1"""Utilities for with-statement contexts.  See PEP 343."""
2import abc
3import sys
4import _collections_abc
5from collections import deque
6from functools import wraps
7from types import MethodType, GenericAlias
8
9__all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext",
10           "AbstractContextManager", "AbstractAsyncContextManager",
11           "AsyncExitStack", "ContextDecorator", "ExitStack",
12           "redirect_stdout", "redirect_stderr", "suppress"]
13
14
15class AbstractContextManager(abc.ABC):
16
17    """An abstract base class for context managers."""
18
19    __class_getitem__ = classmethod(GenericAlias)
20
21    def __enter__(self):
22        """Return `self` upon entering the runtime context."""
23        return self
24
25    @abc.abstractmethod
26    def __exit__(self, exc_type, exc_value, traceback):
27        """Raise any exception triggered within the runtime context."""
28        return None
29
30    @classmethod
31    def __subclasshook__(cls, C):
32        if cls is AbstractContextManager:
33            return _collections_abc._check_methods(C, "__enter__", "__exit__")
34        return NotImplemented
35
36
37class AbstractAsyncContextManager(abc.ABC):
38
39    """An abstract base class for asynchronous context managers."""
40
41    __class_getitem__ = classmethod(GenericAlias)
42
43    async def __aenter__(self):
44        """Return `self` upon entering the runtime context."""
45        return self
46
47    @abc.abstractmethod
48    async def __aexit__(self, exc_type, exc_value, traceback):
49        """Raise any exception triggered within the runtime context."""
50        return None
51
52    @classmethod
53    def __subclasshook__(cls, C):
54        if cls is AbstractAsyncContextManager:
55            return _collections_abc._check_methods(C, "__aenter__",
56                                                   "__aexit__")
57        return NotImplemented
58
59
60class ContextDecorator(object):
61    "A base class or mixin that enables context managers to work as decorators."
62
63    def _recreate_cm(self):
64        """Return a recreated instance of self.
65
66        Allows an otherwise one-shot context manager like
67        _GeneratorContextManager to support use as
68        a decorator via implicit recreation.
69
70        This is a private interface just for _GeneratorContextManager.
71        See issue #11647 for details.
72        """
73        return self
74
75    def __call__(self, func):
76        @wraps(func)
77        def inner(*args, **kwds):
78            with self._recreate_cm():
79                return func(*args, **kwds)
80        return inner
81
82
83class _GeneratorContextManagerBase:
84    """Shared functionality for @contextmanager and @asynccontextmanager."""
85
86    def __init__(self, func, args, kwds):
87        self.gen = func(*args, **kwds)
88        self.func, self.args, self.kwds = func, args, kwds
89        # Issue 19330: ensure context manager instances have good docstrings
90        doc = getattr(func, "__doc__", None)
91        if doc is None:
92            doc = type(self).__doc__
93        self.__doc__ = doc
94        # Unfortunately, this still doesn't provide good help output when
95        # inspecting the created context manager instances, since pydoc
96        # currently bypasses the instance docstring and shows the docstring
97        # for the class instead.
98        # See http://bugs.python.org/issue19404 for more details.
99
100
101class _GeneratorContextManager(_GeneratorContextManagerBase,
102                               AbstractContextManager,
103                               ContextDecorator):
104    """Helper for @contextmanager decorator."""
105
106    def _recreate_cm(self):
107        # _GCM instances are one-shot context managers, so the
108        # CM must be recreated each time a decorated function is
109        # called
110        return self.__class__(self.func, self.args, self.kwds)
111
112    def __enter__(self):
113        # do not keep args and kwds alive unnecessarily
114        # they are only needed for recreation, which is not possible anymore
115        del self.args, self.kwds, self.func
116        try:
117            return next(self.gen)
118        except StopIteration:
119            raise RuntimeError("generator didn't yield") from None
120
121    def __exit__(self, type, value, traceback):
122        if type is None:
123            try:
124                next(self.gen)
125            except StopIteration:
126                return False
127            else:
128                raise RuntimeError("generator didn't stop")
129        else:
130            if value is None:
131                # Need to force instantiation so we can reliably
132                # tell if we get the same exception back
133                value = type()
134            try:
135                self.gen.throw(type, value, traceback)
136            except StopIteration as exc:
137                # Suppress StopIteration *unless* it's the same exception that
138                # was passed to throw().  This prevents a StopIteration
139                # raised inside the "with" statement from being suppressed.
140                return exc is not value
141            except RuntimeError as exc:
142                # Don't re-raise the passed in exception. (issue27122)
143                if exc is value:
144                    return False
145                # Likewise, avoid suppressing if a StopIteration exception
146                # was passed to throw() and later wrapped into a RuntimeError
147                # (see PEP 479).
148                if type is StopIteration and exc.__cause__ is value:
149                    return False
150                raise
151            except:
152                # only re-raise if it's *not* the exception that was
153                # passed to throw(), because __exit__() must not raise
154                # an exception unless __exit__() itself failed.  But throw()
155                # has to raise the exception to signal propagation, so this
156                # fixes the impedance mismatch between the throw() protocol
157                # and the __exit__() protocol.
158                #
159                # This cannot use 'except BaseException as exc' (as in the
160                # async implementation) to maintain compatibility with
161                # Python 2, where old-style class exceptions are not caught
162                # by 'except BaseException'.
163                if sys.exc_info()[1] is value:
164                    return False
165                raise
166            raise RuntimeError("generator didn't stop after throw()")
167
168
169class _AsyncGeneratorContextManager(_GeneratorContextManagerBase,
170                                    AbstractAsyncContextManager):
171    """Helper for @asynccontextmanager."""
172
173    async def __aenter__(self):
174        try:
175            return await self.gen.__anext__()
176        except StopAsyncIteration:
177            raise RuntimeError("generator didn't yield") from None
178
179    async def __aexit__(self, typ, value, traceback):
180        if typ is None:
181            try:
182                await self.gen.__anext__()
183            except StopAsyncIteration:
184                return
185            else:
186                raise RuntimeError("generator didn't stop")
187        else:
188            if value is None:
189                value = typ()
190            # See _GeneratorContextManager.__exit__ for comments on subtleties
191            # in this implementation
192            try:
193                await self.gen.athrow(typ, value, traceback)
194                raise RuntimeError("generator didn't stop after athrow()")
195            except StopAsyncIteration as exc:
196                return exc is not value
197            except RuntimeError as exc:
198                if exc is value:
199                    return False
200                # Avoid suppressing if a StopIteration exception
201                # was passed to throw() and later wrapped into a RuntimeError
202                # (see PEP 479 for sync generators; async generators also
203                # have this behavior). But do this only if the exception wrapped
204                # by the RuntimeError is actully Stop(Async)Iteration (see
205                # issue29692).
206                if isinstance(value, (StopIteration, StopAsyncIteration)):
207                    if exc.__cause__ is value:
208                        return False
209                raise
210            except BaseException as exc:
211                if exc is not value:
212                    raise
213
214
215def contextmanager(func):
216    """@contextmanager decorator.
217
218    Typical usage:
219
220        @contextmanager
221        def some_generator(<arguments>):
222            <setup>
223            try:
224                yield <value>
225            finally:
226                <cleanup>
227
228    This makes this:
229
230        with some_generator(<arguments>) as <variable>:
231            <body>
232
233    equivalent to this:
234
235        <setup>
236        try:
237            <variable> = <value>
238            <body>
239        finally:
240            <cleanup>
241    """
242    @wraps(func)
243    def helper(*args, **kwds):
244        return _GeneratorContextManager(func, args, kwds)
245    return helper
246
247
248def asynccontextmanager(func):
249    """@asynccontextmanager decorator.
250
251    Typical usage:
252
253        @asynccontextmanager
254        async def some_async_generator(<arguments>):
255            <setup>
256            try:
257                yield <value>
258            finally:
259                <cleanup>
260
261    This makes this:
262
263        async with some_async_generator(<arguments>) as <variable>:
264            <body>
265
266    equivalent to this:
267
268        <setup>
269        try:
270            <variable> = <value>
271            <body>
272        finally:
273            <cleanup>
274    """
275    @wraps(func)
276    def helper(*args, **kwds):
277        return _AsyncGeneratorContextManager(func, args, kwds)
278    return helper
279
280
281class closing(AbstractContextManager):
282    """Context to automatically close something at the end of a block.
283
284    Code like this:
285
286        with closing(<module>.open(<arguments>)) as f:
287            <block>
288
289    is equivalent to this:
290
291        f = <module>.open(<arguments>)
292        try:
293            <block>
294        finally:
295            f.close()
296
297    """
298    def __init__(self, thing):
299        self.thing = thing
300    def __enter__(self):
301        return self.thing
302    def __exit__(self, *exc_info):
303        self.thing.close()
304
305
306class _RedirectStream(AbstractContextManager):
307
308    _stream = None
309
310    def __init__(self, new_target):
311        self._new_target = new_target
312        # We use a list of old targets to make this CM re-entrant
313        self._old_targets = []
314
315    def __enter__(self):
316        self._old_targets.append(getattr(sys, self._stream))
317        setattr(sys, self._stream, self._new_target)
318        return self._new_target
319
320    def __exit__(self, exctype, excinst, exctb):
321        setattr(sys, self._stream, self._old_targets.pop())
322
323
324class redirect_stdout(_RedirectStream):
325    """Context manager for temporarily redirecting stdout to another file.
326
327        # How to send help() to stderr
328        with redirect_stdout(sys.stderr):
329            help(dir)
330
331        # How to write help() to a file
332        with open('help.txt', 'w') as f:
333            with redirect_stdout(f):
334                help(pow)
335    """
336
337    _stream = "stdout"
338
339
340class redirect_stderr(_RedirectStream):
341    """Context manager for temporarily redirecting stderr to another file."""
342
343    _stream = "stderr"
344
345
346class suppress(AbstractContextManager):
347    """Context manager to suppress specified exceptions
348
349    After the exception is suppressed, execution proceeds with the next
350    statement following the with statement.
351
352         with suppress(FileNotFoundError):
353             os.remove(somefile)
354         # Execution still resumes here if the file was already removed
355    """
356
357    def __init__(self, *exceptions):
358        self._exceptions = exceptions
359
360    def __enter__(self):
361        pass
362
363    def __exit__(self, exctype, excinst, exctb):
364        # Unlike isinstance and issubclass, CPython exception handling
365        # currently only looks at the concrete type hierarchy (ignoring
366        # the instance and subclass checking hooks). While Guido considers
367        # that a bug rather than a feature, it's a fairly hard one to fix
368        # due to various internal implementation details. suppress provides
369        # the simpler issubclass based semantics, rather than trying to
370        # exactly reproduce the limitations of the CPython interpreter.
371        #
372        # See http://bugs.python.org/issue12029 for more details
373        return exctype is not None and issubclass(exctype, self._exceptions)
374
375
376class _BaseExitStack:
377    """A base class for ExitStack and AsyncExitStack."""
378
379    @staticmethod
380    def _create_exit_wrapper(cm, cm_exit):
381        return MethodType(cm_exit, cm)
382
383    @staticmethod
384    def _create_cb_wrapper(callback, /, *args, **kwds):
385        def _exit_wrapper(exc_type, exc, tb):
386            callback(*args, **kwds)
387        return _exit_wrapper
388
389    def __init__(self):
390        self._exit_callbacks = deque()
391
392    def pop_all(self):
393        """Preserve the context stack by transferring it to a new instance."""
394        new_stack = type(self)()
395        new_stack._exit_callbacks = self._exit_callbacks
396        self._exit_callbacks = deque()
397        return new_stack
398
399    def push(self, exit):
400        """Registers a callback with the standard __exit__ method signature.
401
402        Can suppress exceptions the same way __exit__ method can.
403        Also accepts any object with an __exit__ method (registering a call
404        to the method instead of the object itself).
405        """
406        # We use an unbound method rather than a bound method to follow
407        # the standard lookup behaviour for special methods.
408        _cb_type = type(exit)
409
410        try:
411            exit_method = _cb_type.__exit__
412        except AttributeError:
413            # Not a context manager, so assume it's a callable.
414            self._push_exit_callback(exit)
415        else:
416            self._push_cm_exit(exit, exit_method)
417        return exit  # Allow use as a decorator.
418
419    def enter_context(self, cm):
420        """Enters the supplied context manager.
421
422        If successful, also pushes its __exit__ method as a callback and
423        returns the result of the __enter__ method.
424        """
425        # We look up the special methods on the type to match the with
426        # statement.
427        _cm_type = type(cm)
428        _exit = _cm_type.__exit__
429        result = _cm_type.__enter__(cm)
430        self._push_cm_exit(cm, _exit)
431        return result
432
433    def callback(self, callback, /, *args, **kwds):
434        """Registers an arbitrary callback and arguments.
435
436        Cannot suppress exceptions.
437        """
438        _exit_wrapper = self._create_cb_wrapper(callback, *args, **kwds)
439
440        # We changed the signature, so using @wraps is not appropriate, but
441        # setting __wrapped__ may still help with introspection.
442        _exit_wrapper.__wrapped__ = callback
443        self._push_exit_callback(_exit_wrapper)
444        return callback  # Allow use as a decorator
445
446    def _push_cm_exit(self, cm, cm_exit):
447        """Helper to correctly register callbacks to __exit__ methods."""
448        _exit_wrapper = self._create_exit_wrapper(cm, cm_exit)
449        self._push_exit_callback(_exit_wrapper, True)
450
451    def _push_exit_callback(self, callback, is_sync=True):
452        self._exit_callbacks.append((is_sync, callback))
453
454
455# Inspired by discussions on http://bugs.python.org/issue13585
456class ExitStack(_BaseExitStack, AbstractContextManager):
457    """Context manager for dynamic management of a stack of exit callbacks.
458
459    For example:
460        with ExitStack() as stack:
461            files = [stack.enter_context(open(fname)) for fname in filenames]
462            # All opened files will automatically be closed at the end of
463            # the with statement, even if attempts to open files later
464            # in the list raise an exception.
465    """
466
467    def __enter__(self):
468        return self
469
470    def __exit__(self, *exc_details):
471        received_exc = exc_details[0] is not None
472
473        # We manipulate the exception state so it behaves as though
474        # we were actually nesting multiple with statements
475        frame_exc = sys.exc_info()[1]
476        def _fix_exception_context(new_exc, old_exc):
477            # Context may not be correct, so find the end of the chain
478            while 1:
479                exc_context = new_exc.__context__
480                if exc_context is old_exc:
481                    # Context is already set correctly (see issue 20317)
482                    return
483                if exc_context is None or exc_context is frame_exc:
484                    break
485                new_exc = exc_context
486            # Change the end of the chain to point to the exception
487            # we expect it to reference
488            new_exc.__context__ = old_exc
489
490        # Callbacks are invoked in LIFO order to match the behaviour of
491        # nested context managers
492        suppressed_exc = False
493        pending_raise = False
494        while self._exit_callbacks:
495            is_sync, cb = self._exit_callbacks.pop()
496            assert is_sync
497            try:
498                if cb(*exc_details):
499                    suppressed_exc = True
500                    pending_raise = False
501                    exc_details = (None, None, None)
502            except:
503                new_exc_details = sys.exc_info()
504                # simulate the stack of exceptions by setting the context
505                _fix_exception_context(new_exc_details[1], exc_details[1])
506                pending_raise = True
507                exc_details = new_exc_details
508        if pending_raise:
509            try:
510                # bare "raise exc_details[1]" replaces our carefully
511                # set-up context
512                fixed_ctx = exc_details[1].__context__
513                raise exc_details[1]
514            except BaseException:
515                exc_details[1].__context__ = fixed_ctx
516                raise
517        return received_exc and suppressed_exc
518
519    def close(self):
520        """Immediately unwind the context stack."""
521        self.__exit__(None, None, None)
522
523
524# Inspired by discussions on https://bugs.python.org/issue29302
525class AsyncExitStack(_BaseExitStack, AbstractAsyncContextManager):
526    """Async context manager for dynamic management of a stack of exit
527    callbacks.
528
529    For example:
530        async with AsyncExitStack() as stack:
531            connections = [await stack.enter_async_context(get_connection())
532                for i in range(5)]
533            # All opened connections will automatically be released at the
534            # end of the async with statement, even if attempts to open a
535            # connection later in the list raise an exception.
536    """
537
538    @staticmethod
539    def _create_async_exit_wrapper(cm, cm_exit):
540        return MethodType(cm_exit, cm)
541
542    @staticmethod
543    def _create_async_cb_wrapper(callback, /, *args, **kwds):
544        async def _exit_wrapper(exc_type, exc, tb):
545            await callback(*args, **kwds)
546        return _exit_wrapper
547
548    async def enter_async_context(self, cm):
549        """Enters the supplied async context manager.
550
551        If successful, also pushes its __aexit__ method as a callback and
552        returns the result of the __aenter__ method.
553        """
554        _cm_type = type(cm)
555        _exit = _cm_type.__aexit__
556        result = await _cm_type.__aenter__(cm)
557        self._push_async_cm_exit(cm, _exit)
558        return result
559
560    def push_async_exit(self, exit):
561        """Registers a coroutine function with the standard __aexit__ method
562        signature.
563
564        Can suppress exceptions the same way __aexit__ method can.
565        Also accepts any object with an __aexit__ method (registering a call
566        to the method instead of the object itself).
567        """
568        _cb_type = type(exit)
569        try:
570            exit_method = _cb_type.__aexit__
571        except AttributeError:
572            # Not an async context manager, so assume it's a coroutine function
573            self._push_exit_callback(exit, False)
574        else:
575            self._push_async_cm_exit(exit, exit_method)
576        return exit  # Allow use as a decorator
577
578    def push_async_callback(self, callback, /, *args, **kwds):
579        """Registers an arbitrary coroutine function and arguments.
580
581        Cannot suppress exceptions.
582        """
583        _exit_wrapper = self._create_async_cb_wrapper(callback, *args, **kwds)
584
585        # We changed the signature, so using @wraps is not appropriate, but
586        # setting __wrapped__ may still help with introspection.
587        _exit_wrapper.__wrapped__ = callback
588        self._push_exit_callback(_exit_wrapper, False)
589        return callback  # Allow use as a decorator
590
591    async def aclose(self):
592        """Immediately unwind the context stack."""
593        await self.__aexit__(None, None, None)
594
595    def _push_async_cm_exit(self, cm, cm_exit):
596        """Helper to correctly register coroutine function to __aexit__
597        method."""
598        _exit_wrapper = self._create_async_exit_wrapper(cm, cm_exit)
599        self._push_exit_callback(_exit_wrapper, False)
600
601    async def __aenter__(self):
602        return self
603
604    async def __aexit__(self, *exc_details):
605        received_exc = exc_details[0] is not None
606
607        # We manipulate the exception state so it behaves as though
608        # we were actually nesting multiple with statements
609        frame_exc = sys.exc_info()[1]
610        def _fix_exception_context(new_exc, old_exc):
611            # Context may not be correct, so find the end of the chain
612            while 1:
613                exc_context = new_exc.__context__
614                if exc_context is old_exc:
615                    # Context is already set correctly (see issue 20317)
616                    return
617                if exc_context is None or exc_context is frame_exc:
618                    break
619                new_exc = exc_context
620            # Change the end of the chain to point to the exception
621            # we expect it to reference
622            new_exc.__context__ = old_exc
623
624        # Callbacks are invoked in LIFO order to match the behaviour of
625        # nested context managers
626        suppressed_exc = False
627        pending_raise = False
628        while self._exit_callbacks:
629            is_sync, cb = self._exit_callbacks.pop()
630            try:
631                if is_sync:
632                    cb_suppress = cb(*exc_details)
633                else:
634                    cb_suppress = await cb(*exc_details)
635
636                if cb_suppress:
637                    suppressed_exc = True
638                    pending_raise = False
639                    exc_details = (None, None, None)
640            except:
641                new_exc_details = sys.exc_info()
642                # simulate the stack of exceptions by setting the context
643                _fix_exception_context(new_exc_details[1], exc_details[1])
644                pending_raise = True
645                exc_details = new_exc_details
646        if pending_raise:
647            try:
648                # bare "raise exc_details[1]" replaces our carefully
649                # set-up context
650                fixed_ctx = exc_details[1].__context__
651                raise exc_details[1]
652            except BaseException:
653                exc_details[1].__context__ = fixed_ctx
654                raise
655        return received_exc and suppressed_exc
656
657
658class nullcontext(AbstractContextManager):
659    """Context manager that does no additional processing.
660
661    Used as a stand-in for a normal context manager, when a particular
662    block of code is only sometimes used with a normal context manager:
663
664    cm = optional_cm if condition else nullcontext()
665    with cm:
666        # Perform operation, using optional_cm if condition is True
667    """
668
669    def __init__(self, enter_result=None):
670        self.enter_result = enter_result
671
672    def __enter__(self):
673        return self.enter_result
674
675    def __exit__(self, *excinfo):
676        pass
677