New version from Greg.
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Guido van Rossum
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162
pep-0380.txt
162
pep-0380.txt
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@ -40,8 +40,8 @@ much difficulty using a loop such as
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::
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for v in g:
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yield v
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for v in g:
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yield v
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However, if the subgenerator is to interact properly with the caller
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in the case of calls to ``send()``, ``throw()`` and ``close()``, things
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@ -63,7 +63,7 @@ generator:
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::
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yield from <expr>
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yield from <expr>
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where <expr> is an expression evaluating to an iterable, from which an
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iterator is extracted. The iterator is run to exhaustion, during which
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@ -78,30 +78,30 @@ becomes the value of the ``yield from`` expression.
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In general, the semantics can be described in terms of the iterator
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protocol as follows:
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* Any values that the iterator yields are passed directly to the
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caller.
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* Any values that the iterator yields are passed directly to the
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caller.
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* Any values sent to the delegating generator using ``send()``
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are passed directly to the iterator. If the sent value is None,
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the iterator's ``next()`` method is called. If the sent value is
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not None, the iterator's ``send()`` method is called. Any exception
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resulting from attempting to call ``next`` or ``send`` is raised
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in the delegating generator.
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* Any values sent to the delegating generator using ``send()``
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are passed directly to the iterator. If the sent value is None,
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the iterator's ``next()`` method is called. If the sent value is
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not None, the iterator's ``send()`` method is called. Any exception
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resulting from attempting to call ``next`` or ``send`` is raised
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in the delegating generator.
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* Exceptions passed to the ``throw()`` method of the delegating
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generator are forwarded to the ``throw()`` method of the iterator.
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If the iterator does not have a ``throw()`` method, its ``close()``
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method is called if it has one, then the thrown-in exception is
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raised in the delegating generator. Any exception resulting from
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attempting to call these methods (apart from one case noted below)
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is raised in the delegating generator.
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* Exceptions passed to the ``throw()`` method of the delegating
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generator are forwarded to the ``throw()`` method of the iterator.
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If the iterator does not have a ``throw()`` method, its ``close()``
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method is called if it has one, then the thrown-in exception is
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raised in the delegating generator. Any exception resulting from
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attempting to call these methods (apart from one case noted below)
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is raised in the delegating generator.
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* The value of the ``yield from`` expression is the first argument
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to the ``StopIteration`` exception raised by the iterator when it
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terminates.
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* The value of the ``yield from`` expression is the first argument
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to the ``StopIteration`` exception raised by the iterator when it
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terminates.
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* ``return expr`` in a generator causes ``StopIteration(expr)`` to
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be raised.
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* ``return expr`` in a generator causes ``StopIteration(expr)`` to
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be raised.
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Fine Details
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@ -113,8 +113,9 @@ An iterator having a ``throw()`` method is expected to recognize
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this as a request to finalize itself.
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If a call to the iterator's ``throw()`` method raises a StopIteration
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exception, and it is *not* the same exception object that was thrown
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in, its value is returned as the value of the ``yield from`` expression
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exception, and it is *not* the same exception object that was thrown in,
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and the original exception was not GeneratorExit, then the value of the
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new exception is returned as the value of the ``yield from`` expression
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and the delegating generator is resumed.
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@ -133,51 +134,48 @@ Formal Semantics
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::
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RESULT = yield from EXPR
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RESULT = yield from EXPR
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is semantically equivalent to
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::
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_i = iter(EXPR)
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try:
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try:
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_y = _i.next()
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except StopIteration, _e:
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_r = _e.value
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else:
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while 1:
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try:
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_s = yield _y
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except:
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_m = getattr(_i, 'throw', None)
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if _m is not None:
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_x = sys.exc_info()
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try:
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_y = _m(*_x)
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except StopIteration, _e:
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if _e is _x[1]:
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raise
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else:
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_r = _e.value
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break
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else:
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_m = getattr(_i, 'close', None)
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if _m is not None:
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_m()
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raise
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else:
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try:
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if _s is None:
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_y = _i.next()
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else:
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_y = _i.send(_s)
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except StopIteration, _e:
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_r = _e.value
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break
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finally:
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del _i
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RESULT = _r
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_i = iter(EXPR)
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try:
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_y = _i.next()
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except StopIteration, _e:
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_r = _e.value
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else:
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while 1:
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try:
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_s = yield _y
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except:
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_m = getattr(_i, 'throw', None)
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if _m is not None:
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_x = sys.exc_info()
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try:
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_y = _m(*_x)
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except StopIteration, _e:
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if _e is _x[1] or isinstance(_x[1], GeneratorExit):
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raise
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else:
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_r = _e.value
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break
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else:
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_m = getattr(_i, 'close', None)
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if _m is not None:
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_m()
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raise
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else:
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try:
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if _s is None:
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_y = _i.next()
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else:
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_y = _i.send(_s)
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except StopIteration, _e:
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_r = _e.value
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break
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RESULT = _r
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except that implementations are free to cache bound methods for the 'next',
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'send' and 'throw' methods of the iterator upon first use.
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@ -186,13 +184,13 @@ except that implementations are free to cache bound methods for the 'next',
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::
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return value
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return value
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is semantically equivalent to
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::
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raise StopIteration(value)
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raise StopIteration(value)
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except that, as currently, the exception cannot be caught by ``except``
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clauses within the returning generator.
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@ -201,14 +199,14 @@ clauses within the returning generator.
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::
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class StopIteration(Exception):
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class StopIteration(Exception):
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def __init__(self, *args):
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if len(args) > 0:
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self.value = args[0]
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else:
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self.value = None
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Exception.__init__(self, *args)
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def __init__(self, *args):
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if len(args) > 0:
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self.value = args[0]
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else:
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self.value = None
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Exception.__init__(self, *args)
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Rationale
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@ -249,7 +247,7 @@ so to an unfactored one in all Python implementations.
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The assumption made is that, in the majority of use cases, the subiterator
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will not be shared. The rare case of a shared subiterator can be
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accommodated by means of a wrapper that blocks ``throw()`` and ``send()``
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accommodated by means of a wrapper that blocks ``throw()`` and ``close()``
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calls, or by using a means other than ``yield from`` to call the
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subiterator.
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@ -269,14 +267,14 @@ Using the proposed syntax, a statement such as
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::
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y = f(x)
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y = f(x)
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where f is an ordinary function, can be transformed into a delegation
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call
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::
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y = yield from g(x)
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y = yield from g(x)
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where g is a generator. One can reason about the behaviour of the
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resulting code by thinking of g as an ordinary function that can be
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@ -341,13 +339,13 @@ value of the ``close()`` call to the subgenerator. However, the proposed
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mechanism is attractive for a couple of reasons:
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* Using the StopIteration exception makes it easy for other kinds
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of iterators to participate in the protocol without having to
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grow an extra attribute or a close() method.
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of iterators to participate in the protocol without having to
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grow an extra attribute or a close() method.
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* It simplifies the implementation, because the point at which the
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return value from the subgenerator becomes available is the same
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point at which StopIteration is raised. Delaying until any later
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time would require storing the return value somewhere.
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return value from the subgenerator becomes available is the same
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point at which StopIteration is raised. Delaying until any later
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time would require storing the return value somewhere.
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Criticisms
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