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", "aclosing"] 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 AsyncContextDecorator(object): 84 "A base class or mixin that enables async context managers to work as decorators." 85 86 def _recreate_cm(self): 87 """Return a recreated instance of self. 88 """ 89 return self 90 91 def __call__(self, func): 92 @wraps(func) 93 async def inner(*args, **kwds): 94 async with self._recreate_cm(): 95 return await func(*args, **kwds) 96 return inner 97 98 99class _GeneratorContextManagerBase: 100 """Shared functionality for @contextmanager and @asynccontextmanager.""" 101 102 def __init__(self, func, args, kwds): 103 self.gen = func(*args, **kwds) 104 self.func, self.args, self.kwds = func, args, kwds 105 # Issue 19330: ensure context manager instances have good docstrings 106 doc = getattr(func, "__doc__", None) 107 if doc is None: 108 doc = type(self).__doc__ 109 self.__doc__ = doc 110 # Unfortunately, this still doesn't provide good help output when 111 # inspecting the created context manager instances, since pydoc 112 # currently bypasses the instance docstring and shows the docstring 113 # for the class instead. 114 # See http://bugs.python.org/issue19404 for more details. 115 116 def _recreate_cm(self): 117 # _GCMB instances are one-shot context managers, so the 118 # CM must be recreated each time a decorated function is 119 # called 120 return self.__class__(self.func, self.args, self.kwds) 121 122 123class _GeneratorContextManager( 124 _GeneratorContextManagerBase, 125 AbstractContextManager, 126 ContextDecorator, 127): 128 """Helper for @contextmanager decorator.""" 129 130 def __enter__(self): 131 # do not keep args and kwds alive unnecessarily 132 # they are only needed for recreation, which is not possible anymore 133 del self.args, self.kwds, self.func 134 try: 135 return next(self.gen) 136 except StopIteration: 137 raise RuntimeError("generator didn't yield") from None 138 139 def __exit__(self, typ, value, traceback): 140 if typ is None: 141 try: 142 next(self.gen) 143 except StopIteration: 144 return False 145 else: 146 raise RuntimeError("generator didn't stop") 147 else: 148 if value is None: 149 # Need to force instantiation so we can reliably 150 # tell if we get the same exception back 151 value = typ() 152 try: 153 self.gen.throw(typ, value, traceback) 154 except StopIteration as exc: 155 # Suppress StopIteration *unless* it's the same exception that 156 # was passed to throw(). This prevents a StopIteration 157 # raised inside the "with" statement from being suppressed. 158 return exc is not value 159 except RuntimeError as exc: 160 # Don't re-raise the passed in exception. (issue27122) 161 if exc is value: 162 return False 163 # Avoid suppressing if a StopIteration exception 164 # was passed to throw() and later wrapped into a RuntimeError 165 # (see PEP 479 for sync generators; async generators also 166 # have this behavior). But do this only if the exception wrapped 167 # by the RuntimeError is actually Stop(Async)Iteration (see 168 # issue29692). 169 if ( 170 isinstance(value, StopIteration) 171 and exc.__cause__ is value 172 ): 173 return False 174 raise 175 except BaseException as exc: 176 # only re-raise if it's *not* the exception that was 177 # passed to throw(), because __exit__() must not raise 178 # an exception unless __exit__() itself failed. But throw() 179 # has to raise the exception to signal propagation, so this 180 # fixes the impedance mismatch between the throw() protocol 181 # and the __exit__() protocol. 182 if exc is not value: 183 raise 184 return False 185 raise RuntimeError("generator didn't stop after throw()") 186 187class _AsyncGeneratorContextManager( 188 _GeneratorContextManagerBase, 189 AbstractAsyncContextManager, 190 AsyncContextDecorator, 191): 192 """Helper for @asynccontextmanager decorator.""" 193 194 async def __aenter__(self): 195 # do not keep args and kwds alive unnecessarily 196 # they are only needed for recreation, which is not possible anymore 197 del self.args, self.kwds, self.func 198 try: 199 return await anext(self.gen) 200 except StopAsyncIteration: 201 raise RuntimeError("generator didn't yield") from None 202 203 async def __aexit__(self, typ, value, traceback): 204 if typ is None: 205 try: 206 await anext(self.gen) 207 except StopAsyncIteration: 208 return False 209 else: 210 raise RuntimeError("generator didn't stop") 211 else: 212 if value is None: 213 # Need to force instantiation so we can reliably 214 # tell if we get the same exception back 215 value = typ() 216 try: 217 await self.gen.athrow(typ, value, traceback) 218 except StopAsyncIteration as exc: 219 # Suppress StopIteration *unless* it's the same exception that 220 # was passed to throw(). This prevents a StopIteration 221 # raised inside the "with" statement from being suppressed. 222 return exc is not value 223 except RuntimeError as exc: 224 # Don't re-raise the passed in exception. (issue27122) 225 if exc is value: 226 return False 227 # Avoid suppressing if a Stop(Async)Iteration exception 228 # was passed to athrow() and later wrapped into a RuntimeError 229 # (see PEP 479 for sync generators; async generators also 230 # have this behavior). But do this only if the exception wrapped 231 # by the RuntimeError is actully Stop(Async)Iteration (see 232 # issue29692). 233 if ( 234 isinstance(value, (StopIteration, StopAsyncIteration)) 235 and exc.__cause__ is value 236 ): 237 return False 238 raise 239 except BaseException as exc: 240 # only re-raise if it's *not* the exception that was 241 # passed to throw(), because __exit__() must not raise 242 # an exception unless __exit__() itself failed. But throw() 243 # has to raise the exception to signal propagation, so this 244 # fixes the impedance mismatch between the throw() protocol 245 # and the __exit__() protocol. 246 if exc is not value: 247 raise 248 return False 249 raise RuntimeError("generator didn't stop after athrow()") 250 251 252def contextmanager(func): 253 """@contextmanager decorator. 254 255 Typical usage: 256 257 @contextmanager 258 def some_generator(<arguments>): 259 <setup> 260 try: 261 yield <value> 262 finally: 263 <cleanup> 264 265 This makes this: 266 267 with some_generator(<arguments>) as <variable>: 268 <body> 269 270 equivalent to this: 271 272 <setup> 273 try: 274 <variable> = <value> 275 <body> 276 finally: 277 <cleanup> 278 """ 279 @wraps(func) 280 def helper(*args, **kwds): 281 return _GeneratorContextManager(func, args, kwds) 282 return helper 283 284 285def asynccontextmanager(func): 286 """@asynccontextmanager decorator. 287 288 Typical usage: 289 290 @asynccontextmanager 291 async def some_async_generator(<arguments>): 292 <setup> 293 try: 294 yield <value> 295 finally: 296 <cleanup> 297 298 This makes this: 299 300 async with some_async_generator(<arguments>) as <variable>: 301 <body> 302 303 equivalent to this: 304 305 <setup> 306 try: 307 <variable> = <value> 308 <body> 309 finally: 310 <cleanup> 311 """ 312 @wraps(func) 313 def helper(*args, **kwds): 314 return _AsyncGeneratorContextManager(func, args, kwds) 315 return helper 316 317 318class closing(AbstractContextManager): 319 """Context to automatically close something at the end of a block. 320 321 Code like this: 322 323 with closing(<module>.open(<arguments>)) as f: 324 <block> 325 326 is equivalent to this: 327 328 f = <module>.open(<arguments>) 329 try: 330 <block> 331 finally: 332 f.close() 333 334 """ 335 def __init__(self, thing): 336 self.thing = thing 337 def __enter__(self): 338 return self.thing 339 def __exit__(self, *exc_info): 340 self.thing.close() 341 342 343class aclosing(AbstractAsyncContextManager): 344 """Async context manager for safely finalizing an asynchronously cleaned-up 345 resource such as an async generator, calling its ``aclose()`` method. 346 347 Code like this: 348 349 async with aclosing(<module>.fetch(<arguments>)) as agen: 350 <block> 351 352 is equivalent to this: 353 354 agen = <module>.fetch(<arguments>) 355 try: 356 <block> 357 finally: 358 await agen.aclose() 359 360 """ 361 def __init__(self, thing): 362 self.thing = thing 363 async def __aenter__(self): 364 return self.thing 365 async def __aexit__(self, *exc_info): 366 await self.thing.aclose() 367 368 369class _RedirectStream(AbstractContextManager): 370 371 _stream = None 372 373 def __init__(self, new_target): 374 self._new_target = new_target 375 # We use a list of old targets to make this CM re-entrant 376 self._old_targets = [] 377 378 def __enter__(self): 379 self._old_targets.append(getattr(sys, self._stream)) 380 setattr(sys, self._stream, self._new_target) 381 return self._new_target 382 383 def __exit__(self, exctype, excinst, exctb): 384 setattr(sys, self._stream, self._old_targets.pop()) 385 386 387class redirect_stdout(_RedirectStream): 388 """Context manager for temporarily redirecting stdout to another file. 389 390 # How to send help() to stderr 391 with redirect_stdout(sys.stderr): 392 help(dir) 393 394 # How to write help() to a file 395 with open('help.txt', 'w') as f: 396 with redirect_stdout(f): 397 help(pow) 398 """ 399 400 _stream = "stdout" 401 402 403class redirect_stderr(_RedirectStream): 404 """Context manager for temporarily redirecting stderr to another file.""" 405 406 _stream = "stderr" 407 408 409class suppress(AbstractContextManager): 410 """Context manager to suppress specified exceptions 411 412 After the exception is suppressed, execution proceeds with the next 413 statement following the with statement. 414 415 with suppress(FileNotFoundError): 416 os.remove(somefile) 417 # Execution still resumes here if the file was already removed 418 """ 419 420 def __init__(self, *exceptions): 421 self._exceptions = exceptions 422 423 def __enter__(self): 424 pass 425 426 def __exit__(self, exctype, excinst, exctb): 427 # Unlike isinstance and issubclass, CPython exception handling 428 # currently only looks at the concrete type hierarchy (ignoring 429 # the instance and subclass checking hooks). While Guido considers 430 # that a bug rather than a feature, it's a fairly hard one to fix 431 # due to various internal implementation details. suppress provides 432 # the simpler issubclass based semantics, rather than trying to 433 # exactly reproduce the limitations of the CPython interpreter. 434 # 435 # See http://bugs.python.org/issue12029 for more details 436 return exctype is not None and issubclass(exctype, self._exceptions) 437 438 439class _BaseExitStack: 440 """A base class for ExitStack and AsyncExitStack.""" 441 442 @staticmethod 443 def _create_exit_wrapper(cm, cm_exit): 444 return MethodType(cm_exit, cm) 445 446 @staticmethod 447 def _create_cb_wrapper(callback, /, *args, **kwds): 448 def _exit_wrapper(exc_type, exc, tb): 449 callback(*args, **kwds) 450 return _exit_wrapper 451 452 def __init__(self): 453 self._exit_callbacks = deque() 454 455 def pop_all(self): 456 """Preserve the context stack by transferring it to a new instance.""" 457 new_stack = type(self)() 458 new_stack._exit_callbacks = self._exit_callbacks 459 self._exit_callbacks = deque() 460 return new_stack 461 462 def push(self, exit): 463 """Registers a callback with the standard __exit__ method signature. 464 465 Can suppress exceptions the same way __exit__ method can. 466 Also accepts any object with an __exit__ method (registering a call 467 to the method instead of the object itself). 468 """ 469 # We use an unbound method rather than a bound method to follow 470 # the standard lookup behaviour for special methods. 471 _cb_type = type(exit) 472 473 try: 474 exit_method = _cb_type.__exit__ 475 except AttributeError: 476 # Not a context manager, so assume it's a callable. 477 self._push_exit_callback(exit) 478 else: 479 self._push_cm_exit(exit, exit_method) 480 return exit # Allow use as a decorator. 481 482 def enter_context(self, cm): 483 """Enters the supplied context manager. 484 485 If successful, also pushes its __exit__ method as a callback and 486 returns the result of the __enter__ method. 487 """ 488 # We look up the special methods on the type to match the with 489 # statement. 490 _cm_type = type(cm) 491 _exit = _cm_type.__exit__ 492 result = _cm_type.__enter__(cm) 493 self._push_cm_exit(cm, _exit) 494 return result 495 496 def callback(self, callback, /, *args, **kwds): 497 """Registers an arbitrary callback and arguments. 498 499 Cannot suppress exceptions. 500 """ 501 _exit_wrapper = self._create_cb_wrapper(callback, *args, **kwds) 502 503 # We changed the signature, so using @wraps is not appropriate, but 504 # setting __wrapped__ may still help with introspection. 505 _exit_wrapper.__wrapped__ = callback 506 self._push_exit_callback(_exit_wrapper) 507 return callback # Allow use as a decorator 508 509 def _push_cm_exit(self, cm, cm_exit): 510 """Helper to correctly register callbacks to __exit__ methods.""" 511 _exit_wrapper = self._create_exit_wrapper(cm, cm_exit) 512 self._push_exit_callback(_exit_wrapper, True) 513 514 def _push_exit_callback(self, callback, is_sync=True): 515 self._exit_callbacks.append((is_sync, callback)) 516 517 518# Inspired by discussions on http://bugs.python.org/issue13585 519class ExitStack(_BaseExitStack, AbstractContextManager): 520 """Context manager for dynamic management of a stack of exit callbacks. 521 522 For example: 523 with ExitStack() as stack: 524 files = [stack.enter_context(open(fname)) for fname in filenames] 525 # All opened files will automatically be closed at the end of 526 # the with statement, even if attempts to open files later 527 # in the list raise an exception. 528 """ 529 530 def __enter__(self): 531 return self 532 533 def __exit__(self, *exc_details): 534 received_exc = exc_details[0] is not None 535 536 # We manipulate the exception state so it behaves as though 537 # we were actually nesting multiple with statements 538 frame_exc = sys.exc_info()[1] 539 def _fix_exception_context(new_exc, old_exc): 540 # Context may not be correct, so find the end of the chain 541 while 1: 542 exc_context = new_exc.__context__ 543 if exc_context is None or exc_context is old_exc: 544 # Context is already set correctly (see issue 20317) 545 return 546 if exc_context is frame_exc: 547 break 548 new_exc = exc_context 549 # Change the end of the chain to point to the exception 550 # we expect it to reference 551 new_exc.__context__ = old_exc 552 553 # Callbacks are invoked in LIFO order to match the behaviour of 554 # nested context managers 555 suppressed_exc = False 556 pending_raise = False 557 while self._exit_callbacks: 558 is_sync, cb = self._exit_callbacks.pop() 559 assert is_sync 560 try: 561 if cb(*exc_details): 562 suppressed_exc = True 563 pending_raise = False 564 exc_details = (None, None, None) 565 except: 566 new_exc_details = sys.exc_info() 567 # simulate the stack of exceptions by setting the context 568 _fix_exception_context(new_exc_details[1], exc_details[1]) 569 pending_raise = True 570 exc_details = new_exc_details 571 if pending_raise: 572 try: 573 # bare "raise exc_details[1]" replaces our carefully 574 # set-up context 575 fixed_ctx = exc_details[1].__context__ 576 raise exc_details[1] 577 except BaseException: 578 exc_details[1].__context__ = fixed_ctx 579 raise 580 return received_exc and suppressed_exc 581 582 def close(self): 583 """Immediately unwind the context stack.""" 584 self.__exit__(None, None, None) 585 586 587# Inspired by discussions on https://bugs.python.org/issue29302 588class AsyncExitStack(_BaseExitStack, AbstractAsyncContextManager): 589 """Async context manager for dynamic management of a stack of exit 590 callbacks. 591 592 For example: 593 async with AsyncExitStack() as stack: 594 connections = [await stack.enter_async_context(get_connection()) 595 for i in range(5)] 596 # All opened connections will automatically be released at the 597 # end of the async with statement, even if attempts to open a 598 # connection later in the list raise an exception. 599 """ 600 601 @staticmethod 602 def _create_async_exit_wrapper(cm, cm_exit): 603 return MethodType(cm_exit, cm) 604 605 @staticmethod 606 def _create_async_cb_wrapper(callback, /, *args, **kwds): 607 async def _exit_wrapper(exc_type, exc, tb): 608 await callback(*args, **kwds) 609 return _exit_wrapper 610 611 async def enter_async_context(self, cm): 612 """Enters the supplied async context manager. 613 614 If successful, also pushes its __aexit__ method as a callback and 615 returns the result of the __aenter__ method. 616 """ 617 _cm_type = type(cm) 618 _exit = _cm_type.__aexit__ 619 result = await _cm_type.__aenter__(cm) 620 self._push_async_cm_exit(cm, _exit) 621 return result 622 623 def push_async_exit(self, exit): 624 """Registers a coroutine function with the standard __aexit__ method 625 signature. 626 627 Can suppress exceptions the same way __aexit__ method can. 628 Also accepts any object with an __aexit__ method (registering a call 629 to the method instead of the object itself). 630 """ 631 _cb_type = type(exit) 632 try: 633 exit_method = _cb_type.__aexit__ 634 except AttributeError: 635 # Not an async context manager, so assume it's a coroutine function 636 self._push_exit_callback(exit, False) 637 else: 638 self._push_async_cm_exit(exit, exit_method) 639 return exit # Allow use as a decorator 640 641 def push_async_callback(self, callback, /, *args, **kwds): 642 """Registers an arbitrary coroutine function and arguments. 643 644 Cannot suppress exceptions. 645 """ 646 _exit_wrapper = self._create_async_cb_wrapper(callback, *args, **kwds) 647 648 # We changed the signature, so using @wraps is not appropriate, but 649 # setting __wrapped__ may still help with introspection. 650 _exit_wrapper.__wrapped__ = callback 651 self._push_exit_callback(_exit_wrapper, False) 652 return callback # Allow use as a decorator 653 654 async def aclose(self): 655 """Immediately unwind the context stack.""" 656 await self.__aexit__(None, None, None) 657 658 def _push_async_cm_exit(self, cm, cm_exit): 659 """Helper to correctly register coroutine function to __aexit__ 660 method.""" 661 _exit_wrapper = self._create_async_exit_wrapper(cm, cm_exit) 662 self._push_exit_callback(_exit_wrapper, False) 663 664 async def __aenter__(self): 665 return self 666 667 async def __aexit__(self, *exc_details): 668 received_exc = exc_details[0] is not None 669 670 # We manipulate the exception state so it behaves as though 671 # we were actually nesting multiple with statements 672 frame_exc = sys.exc_info()[1] 673 def _fix_exception_context(new_exc, old_exc): 674 # Context may not be correct, so find the end of the chain 675 while 1: 676 exc_context = new_exc.__context__ 677 if exc_context is None or exc_context is old_exc: 678 # Context is already set correctly (see issue 20317) 679 return 680 if exc_context is frame_exc: 681 break 682 new_exc = exc_context 683 # Change the end of the chain to point to the exception 684 # we expect it to reference 685 new_exc.__context__ = old_exc 686 687 # Callbacks are invoked in LIFO order to match the behaviour of 688 # nested context managers 689 suppressed_exc = False 690 pending_raise = False 691 while self._exit_callbacks: 692 is_sync, cb = self._exit_callbacks.pop() 693 try: 694 if is_sync: 695 cb_suppress = cb(*exc_details) 696 else: 697 cb_suppress = await cb(*exc_details) 698 699 if cb_suppress: 700 suppressed_exc = True 701 pending_raise = False 702 exc_details = (None, None, None) 703 except: 704 new_exc_details = sys.exc_info() 705 # simulate the stack of exceptions by setting the context 706 _fix_exception_context(new_exc_details[1], exc_details[1]) 707 pending_raise = True 708 exc_details = new_exc_details 709 if pending_raise: 710 try: 711 # bare "raise exc_details[1]" replaces our carefully 712 # set-up context 713 fixed_ctx = exc_details[1].__context__ 714 raise exc_details[1] 715 except BaseException: 716 exc_details[1].__context__ = fixed_ctx 717 raise 718 return received_exc and suppressed_exc 719 720 721class nullcontext(AbstractContextManager, AbstractAsyncContextManager): 722 """Context manager that does no additional processing. 723 724 Used as a stand-in for a normal context manager, when a particular 725 block of code is only sometimes used with a normal context manager: 726 727 cm = optional_cm if condition else nullcontext() 728 with cm: 729 # Perform operation, using optional_cm if condition is True 730 """ 731 732 def __init__(self, enter_result=None): 733 self.enter_result = enter_result 734 735 def __enter__(self): 736 return self.enter_result 737 738 def __exit__(self, *excinfo): 739 pass 740 741 async def __aenter__(self): 742 return self.enter_result 743 744 async def __aexit__(self, *excinfo): 745 pass 746