Context managers (with)

with x as y: guarantees cleanup (closing files, releasing locks) even if an exception is raised.

Code example

with open('data.csv') as f:
    rows = f.read().splitlines()
# f is closed here even if read() threw

with session.lock:
    session.namespace['x'] = 42

How it works

A context manager implements `__enter__` and `__exit__`. The `with` statement calls `__enter__`, runs the block, and always calls `__exit__` — even on exception. This is the right way to acquire locks (the notebook session lock), open files, or hold DB transactions. The notebook uses `with session.lock:` to ensure only one cell runs at a time per user.