Lib/contextlib.py

Source:

cpython 3.14 @ ab2d84fe1023/Lib/contextlib.py ↗

Lib/contextlib.py provides utilities for creating and working with context managers. The contextmanager decorator turns a generator function into a context manager. ExitStack enables composing multiple context managers dynamically. suppress and nullcontext cover common trivial patterns.

Map

Lines	Symbol	Role
1-50	imports, AbstractContextManager	ABC with default __enter__/__exit__
51-200	_GeneratorContextManagerBase, contextmanager	Generator-based CM decorator
201-320	asynccontextmanager	Async generator-based CM
321-420	closing, nullcontext, suppress	Simple CM wrappers
421-600	redirect_stdout, redirect_stderr	Stream redirection CMs
601-800	ExitStack, AsyncExitStack	Dynamic CM composition

Reading

contextmanager: generator protocol

contextmanager wraps a generator function. The __enter__ method calls next(gen) to advance the generator to the yield statement and returns the yielded value. The __exit__ method either calls next(gen) to continue past the yield (no exception) or gen.throw(exc_type, exc_val, exc_tb) to inject the exception into the generator (with exception).

# CPython: Lib/contextlib.py:51 _GeneratorContextManager.__enter__
def __enter__(self):
    try:
        return next(self.gen)
    except StopIteration:
        raise RuntimeError("generator didn't yield") from None

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:
        try:
            self.gen.throw(typ, value, traceback)
        except StopIteration as exc:
            return exc is not value
        except RuntimeError as exc:
            ...

suppress

suppress(*exceptions) returns a context manager whose __exit__ returns True (suppressing the exception) if the exception type matches any of the specified types.

# CPython: Lib/contextlib.py:321 suppress.__exit__
class suppress(AbstractContextManager):
    def __exit__(self, exctype, excinst, exctb):
        return exctype is not None and issubclass(exctype, self._exceptions)

ExitStack

ExitStack maintains a stack of callbacks and context managers. enter_context(cm) calls cm.__enter__() and pushes cm.__exit__ onto the stack. callback(fn, *args) pushes a plain callable. On __exit__, the stack is unwound in reverse order, with exception chaining if multiple __exit__ calls raise.

gopy notes

Not yet ported. The planned package path is module/contextlib/. The contextmanager decorator requires generator support; gopy's generator implementation in vm/eval_gen.go provides the necessary infrastructure. ExitStack maps to a []func(error) error slice in Go with deferred cleanup.