Exception handling

A try block contains code that might raise an exception. One or more except clauses catch specific exceptions and handle them. An optional else runs if no exception occurred. An optional finally runs no matter what, used for cleanup. The exception object is available in the except clause via as.

A bare except catches every exception, including KeyboardInterrupt and SystemExit, which is rarely what you want. Except Exception catches all normal exceptions but lets system exits and interrupts pass through. As a rule, never use bare except; always catch a specific class.

BaseException is the root of the exception hierarchy. Exception inherits from it and is the parent of normal errors. SystemExit, KeyboardInterrupt, and GeneratorExit inherit directly from BaseException, not from Exception, so they bypass except Exception clauses.

The raise statement raises an exception. You can raise an exception class, which Python instantiates with no arguments, or an instance you create explicitly. Inside an except clause, a bare raise re-raises the current exception, preserving the original traceback.

When one exception is raised while handling another, Python links them via the dunder context attribute, showing both in the traceback. You can also use raise NewException from original to set dunder cause, indicating an explicit causal link. Chaining helps debugging by preserving context.

An exception group, introduced in Python 3.11 via PEP 654, is a way to raise and handle multiple exceptions at once. It is essential for concurrent code where several tasks may fail simultaneously. The except star syntax catches exceptions inside a group while letting unmatched ones propagate.

The finally block runs whether or not an exception occurred and whether or not it was caught. It is the right place for cleanup actions like closing files, releasing locks, or restoring state. Even a return or break in the try or except runs the finally before exiting. ---