Catching Exceptions

What Does It Mean to "Catch" an Exception?

An exception is said to be caught when a piece of code specifically designed to handle that exception actually runs. The whole point of catching is to prevent the program from crashing and to give a controlled response instead.

The try and except Blocks: The Core Mechanism

The book introduces two blocks that work as a pair:

• try block: You place any "suspicious" lines of code here — lines where you suspect an error might occur. While writing or debugging, if you doubt an exception in a particular part of the code, that part goes inside try.
• except block: Every try block must be followed by at least one except block. Inside the except clause, you write the code that should run if a specific exception occurs.

How execution flows:

1. Python starts executing the code inside the try block normally.
2. If no exception occurs, the except block is skipped entirely, and execution continues with the statement after the entire try..except structure.
3. If an exception does occur inside the try block, the remaining code inside try is immediately stopped. Control jumps straight to the matching except block.
4. After the except block finishes, execution continues with the next statement after the try..except block.

Syntax of try..except

try:
    [program statements where exceptions might occur]
except [exception-name]:
    [code for exception handling if the exception-name error is encountered]

Example from the Book: Handling ZeroDivisionError

Program 1-2: Using try..except block

print("Practicing for try block")
try:
    numerator = 50
    denom = int(input("Enter the denominator"))
    quotient = (numerator / denom)
    print(quotient)
    print("Division performed successfully")
except ZeroDivisionError:
    print("Denominator as ZERO.... not allowed")
print("OUTSIDE try..except block")

What happens in two cases:

• User enters a non-zero denominator: The division works, quotient is printed, "Division performed successfully" is printed, the except block is skipped, and finally "OUTSIDE try..except block" is printed.
• User enters 0: The division raises ZeroDivisionError. Execution inside try stops immediately (the print(quotient) and the success message are never reached). Control goes to the except block, which prints "Denominator as ZERO.... not allowed". After that, "OUTSIDE try..except block" is printed.

Handling Multiple Exceptions with Multiple except Blocks

A single piece of code might have more than one type of error. For such situations, you can have multiple except blocks for one try block.

Program 1-3: Use of multiple except clauses

print("Handling multiple exceptions")
try:
    numerator = 50
    denom = int(input("Enter the denominator: "))
    print(numerator / denom)
    print("Division performed successfully")
except ZeroDivisionError:
    print("Denominator as ZERO is not allowed")
except ValueError:
    print("Only INTEGERS should be entered")

Here, two exceptions are handled:

How Python decides which except block to run: When an exception is raised, Python searches through the except blocks in the order they appear. It looks for a matching exception name. If a match is found, that block runs. If no match is found, the program terminates with an unhandled exception error.

The Catch-All except Clause (Without Specifying an Exception)

What if an exception occurs that you didn't anticipate? The book provides a safety net: you can add an except clause without specifying any exception name. This acts as a default handler for any exception not caught by earlier except blocks.

Important rule: This bare except clause must be added as the last clause of the try..except block. If you put it first, it will catch everything and the specific handlers below it will never run.

Program 1-4: Use of except without specifying an exception

print("Handling exceptions without naming them")
try:
    numerator = 50 …