Context managers and the with statement

A context manager is an object that defines runtime context for a block of code, with setup and cleanup actions. It is used with the with statement. Context managers implement dunder enter and dunder exit. The most common example is opening a file: with open as f ensures the file is closed.

Define a class with dunder enter and dunder exit methods. The dunder enter method is called when entering the with block and its return value is bound to the as variable. The dunder exit method is called when leaving the block, receiving exception info if one occurred.

The contextlib.contextmanager decorator turns a generator function into a context manager. The function should yield exactly once. Code before the yield is the setup, code after is the cleanup. Wrap the yield in a try-finally to ensure cleanup runs even if an exception occurs in the with block.

Suppress is a context manager that swallows specified exceptions. It is a clean alternative to a try-except block where you want to ignore certain errors. For example, suppress FileNotFoundError around an os.remove call avoids cluttering the code with empty handlers.

They achieve the same goal of guaranteed cleanup but with different syntax and semantics. The with statement is more concise and requires the resource to be a context manager. Try-finally is more flexible but more verbose. For files, locks, and database connections, with is the preferred idiom. ---