🤝 Advanced Context Managers — Custom Implementation

Create custom context managers and understand exception handling in with blocks.

🎯 Custom Context Managers

from contextlib import contextmanager

@contextmanager
def open_and_lock_file(filename):
    """Custom context manager with cleanup"""
    file = open(filename, "r")
    # Acquire resource
    try:
        yield file
    finally:
        # Release resource
        file.close()

# Usage
with open_and_lock_file("data.txt") as f:
    content = f.read()
# File guaranteed to close

💡 Exception Handling in Context Managers

@contextmanager
def managed_transaction(db):
    """Transaction context manager"""
    try:
        yield db
        db.commit()  # Success
    except Exception:
        db.rollback()  # Error
        raise

# All-or-nothing operation
with managed_transaction(db) as transaction:
    transaction.insert("users", data)

🔑 Key Takeaways

• ✅ `@contextmanager` decorator simplifies context manager creation

• ✅ try/finally ensures cleanup even on exceptions

• ✅ Resources released in LIFO order

• ✅ Exception propagates after cleanup

• ✅ Combine multiple context managers: `with A() as a, B() as b:`

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