"""

Python error handling: try/except, else/finally, custom exceptions, and cleanup.

Run this file to see practical examples and output.

"""

from typing import List

class ValidationError(Exception):

"""Custom exception for validation failures."""

pass

class NotFoundError(Exception):

"""Custom exception when an item is not found."""

pass

def divide_numbers(a: float, b: float) -> float:

"""Divide two numbers, handling division by zero."""

try:

return a / b

except ZeroDivisionError as exc:

print('Caught ZeroDivisionError')

raise ValueError('Cannot divide by zero') from exc

def parse_int(value: str) -> int:

"""Parse an integer, handling invalid input."""

try:

return int(value)

except ValueError as exc:

raise ValidationError(f'Invalid integer: {value}') from exc

def find_item(items: List[str], target: str) -> str:

"""Find an item and raise NotFoundError when missing."""

for item in items:

if item == target:

return item

raise NotFoundError(f'Item {target!r} not found')

def safe_open(path: str):

"""Demonstrate else and finally in exception handling."""

file = None

try:

file = open(path, 'r')

except FileNotFoundError:

print('File not found:', path)

return None

else:

print('File opened successfully')

content = file.read().strip()

return content

finally:

if file:

print('Closing file in finally')

file.close()

def nested_exception_example(value: int) -> None:

"""Demonstrate exception chaining and cleanup."""

try:

if value < 0:

raise ValidationError('Value must be non-negative')

result = 10 / value

print('Result:', result)

except ValidationError as exc:

print('Validation error:', exc)

except ZeroDivisionError as exc:

print('Zero division error, chaining to ValueError')

raise ValueError('Cannot compute with zero') from exc

finally:

print('Nested exception cleanup complete')

def run_demo() -> None:

# Division

try:

print('Divide 10 by 2:', divide_numbers(10, 2))

print('Divide 10 by 0:')

divide_numbers(10, 0)

except ValueError as exc:

print('Handled error:', exc)

# Parsing

for text in ['5', 'abc', '42']:

try:

value = parse_int(text)

print(f'Parsed {text}:', value)

except ValidationError as exc:

print('Validation error:', exc)

# Not found

try:

print('Find item:', find_item(['apple', 'banana'], 'banana'))

find_item(['apple', 'banana'], 'orange')

except NotFoundError as exc:

print('Not found error:', exc)

# Safe open

print('Safe open:', safe_open('context_demo.txt'))

print('Safe open missing:', safe_open('missing-file.txt'))

# Nested exception example

try:

nested_exception_example(0)

except ValueError as exc:

print('Chained error:', exc)

if __name__ == '__main__':

run_demo()