = try (monitor) + except (respond). The goal is to replace technical crashes with messages users can understand.

What you will learn

• Understand the try and except syntax
• Learn how Python monitors risky code
• See how PVM exits a try block on error
• Build a safe division program
• Understand else, finally, and raise at a high level

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The five exception keywords

Think of exception handling as a team where each keyword has one job.

Quick summary

Keyword	Purpose
try	Monitor risky statements
except	Catch matching exceptions
else	Execute only when no exception occurs
finally	Always runs (cleanup)
raise	Manually create an exception

This lesson focuses on try and except.

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1. Basic syntax

Every exception handling block follows the same structure.

try:
    # risky statements
    a = int("10")
    b = int("0")
    result = a / b

except ZeroDivisionError:
    print("Denominator cannot be zero")

except ValueError:
    print("Please enter valid numbers")

Visual structure

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2. What does the try block do?

The try block is called the Exception Monitoring Block.

It tells Python:

"Watch these lines carefully. They may produce a runtime exception."

Example

try:
    print("Step 1")
    result = 10 / 0
    print("Step 2")  # never runs

What happens internally?

Important rule

When an exception occurs:

• ✅ Current line stops immediately
• ✅ PVM exits the try block
• ❌ Remaining lines inside try are skipped

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3. What does the except block do?

The except block is called the Exception Processing Block.

Its job is to replace this:

ZeroDivisionError: division by zero

with this:

Please don't enter zero.

Visual comparison

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4. Multiple except blocks

Different errors deserve different messages.

# Simulated user input — no input() in browser playground
a = int("10")
b = int("0")

try:
    print(a / b)
except ValueError:
    print("Enter numbers only")
except ZeroDivisionError:
    print("Don't divide by zero")

Try changing a = int("abc") in the playground to trigger ValueError instead.

Decision flow

Python executes only the matching except block.

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5. Safe division program

This is how real applications validate user input — here with fixed values instead of input().

def safe_div(a, b):
    print("Program started")
    print("Trying division:", a, "/", b)
    try:
        result = a / b
        print("Division OK:", result)
    except ZeroDivisionError:
        print("Cannot divide by zero")
    except ValueError:
        print("Invalid numbers")
    print("Program ended")

safe_div(10, 2)   # Division OK: 5.0
safe_div(10, 0)   # Cannot divide by zero

Change a or b in the playground to simulate different runtime situations.

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6. Step-by-step program execution

Let's trace a real execution.

print("Program started")

try:
    result = 10 / 0
    print(result)  # skipped

except ZeroDivisionError:
    print("Don't enter zero")

print("Program ended")

Execution timeline

Final output

Program started
Don't enter zero
Program ended

Notice that print(result) never executes.

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7. What if there is NO exception?

The try block simply finishes normally.

Case 1 — Success:

try:
    print(10 / 2)
except ZeroDivisionError:
    print("Error")

Output: 5.0

Case 2 — Failure:

try:
    print(10 / 0)
except ZeroDivisionError:
    print("Error")

Output: Error

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8. Common beginner mistakes

Mistake 1 — try without except:

try:
    print(10 / 2)
# ❌ Invalid syntax — need at least one except or finally

Mistake 2 — Wrong exception type:

try:
    int("abc")
except KeyError:
    print("Handled")  # ❌ ValueError is raised, not KeyError

Correct approach:

try:
    int("abc")
except ValueError:
    print("Please enter numbers")  # ✅ Match the class exactly

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Memory map

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Interview questions

Q1. What is the purpose of a try block?

Answer: To monitor statements that may raise runtime exceptions.

Q2. What happens when an exception occurs inside try?

Answer: PVM immediately exits the try block, skips the remaining statements, and searches for a matching except.

Q3. Can one try have multiple except blocks?

Answer: Yes. Each except handles a different exception class with its own user-friendly message.

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Key takeaways

• try = Exception Monitoring Block
• except = Exception Processing Block
• When an exception occurs, PVM exits the try block immediately
• Multiple except blocks allow different messages for different errors
• Exception handling makes applications robust by preventing crashes
• Next: else and finally Blocks — when else runs, why finally always executes, and resource cleanup patterns