UNIT-4: Exception Handling

1. What is an exception?

Definition: An event that occurs during program execution, disrupting normal flow.
Distinction: Occurs at runtime (unlike syntax errors). Python raises an exception when it encounters an unhandleable situation.

2. Explain try-except-else-finally block.

Structure for handling exceptions gracefully:

try Block: Code that might raise an exception.
except Block: Handles exceptions from the try block. Can specify exception type (e.g., ZeroDivisionError).
else Block: Executes only if the try block completes successfully (no exception).
finally Block: Executes always, regardless of exception occurrence or handling. Used for cleanup operations (e.g., closing files).

Example:

try:
  result = 10 / 2 # Code to monitor
except ZeroDivisionError:
  print("Cannot divide by zero.") # Code to handle exception
else:
  print(f"Result: {result}") # Code if no exception occurred
finally:
  print("Execution attempt finished.") # Code that always runs

3. List built-in exceptions in Python.

ZeroDivisionError: Second operand of division/modulo is zero.
TypeError: Operation/function applied to inappropriate object type.
NameError: Local or global name not found.
FileNotFoundError: File or directory not found.
IndexError: Sequence index (list, tuple) out of range.
KeyError: Dictionary key not found.
ValueError: Function gets argument of correct type but inappropriate value.

4. Explain user-defined exceptions with example.

Definition: Exceptions created by the programmer to handle specific application-logic error conditions.
Creation: Define a new class inheriting from Exception or its subclasses.
Raising: Using the raise keyword.

Example (Age Validation):

class InvalidAgeError(Exception):
  "Raised when the input age is less than 18"
  pass

def validate_age(age):
  if age < 18:
    raise InvalidAgeError("Age is below the legal limit of 18") # Raising the exception
  print("Age is valid.")

try:
  validate_age(16)
except InvalidAgeError as e:
  print(f"Error: {e}") # Handling the exception

UNIT-5: File Processing

1. What is a file? Explain different file modes.

Definition: A named location on disk/storage medium used to store data permanently.

File Modes: Specify purpose for opening a file.

Mode	Description	If File Exists	If File Doesn't Exist
`'r'` (Read)	Opens for reading only. Pointer at beginning.	Reading starts.	Raises `FileNotFoundError`.
`'w'` (Write)	Opens for writing. Overwrites existing content.	Content is erased.	Creates a new file.
`'a'` (Append)	Opens for writing. New content added to the end.	New data is appende