Fix two examples that were broken by the new semantics.
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Guido van Rossum
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124
pep-0343.txt
124
pep-0343.txt
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@ -576,7 +576,7 @@ Examples
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The tense used in the names of the example context managers is not
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arbitrary. Past tense ("-ed") is used when the name refers to an
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action which is done in the __enter__ method and undone in the
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__exit__ method. Progressive tense ("-ing") is used when the name
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__exit__ method. Progressive tense ("-ing") is used when the name
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refers to an action which is to be done in the __exit__ method.
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1. A template for ensuring that a lock, acquired at the start of a
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@ -643,6 +643,8 @@ Examples
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self.lock.acquire()
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def __exit__(self, type, value, tb):
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self.lock.release()
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if type is not None:
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raise type, value, tb
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(This example is easily modified to implement the other
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relatively stateless examples; it shows that it is easy to avoid
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@ -769,34 +771,35 @@ Examples
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# General Decimal Arithmetic Specification
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return +s # Convert result to normal context
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10. A generic "object-closing" template:
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10. A generic "object-closing" template:
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@contextmanager
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def closing(obj):
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try:
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yield obj
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finally:
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try:
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close = obj.close
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except AttributeError:
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pass
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else:
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close()
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@contextmanager
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def closing(obj):
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try:
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yield obj
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finally:
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try:
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close = obj.close
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except AttributeError:
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pass
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else:
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close()
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This can be used to deterministically close anything with a
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close method, be it file, generator, or something else. It can
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even be used when the object isn't guaranteed to require
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closing (e.g., a function that accepts an arbitrary iterable):
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This can be used to deterministically close anything with a
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close method, be it file, generator, or something else. It
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can even be used when the object isn't guaranteed to require
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closing (e.g., a function that accepts an arbitrary
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iterable):
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# emulate opening():
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with closing(open("argument.txt")) as contradiction:
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for line in contradiction:
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print line
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# emulate opening():
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with closing(open("argument.txt")) as contradiction:
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for line in contradiction:
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print line
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# deterministically finalize an iterator:
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with closing(iter(data_source)) as data:
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for datum in data:
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process(datum)
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# deterministically finalize an iterator:
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with closing(iter(data_source)) as data:
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for datum in data:
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process(datum)
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11. Native contexts for objects with acquire/release methods:
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@ -807,7 +810,7 @@ Examples
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def released(self):
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return unlocked(self)
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Sample usage:
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Sample usage:
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with my_lock:
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# Operations with the lock held
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@ -820,54 +823,43 @@ Examples
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supplied contexts from left-to-right to avoid excessive
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indentation:
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class nested(object):
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def __init__(self, *contexts):
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self.contexts = contexts
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self.entered = None
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def __context__(self):
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return self
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def __enter__(self):
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if self.entered is not None:
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raise RuntimeError("Context is not reentrant")
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self.entered = deque()
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vars = []
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@contextmanager
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def nested(*contexts):
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exits = []
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vars = []
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exc = (None, None, None)
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try:
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try:
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for context in self.contexts:
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for context in contexts:
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mgr = context.__context__()
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vars.append(mgr.__enter__())
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self.entered.appendleft(mgr)
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exit = mgr.__exit__
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enter = mgr.__enter__
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vars.append(enter())
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exits.append(exit)
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yield vars
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except:
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self.__exit__(*sys.exc_info())
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raise
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return vars
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def __exit__(self, *exc_info):
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# Behave like nested with statements
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# first in, last out
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# New exceptions override old ones
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ex = exc_info
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for mgr in self.entered:
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exc = sys.exc_info()
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finally:
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while exits:
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exit = exits.pop()
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try:
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mgr.__exit__(*ex)
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exit(*exc)
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except:
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ex = sys.exc_info()
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self.entered = None
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if ex is not exc_info:
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raise ex[0], ex[1], ex[2]
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exc = sys.exc_info()
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if exc != (None, None, None):
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raise
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Sample usage:
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Sample usage:
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with nested(a, b, c) as (x, y, z):
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# Perform operation
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with nested(a, b, c) as (x, y, z):
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# Perform operation
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Is equivalent to:
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Is equivalent to:
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with a as x:
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with b as y:
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with c as z:
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# Perform operation
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with a as x:
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with b as y:
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with c as z:
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# Perform operation
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Reference Implementation
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@ -911,6 +903,8 @@ References
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[12]
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http://sourceforge.net/tracker/index.php?func=detail&aid=1223381&group_id=5470&atid=305470
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[13]
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http://mail.python.org/pipermail/python-dev/2006-February/061903.html
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Copyright
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