[Python-ideas] Cofunctions PEP - Revision 4

[Greg Ewing]

Here's an updated version of the PEP reflecting my
recent suggestions on how to eliminate 'codef'.

PEP: XXX
Title: Cofunctions
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: 3.x
Post-History:


Abstract
========

A syntax is proposed for defining and calling a special type of generator
called a 'cofunction'.  It is designed to provide a streamlined way of
writing generator-based coroutines, and allow the early detection of
certain kinds of error that are easily made when writing such code, which
otherwise tend to cause hard-to-diagnose symptoms.

This proposal builds on the 'yield from' mechanism described in PEP 380,
and describes some of the semantics of cofunctions in terms of it. However,
it would be possible to define and implement cofunctions independently of
PEP 380 if so desired.


Specification
=============

Cofunction definitions
----------------------

A cofunction is a special kind of generator, distinguished by the presence
of the keyword ``cocall`` (defined below) at least once in its body. It may
also contain ``yield`` and/or ``yield from`` expressions, which behave as
they do in other generators.

 From the outside, the distinguishing feature of a cofunction is that it cannot
be called the same way as an ordinary function. An exception is raised if an
ordinary call to a cofunction is attempted.

Cocalls
-------

Calls from one cofunction to another are made by marking the call with
a new keyword ``cocall``. The expression

::

     cocall f(*args, **kwds)

is evaluated by first checking whether the object ``f`` implements
a ``__cocall__`` method. If it does, the cocall expression is
equivalent to

::

     yield from f.__cocall__(*args, **kwds)

except that the object returned by __cocall__ is expected to be an
iterator, so the step of calling iter() on it is skipped.

If ``f`` does not have a ``__cocall__`` method, or the ``__cocall__``
method returns ``NotImplemented``, then the cocall expression is
treated as an ordinary call, and the ``__call__`` method of ``f``
is invoked.

Objects which implement __cocall__ are expected to return an object
obeying the iterator protocol. Cofunctions respond to __cocall__ the
same way as ordinary generator functions respond to __call__, i.e. by
returning a generator-iterator.

Certain objects that wrap other callable objects, notably bound methods,
will be given __cocall__ implementations that delegate to the underlying
object.

Grammar
-------

The full syntax of a cocall expression is described by the following
grammar lines:

::

     atom: cocall | <existing alternatives for atom>
     cocall: 'cocall' atom cotrailer* '(' [arglist] ')'
     cotrailer: '[' subscriptlist ']' | '.' NAME

Note that this syntax allows cocalls to methods and elements of sequences
or mappings to be expressed naturally. For example, the following are valid:

::

     y = cocall self.foo(x)
     y = cocall funcdict[key](x)
     y = cocall a.b.c[i].d(x)

Also note that the final calling parentheses are mandatory, so that for example
the following is invalid syntax:

::

     y = cocall f     # INVALID

New builtins, attributes and C API functions
--------------------------------------------

To facilitate interfacing cofunctions with non-coroutine code, there will
be a built-in function ``costart`` whose definition is equivalent to

::

     def costart(obj, *args, **kwds):
         try:
             m = obj.__cocall__
         except AttributeError:
             result = NotImplemented
         else:
             result = m(*args, **kwds)
         if result is NotImplemented:
             raise TypeError("Object does not support cocall")
         return result

There will also be a corresponding C API function

::

     PyObject *PyObject_CoCall(PyObject *obj, PyObject *args, PyObject *kwds)

It is left unspecified for now whether a cofunction is a distinct type
of object or, like a generator function, is simply a specially-marked
function instance. If the latter, a read-only boolean attribute
``__iscofunction__`` should be provided to allow testing whether a given
function object is a cofunction.


Motivation and Rationale
========================

The ``yield from`` syntax is reasonably self-explanatory when used for the
purpose of delegating part of the work of a generator to another function. It
can also be used to good effect in the implementation of generator-based
coroutines, but it reads somewhat awkwardly when used for that purpose, and
tends to obscure the true intent of the code.

Furthermore, using generators as coroutines is somewhat error-prone. If one
forgets to use ``yield from`` when it should have been used, or uses it when it
shouldn't have, the symptoms that result can be extremely obscure and confusing.

Finally, sometimes there is a need for a function to be a coroutine even though
it does not yield anything, and in these cases it is necessary to resort to
kludges such as ``if 0: yield`` to force it to be a generator.

The ``cocall`` construct address the first issue by making the syntax directly
reflect the intent, that is, that the function being called forms part of a
coroutine.

The second issue is addressed by making it impossible to mix coroutine and
non-coroutine code in ways that don't make sense. If the rules are violated, an
exception is raised that points out exactly what and where the problem is.

Lastly, the need for dummy yields is eliminated by making it possible for a
cofunction to call both cofunctions and ordinary functions with the same syntax,
so that an ordinary function can be used in place of a cofunction that yields
zero times.


Record of Discussion
====================

An earlier version of this proposal required a special keyword ``codef`` to be
used in place of ``def`` when defining a cofunction, and disallowed calling an
ordinary function using ``cocall``.  However, it became evident that these
features were not necessary, and the ``codef`` keyword was dropped in the
interests of minimising the number of new keywords required.

The use of a decorator instead of ``codef`` was also suggested, but the current
proposal makes this unnecessary as well.

It has been questioned whether some combination of decorators and functions
could be used instead of a dedicated ``cocall`` syntax. While this might be
possible, to achieve equivalent error-detecting power it would be necessary
to write cofunction calls as something like

::

     yield from cocall(f)(args)

making them even more verbose and inelegant than an unadorned ``yield from``.
It is also not clear whether it is possible to achieve all of the benefits of
the cocall syntax using this kind of approach.


Prototype Implementation
========================

An implementation of an earlier version of this proposal in the form of patches
to Python 3.1.2 can be found here:

http://www.cosc.canterbury.ac.nz/greg.ewing/python/generators/cofunctions.html

If this version of the proposal is received favourably, the implementation will
be updated to match.


Copyright
=========

This document has been placed in the public domain.



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