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| import sys | |
| from functools import wraps | |
| from warnings import warn | |
| class GeneratorContextManager(object): | |
| """Helper for @contextmanager decorator.""" | |
| def __init__(self, gen): | |
| self.gen = gen | |
| def __enter__(self): | |
| try: | |
| return next(self.gen) | |
| except StopIteration: | |
| raise RuntimeError("generator didn't yield") | |
| def pair(self): | |
| with self: | |
| pass | |
| def __exit__(self, type, value, traceback): | |
| if type is None: | |
| try: | |
| next(self.gen) | |
| except StopIteration: | |
| return | |
| else: | |
| raise RuntimeError("generator didn't stop") | |
| else: | |
| if value is None: | |
| # Need to force instantiation so we can reliably | |
| # tell if we get the same exception back | |
| value = type() | |
| try: | |
| self.gen.throw(type, value, traceback) | |
| raise RuntimeError("generator didn't stop after throw()") | |
| except StopIteration as exc: | |
| # Suppress the exception *unless* it's the same exception that | |
| # was passed to throw(). This prevents a StopIteration | |
| # raised inside the "with" statement from being suppressed | |
| return exc is not value | |
| except: | |
| # only re-raise if it's *not* the exception that was | |
| # passed to throw(), because __exit__() must not raise | |
| # an exception unless __exit__() itself failed. But throw() | |
| # has to raise the exception to signal propagation, so this | |
| # fixes the impedance mismatch between the throw() protocol | |
| # and the __exit__() protocol. | |
| # | |
| if sys.exc_info()[1] is not value: | |
| raise | |
| def contextmanager(func): | |
| """@contextmanager decorator. | |
| Typical usage: | |
| @contextmanager | |
| def some_generator(<arguments>): | |
| <setup> | |
| try: | |
| yield <value> | |
| finally: | |
| <cleanup> | |
| This makes this: | |
| with some_generator(<arguments>) as <variable>: | |
| <body> | |
| equivalent to this: | |
| <setup> | |
| try: | |
| <variable> = <value> | |
| <body> | |
| finally: | |
| <cleanup> | |
| """ | |
| #@wraps(func) | |
| def helper(*args, **kwds): | |
| return GeneratorContextManager(func(*args, **kwds)) | |
| return helper | |
| @contextmanager | |
| def tag(name): | |
| print( "<%s>" % name) | |
| yield | |
| print ("</%s>" % name) | |
| with tag('main'): | |
| with tag('sub'): | |
| print('hi') | |
| tag('foo').pair() | |
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