[Python-ideas] Revised**6 PEP on yield-from

Sun Feb 22 03:04:04 CET 2009

I haven't been following the discussion too closely, but shouldn't it be
"return value" is semantically equivalent to raise
StopIteration(value) instead of "raise value"? in bullet 2 of formal
semantics?
=====
--Ryan E. Freckleton
On Sat, Feb 21, 2009 at 3:43 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> I've re-worded things yet again to nail the semantics
> down as per recent discussions. Briefly:
>> * send(None) converted to next() upon delegation
> * send(not_None) raises exception if no send() method
> * throw() and close() ignore missing methods
>> No longer describing semantics in terms of "direct
> communication".
>> Fixed a bug in the expansion (return value of
> throw() was getting lost).
>>> PEP: XXX
> Title: Syntax for Delegating to a Subgenerator
> Version: $Revision$
> Last-Modified: $Date$
> Author: Gregory Ewing <greg.ewing at canterbury.ac.nz>
> Status: Draft
> Type: Standards Track
> Content-Type: text/x-rst
> Created: 13-Feb-2009
> Python-Version: 2.7
> Post-History:
>>> Abstract
> ========
>> A syntax is proposed for a generator to delegate part of its
> operations to another generator. This allows a section of code
> containing 'yield' to be factored out and placed in another
> generator. Additionally, the subgenerator is allowed to return with a
> value, and the value is made available to the delegating generator.
>> The new syntax also opens up some opportunities for optimisation when
> one generator re-yields values produced by another.
>>> Proposal
> ========
>> The following new expression syntax will be allowed in the body of a
> generator:
>> ::
>> yield from <expr>
>> where <expr> is an expression evaluating to an iterable, from which an
> iterator is extracted. The iterator is run to exhaustion, during which
> time it yields and receives values directly to or from the caller of
> the generator containing the ``yield from`` expression (the
> "delegating generator").
>> When the iterator is another generator, the effect is the same as if
> the body of the subgenerator were inlined at the point of the ``yield
> from`` expression. Furthermore, the subgenerator is allowed to execute
> a ``return`` statement with a value, and that value becomes the value of
> the ``yield from`` expression.
>> In general, the semantics can be understood in terms of the iterator
> protocol as follows:
>> * Any values that the iterator yields are passed directly to the
> caller.
>> * Any values sent to the delegating generator using ``send()``
> are passed directly to the iterator. If the sent value is None,
> the iterator's ``next()`` method is called. If the sent value is
> not None, the iterator's ``send()`` method is called if it has
> one, otherwise an exception is raised in the delegating generator.
>> * Calls to the ``throw()`` method of the delegating generator are
> forwarded to the iterator. If the iterator does not have a
> ``throw()`` method, the thrown-in exception is raised in the
> delegating generator.
>> * If the delegating generator's ``close()`` method is called, the
> ``close() method of the iterator is called first if it has one,
> then the delegating generator is finalised.
>> * The value of the ``yield from`` expression is the first argument
> to the ``StopIteration`` exception raised by the iterator when it
> terminates.
>> * ``return expr`` in a generator causes ``StopIteration(expr)`` to
> be raised.
>> For convenience, the ``StopIteration`` exception will be given a
> ``value`` attribute that holds its first argument, or None if there
> are no arguments.
>>> Formal Semantics
> ----------------
>> 1. The statement
>> ::
>> result = yield from expr
>> is semantically equivalent to
>> ::
>> _i = iter(expr)
> try:
> _u = _i.next()
> while 1:
> try:
> _v = yield _u
> except Exception, _e:
> if hasattr(_i, 'throw'):
> _u = _i.throw(_e)
> else:
> raise
> else:
> if _v is None:
> _u = _i.next()
> else:
> _u = _i.send(_v)
> except StopIteration, _e:
> result = _e.value
> finally:
> if hasattr(_i, 'close'):
> _i.close()
>> 2. In a generator, the statement
>> ::
>> raise value
>> is semantically equivalent to
>> raise StopIteration(value)
>> except that, as currently, the exception cannot be caught by 'except'
> clauses within the returning generator.
>> 3. The StopIteration exception behaves as though defined thusly:
>> ::
>> class StopIteration(Exception):
>> def __init__(self, *args):
> if len(args) > 0:
> self.value = args[0]
> else:
> self.value = None
> Exception.__init__(self, *args)
>>> Rationale
> =========
>> A Python generator is a form of coroutine, but has the limitation that
> it can only yield to its immediate caller. This means that a piece of
> code containing a ``yield`` cannot be factored out and put into a
> separate function in the same way as other code. Performing such a
> factoring causes the called function to itself become a generator, and
> it is necessary to explicitly iterate over this second generator and
> re-yield any values that it produces.
>> If yielding of values is the only concern, this is not very arduous
> and can be performed with a loop such as
>> ::
>> for v in g:
> yield v
>> However, if the subgenerator is to interact properly with the caller
> in the case of calls to ``send()``, ``throw()`` and ``close()``, things
> become considerably more complicated. As the formal expansion presented
> above illustrates, the necessary code is very longwinded, and it is tricky
> to handle all the corner cases correctly. In this situation, the advantages
> of a specialised syntax should be clear.
>>> Generators as Threads
> ---------------------
>> A motivating use case for generators being able to return values
> concerns the use of generators to implement lightweight threads. When
> using generators in that way, it is reasonable to want to spread the
> computation performed by the lightweight thread over many functions.
> One would like to be able to call a subgenerator as though it were
> an ordinary function, passing it parameters and receiving a returned
> value.
>> Using the proposed syntax, a statement such as
>> ::
>> y = f(x)
>> where f is an ordinary function, can be transformed into a delegation
> call
>> ::
>> y = yield from g(x)
>> where g is a generator. One can reason about the behaviour of the
> resulting code by thinking of g as an ordinary function that can be
> suspended using a ``yield`` statement.
>> When using generators as threads in this way, typically one is not
> interested in the values being passed in or out of the yields.
> However, there are use cases for this as well, where the thread is
> seen as a producer or consumer of items. The ``yield from``
> expression allows the logic of the thread to be spread over as
> many functions as desired, with the production or consumption of
> items occuring in any subfunction, and the items are automatically
> routed to or from their ultimate source or destination.
>> Concerning ``throw()`` and ``close()``, it is reasonable to expect
> that if an exception is thrown into the thread from outside, it should
> first be raised in the innermost generator where the thread is suspended,
> and propagate outwards from there; and that if the thread is terminated
> from outside by calling ``close()``, the chain of active generators
> should be finalised from the innermost outwards.
>>> Syntax
> ------
>> The particular syntax proposed has been chosen as suggestive of its
> meaning, while not introducing any new keywords and clearly standing
> out as being different from a plain ``yield``.
>>> Optimisations
> -------------
>> Using a specialised syntax opens up possibilities for optimisation
> when there is a long chain of generators. Such chains can arise, for
> instance, when recursively traversing a tree structure. The overhead
> of passing ``next()`` calls and yielded values down and up the chain
> can cause what ought to be an O(n) operation to become O(n\*\*2).
>> A possible strategy is to add a slot to generator objects to hold a
> generator being delegated to. When a ``next()`` or ``send()`` call is
> made on the generator, this slot is checked first, and if it is
> nonempty, the generator that it references is resumed instead. If it
> raises StopIteration, the slot is cleared and the main generator is
> resumed.
>> This would reduce the delegation overhead to a chain of C function
> calls involving no Python code execution. A possible enhancement would
> be to traverse the whole chain of generators in a loop and directly
> resume the one at the end, although the handling of StopIteration is
> more complicated then.
>>> Use of StopIteration to return values
> -------------------------------------
>> There are a variety of ways that the return value from the generator
> could be passed back. Some alternatives include storing it as an
> attribute of the generator-iterator object, or returning it as the
> value of the ``close()`` call to the subgenerator. However, the proposed
> mechanism is attractive for a couple of reasons:
>> * Using the StopIteration exception makes it easy for other kinds
> of iterators to participate in the protocol without having to
> grow extra attributes or a close() method.
>> * It simplifies the implementation, because the point at which the
> return value from the subgenerator becomes available is the same
> point at which StopIteration is raised. Delaying until any later
> time would require storing the return value somewhere.
>>> Criticisms
> ==========
>> Under this proposal, the value of a ``yield from`` expression would
> be derived in a very different way from that of an ordinary ``yield``
> expression. This suggests that some other syntax not containing the
> word ``yield`` might be more appropriate, but no acceptable alternative
> has so far been proposed.
>> It has been suggested that some mechanism other than ``return`` in
> the subgenerator should be used to establish the value returned by
> the ``yield from`` expression. However, this would interfere with
> the goal of being able to think of the subgenerator as a suspendable
> function, since it would not be able to return values in the same way
> as other functions.
>> The use of an argument to StopIteration to pass the return value
> has been criticised as an "abuse of exceptions", without any
> concrete justification of this claim. In any case, this is only
> one suggested implementation; another mechanism could be used
> without losing any essential features of the proposal.
>> It has been suggested that a different exception, such as
> GeneratorReturn, should be used instead of StopIteration to return a
> value. However, no convincing practical reason for this has been put
> forward, and the addition of a ``value`` attribute to StopIteration
> mitigates any difficulties in extracting a return value from a
> StopIteration exception that may or may not have one. Also, using a
> different exception would mean that, unlike ordinary functions,
> 'return' without a value in a generator would not be equivalent to
> 'return None'.
>>> Alternative Proposals
> =====================
>> Proposals along similar lines have been made before, some using the
> syntax ``yield *`` instead of ``yield from``. While ``yield *`` is
> more concise, it could be argued that it looks too similar to an
> ordinary ``yield`` and the difference might be overlooked when reading
> code.
>> To the author's knowledge, previous proposals have focused only on
> yielding values, and thereby suffered from the criticism that the
> two-line for-loop they replace is not sufficiently tiresome to write
> to justify a new syntax. By also dealing with calls to ``send()``,
> ``throw()`` and ``close()``, this proposal provides considerably more
> benefit.
>>> Copyright
> =========
>> This document has been placed in the public domain.
>>>> ..
> Local Variables:
> mode: indented-text
> indent-tabs-mode: nil
> sentence-end-double-space: t
> fill-column: 70
> coding: utf-8
> End:
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