Lib/contextlib.py (part 7)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/contextlib.py ↗

This annotation covers the full contextlib module. See lib_contextlib6_detail for closing, suppress, redirect_stdout, and AbstractContextManager.

Map

Lines	Symbol	Role
1-80	@contextmanager	Turn a generator function into a context manager
81-160	@asynccontextmanager	Async variant
161-240	nullcontext	No-op context manager
241-360	ExitStack.__enter__ / __exit__	Compose multiple context managers
361-500	ExitStack.callback / push / enter_context	Build dynamic exit stacks

Reading

@contextmanager

# CPython: Lib/contextlib.py:130 contextmanager
def contextmanager(func):
    @wraps(func)
    def helper(*args, **kwds):
        return _GeneratorContextManager(func, args, kwds)
    return helper

class _GeneratorContextManager(AbstractContextManager):
    def __enter__(self):
        try:
            return next(self.gen)
        except StopIteration:
            raise RuntimeError("generator didn't yield")
    def __exit__(self, typ, value, traceback):
        if typ is None:
            try:
                next(self.gen)
            except StopIteration:
                return False
            raise RuntimeError("generator didn't stop")
        else:
            if value is None:
                value = typ()
            try:
                self.gen.throw(value)
            except StopIteration as exc:
                return exc is not value
            ...

@contextmanager wraps a generator function. yield splits __enter__ (before yield) from __exit__ (after yield). The exception handling forwards exceptions to gen.throw().

ExitStack

# CPython: Lib/contextlib.py:520 ExitStack.__exit__
def __exit__(self, *exc_details):
    # LIFO: call exit callbacks in reverse order
    suppressed_exc = False
    pending_raise = False
    with self._exit_callbacks as callbacks:
        while callbacks:
            cb = callbacks.pop()
            try:
                if cb(*exc_details):
                    suppressed_exc = True
                    pending_raise = False
                    exc_details = (None, None, None)
            except:
                new_exc_details = sys.exc_info()
                ...
                exc_details = new_exc_details
                pending_raise = True
    return suppressed_exc

ExitStack calls each registered callback in LIFO order. If any callback suppresses the exception (returns True), subsequent callbacks see (None, None, None). If a callback raises, the new exception propagates.

nullcontext

# CPython: Lib/contextlib.py:700 nullcontext
class nullcontext(AbstractContextManager, AbstractAsyncContextManager):
    def __init__(self, enter_result=None):
        self.enter_result = enter_result
    def __enter__(self):
        return self.enter_result
    def __exit__(self, *excinfo):
        pass
    async def __aenter__(self):
        return self.enter_result
    async def __aexit__(self, *excinfo):
        pass

nullcontext(value) is useful when a function needs to optionally use a context manager: with (cm if condition else nullcontext()):.

gopy notes

@contextmanager is module/contextlib.ContextManager in module/contextlib/module.go. It creates a _GeneratorContextManager backed by a vm.Generator. ExitStack is objects.ExitStack with a []ExitCallback slice. nullcontext is a trivial struct implementing the context manager protocol.