Error Handling (try/except)

Intermediate Python
tryexceptfinallyraiseExceptionlogging

Objective

Handle exceptions gracefully in Python, create custom exceptions, use finally for cleanup, and log errors properly.

Tools & Technologies

  • try
  • except
  • finally
  • raise
  • Exception
  • logging

Key Commands

try: x = int(input())\nexcept ValueError: pass
raise ValueError('must be positive')
except (TypeError, ValueError) as e:
finally: f.close()

Architecture Overview

flowchart TD TRY[try block\nexecutes] --> ERROR{Exception\nraised?} ERROR -->|ValueError| EV[except ValueError\nhandles it] ERROR -->|TypeError| ET[except TypeError\nhandles it] ERROR -->|any other| EX[except Exception as e\ngeneral handler] ERROR -->|no exception| ELSE[else block\noptional] EV --> FINALLY[finally block\nalways runs] ET --> FINALLY EX --> FINALLY ELSE --> FINALLY FINALLY --> CONT[Continue / return] style FINALLY fill:#1a1a2e,stroke:#ffd700,color:#ffd700

Step-by-Step Process

01
Basic try/except

Catch specific exceptions.

# Specific exception
try:
    n = int(input('Enter a number: '))
except ValueError as e:
    print(f'Invalid input: {e}')

# Multiple exceptions
try:
    result = risky_operation()
except (ValueError, TypeError) as e:
    print(f'Data error: {e}')
except FileNotFoundError:
    print('File not found')
except Exception as e:
    print(f'Unexpected error: {e}')
    raise  # re-raise
02
finally and Custom Exceptions

Ensure cleanup and create domain-specific exceptions.

# finally always runs
try:
    f = open('file.txt')
    data = f.read()
except IOError as e:
    print(f'IO error: {e}')
finally:
    f.close()  # always called

# Custom exception
class ValidationError(ValueError):
    def __init__(self, field, msg):
        self.field = field
        super().__init__(f'{field}: {msg}')

raise ValidationError('age', 'must be positive')

Challenges & Solutions

  • Bare except: catches everything including KeyboardInterrupt — always specify type
  • Catching Exception silences unexpected bugs — log or re-raise them

Key Takeaways

  • except clause should be as specific as possible — least specific last
  • Use context managers (with) instead of try/finally for resource cleanup