2nd PUC Computer Science quick Notes |Exception Handling in Python

Commonly occurring exceptions are usually defined in the compiler/interpreter. These are called built-in exceptions.

Python’s standard library contains built-in exceptions for commonly occurring errors. When a built-in exception occurs, the appropriate exception handler code can display the reason along with the raised exception name.

When an error is detected, the Python interpreter raises or throws an exception. Exception handlers are designed to execute when a specific exception is raised.

Programmers can also forcefully raise exceptions by using the raise and assert statements.

• Raising an exception interrupts the normal flow of program execution.
• Control moves to the exception handler code.
• No further statement in the current block is executed after the exception is raised.

Exception handling helps prevent a program from crashing abruptly.

If an error is detected in the try block and the exception is thrown, the appropriate except block handles it.

If no except clause handles the exception, the exception is re-raised after the execution of the finally block.

print (" Practicing for try block")
try:
    numerator=50
    denom=int(input("Enter the denominator"))
    quotient=(numerator/denom)
    print ("Division performed successfully")
except ZeroDivisionError:
    print ("Denominator as ZERO is not allowed")
else:
    print ("The result of division operation is ", quotient)
finally:
    print ("OVER AND OUT")

Behaviour of Program 1-7

If non-numeric data is entered, the finally block is executed and the message OVER AND OUT is displayed. Thereafter, the exception for which a handler is not present is re-raised.

After execution of the finally block, Python transfers control to a previously entered try or to the next higher level default exception handler.

Overall try…except…else…finally Structure

• Syntax errors or parsing errors occur when programming-language rules are not followed.
• Python displays the name and a short description of a syntax error.
• Program execution starts only after a syntax error is rectified.
• An exception is a Python object representing an error.
• Syntax errors are also handled as exceptions.
• An exception needs to be handled so that the program does not terminate abruptly.
• Python provides built-in exceptions for commonly occurring errors.
• Common built-in exceptions include SyntaxError, ValueError, IOError, KeyboardInterrupt, ImportError, EOFError, ZeroDivisionError, IndexError, NameError, IndentationError, TypeError and OverFlowError.
• Python raises or throws an exception when an error is encountered.
• Exception handlers are codes designed to execute when a particular exception is raised.
• Raising an exception interrupts normal program execution and transfers control to the exception handler.
• raise and assert statements are used to raise exceptions.
• Exception handling adds code that gives proper messages or instructions and prevents abrupt program crashing.
• An exception is caught when the code designed to handle it is executed.
• Exceptions are caught in the try block and handled in the except block.
• Statements inside the finally block are always executed whether an exception occurs or not.

1. Define syntax error.
2. What is an exception?
3. What is a built-in exception?
4. Name two statements used for raising exceptions.
5. What is ZeroDivisionError?
6. What is ValueError?
7. What is IndexError?
8. What is NameError?
9. What is the use of the finally block?

1. Explain syntax errors and parsing errors.
2. Explain the purpose of the raise statement.
3. Explain the assert statement.
4. Define exception handling.
5. Define throwing and catching an exception.
6. Explain the need for exception handling.

1. Explain the process of handling an exception.
2. Explain catching exceptions using try and except.
3. Explain the use of multiple except clauses.
4. Explain the purpose of the else clause.

1. Explain built-in exceptions in Python with their meanings.
2. Explain raising exceptions using raise and assert.
3. Explain exception handling using try...except.
4. Explain try...except...else with the program structure.
5. Explain the finally clause and its behaviour when an exception is not handled.
6. Write a suitable exception-handling program for division and handle the denominator being zero.

Must Remember Before Examination

1. Syntax errors occur when the rules of the programming language are not followed.
2. An exception is a Python object representing an error.
3. Built-in exceptions are commonly occurring exceptions defined in the compiler/interpreter.
4. raise is used to throw an exception.
5. assert tests an expression and raises AssertionError when the expression is false.
6. Exception handling prevents abrupt program termination.
7. The try block contains code where an exception may occur.
8. The except block handles the matching exception.
9. Multiple except blocks can handle different exception types.
10. An unnamed except clause can handle an exception for which a specific handler has not been created and should be the last clause.
11. The else block executes when no exception occurs.
12. The finally block executes regardless of whether an exception occurs or not.
13. If an exception is not handled by an except clause, it can continue to be raised after the finally block.
14. The call stack is searched to find a suitable exception handler.

Built-in Exceptions — One-Glance Revision

1. Justify the statement that every syntax error is an exception but every exception cannot be a syntax error.
2. Explain when ImportError, IOError, NameError and ZeroDivisionError are raised, with suitable examples.
3. Explain the use of the raise statement and prepare a program to accept two numbers and display their quotient, raising an appropriate exception when the denominator is zero.
4. Use an assert statement in the division problem to test the division expression.
5. Define exception handling, throwing an exception and catching an exception.
6. Explain catching exceptions using try and except.
7. Complete a program using the appropriate exceptions for accepting only integers, preventing a zero denominator, an else clause and a finally clause.
8. Using the math module, prepare a program involving an incorrect number of arguments for a method such as sqrt() or pow() and apply exception handling as required by the textbook exercise.
9. Explain the use of the finally clause and apply it to the division exception-handling problem.