Python Context Manager

Basic Usage

@contextmanager lets you write a context manager using a generator — no need to implement __enter__ / __exit__:

from contextlib import contextmanager

@contextmanager
def db_context():
    db.connect(reuse_if_open=True)
    try:
        yield
    finally:
        if not db.is_closed():
            db.close()

# Usage
with db_context():
    result = db.execute(query)

Nested Context Managers

Context managers can be nested. Execution goes from outer to inner when entering, and inner to outer when exiting:

@contextmanager
def get_db_context():
    db_manager = init_db()
    with db_manager.connection_context():  # inner
        yield                              # control returns to outer with block

# Usage
with get_db_context():           # outer
    for item in items:
        process(item)

Execution order:

1. Enter outer get_db_context() → initialize db_manager
2. Enter inner connection_context() → establish DB connection
3. yield → execute code inside the with block
4. Exit inner connection_context() → close DB connection
5. Exit outer get_db_context() → clean up resources

DB Connection Context Manager

Manages the DB connection lifecycle to ensure connections are properly closed:

@contextmanager
def connection_context(self):
    if self.db.is_closed():
        self.db.connect()
    try:
        yield
    except Exception:
        if not self.db.is_closed():
            self.db.rollback()
        raise
    finally:
        if not self.db.is_closed():
            self.db.close()

Why Use Context Managers

Without a context manager, an exception can leave the connection open:

# If an exception occurs, connection won't be closed
db.connect()
result = db.execute(risky_query())  # might raise
db.close()  # never reached

# With context manager, finally always runs
with db_context():
    result = db.execute(risky_query())  # even if it raises, finally executes

Python vs JS Comparison

	Python	JavaScript
Auto-close	with syntax	try/finally
Resource management	Context Manager	Return a release function
Syntactic sugar	@contextmanager	No direct equivalent