Revision 6 of the PEP. * Changed wording slightly to avoid implying that StopIteration(value) can be caught inside the returning generator. * Added a suggestion that StopIteration be given a 'value' attribute. * Mentioned the idea of a GeneratorReturn exception in the Criticisms section. PEP: XXX Title: Syntax for Delegating to a Subgenerator Version: $Revision$ Last-Modified: $Date$ Author: Gregory Ewing <greg.ewing@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 behaves as though it were communicating directly with 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. The subgenerator is allowed to execute a 'return' statement with a value, and that value becomes the value of the 'yield from' expression. In terms of the iterator protocol: * Any values that the iterator yields are passed directly to the caller. * Any values sent to the delegating generator using ``send()`` are sent directly to the iterator. (If the iterator does not have a ``send()`` method, it remains to be decided whether the value sent in is ignored or an exception is raised.) * 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 iterator is finalised before finalising the delegating generator. * 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 ---------------- 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'): _i.throw(_e) else: raise else: if hasattr(_i, 'send'): _u = _i.send(_v) else: _u = _i.next() except StopIteration, _e: _a = _e.args if len(_a) > 0: result = _a[0] else: result = None finally: if hasattr(_i, 'close'): _i.close() 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: