unit-06

Unit: Exception Handling in Python

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1. What is an Exception?

Definition: An exception is an error that occurs during the execution of a program, disrupting the normal flow of the code.

Examples of exceptions:

• ZeroDivisionError (dividing by zero)
• FileNotFoundError (attempting to open a file that doesn’t exist)
• ValueError (invalid value)
• TypeError (mismatched data types)

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2. Handling an Exception: try…except Statement

Purpose: To handle exceptions gracefully so the program does not crash.

Syntax:

try:
    # code that may raise exception
except ExceptionType1:
    # code to handle exception type 1
except ExceptionType2:
    # code to handle exception type 2
except:
    # code to handle any other exception

Example:

try:
    x = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero.")
except:
    print("Some error occurred.")

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3. try…except…else Statement

Definition: else block runs if no exception is raised in the try block.

Syntax:

try:
    # code that may raise exception
except ExceptionType:
    # handle exception
else:
    # code to execute if no exception

Example:

try:
    num = int(input("Enter a number: "))
except ValueError:
    print("Invalid input.")
else:
    print(f"You entered {num}")

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4. try…finally Clause

Definition: finally block runs no matter what, ensuring cleanup actions.

Syntax:

try:
    # risky code
finally:
    # cleanup code that runs always

Example:

try:
    f = open("file.txt", "r")
finally:
    f.close()

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5. Python Standard Exceptions and Raising Exceptions

• Python includes a set of built-in standard exceptions like IndexError, NameError, KeyError, etc.

Raising Exceptions:

raise ExceptionType("Error message")

Example:

def check_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")

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6. User-Defined Exceptions

Definition: Custom exceptions created by subclassing the Exception class.

Syntax:

class CustomError(Exception):
    pass
 
def func(x):
    if x < 0:
        raise CustomError("Invalid value")

Example:

try:
    func(-1)
except CustomError as e:
    print(e)  # Output: Invalid value

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This completes a thorough and syllabus-aligned explanation of Python exception handling with practical examples and essential concepts.

Say “next” to move on to the I/O unit. ^123456789

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Footnotes

1. https://www.w3schools.com/python/python_try_except.asp ↩

2. https://www.geeksforgeeks.org/python/python-try-except/ ↩

3. https://docs.python.org/3/tutorial/errors.html ↩

4. https://www.geeksforgeeks.org/python/python-exception-handling/ ↩

5. https://www.tutorialspoint.com/python/python_tryexcept_block.htm ↩

6. https://www.programiz.com/python-programming/exception-handling ↩

7. https://serveracademy.com/blog/python-try-except/ ↩

8. https://realpython.com/python-exceptions/ ↩

9. https://stackoverflow.com/questions/4990718/how-can-i-write-a-try-except-block-that-catches-all-exceptions ↩