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Last-Modified:2015-04-24 15:14:39 -0700 (Fri, 24 Apr 2015)
Author:Gregory Ewing <greg.ewing at>
Type:Standards Track


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.


Cofunction definitions

A new keyword codef is introduced which is used in place of def to define a cofunction. A cofunction is a special kind of generator having the following characteristics:

  1. A cofunction is always a generator, even if it does not contain any yield or yield from expressions.
  2. A cofunction cannot be called the same way as an ordinary function. An exception is raised if an ordinary call to a cofunction is attempted.


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

cocall f(*args, **kwds)

is semantically 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.

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

The cocall keyword is syntactically valid only inside a cofunction. A SyntaxError will result if it is used in any other context.

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.

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):
    return obj.__cocall__(*args, **kwds)

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 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 codef and cocall constructs address the first issue by making the syntax directly reflect the intent, that is, that the function 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 the form of definition determine whether the function is a coroutine, rather than what it contains.

Prototype Implementation

An implementation in the form of patches to Python 3.1.2 can be found here: