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	PEP: 530
	Title: Asynchronous Comprehensions
	Version: $Revision$
	Last-Modified: $Date$
	Author: Yury Selivanov <yury@edgedb.com>
	Discussions-To: python-dev@python.org
	Status: Final
	Type: Standards Track
	Content-Type: text/x-rst
	Created: 03-Sep-2016
	Python-Version: 3.6
	Post-History: 03-Sep-2016


	Abstract
	========

	:pep:`492` and :pep:`525` introduce support for native coroutines and
	asynchronous generators using ``async`` / ``await`` syntax. This PEP
	proposes to add asynchronous versions of list, set, dict comprehensions
	and generator expressions.


	Rationale and Goals
	===================

	Python has extensive support for synchronous comprehensions, allowing
	to produce lists, dicts, and sets with a simple and concise syntax. We
	propose implementing similar syntactic constructions for the
	asynchronous code.

	To illustrate the readability improvement, consider the following
	example::

	result = []
	async for i in aiter():
	if i % 2:
	result.append(i)

	With the proposed asynchronous comprehensions syntax, the above code
	becomes as short as::

	result = [i async for i in aiter() if i % 2]

	The PEP also makes it possible to use the ``await`` expressions in
	all kinds of comprehensions::

	result = [await fun() for fun in funcs]


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

	Asynchronous Comprehensions
	---------------------------

	We propose to allow using ``async for`` inside list, set and dict
	comprehensions. Pending :pep:`525` approval, we can also allow creation
	of asynchronous generator expressions.

	Examples:

	* set comprehension: ``{i async for i in agen()}``;

	* list comprehension: ``[i async for i in agen()]``;

	* dict comprehension: ``{i: i ** 2 async for i in agen()}``;

	* generator expression: ``(i ** 2 async for i in agen())``.

	It is allowed to use ``async for`` along with ``if`` and ``for``
	clauses in asynchronous comprehensions and generator expressions::

	dataset = {data for line in aiter()
	async for data in line
	if check(data)}

	Asynchronous comprehensions are only allowed inside an ``async def``
	function.

	In principle, asynchronous generator expressions are allowed in
	any context. However, in Python 3.6, due to ``async`` and ``await``
	soft-keyword status, asynchronous generator expressions are only
	allowed in an ``async def`` function. Once ``async`` and ``await``
	become reserved keywords in Python 3.7, this restriction will be
	removed.


	``await`` in Comprehensions
	---------------------------

	We propose to allow the use of ``await`` expressions in both
	asynchronous and synchronous comprehensions::

	result = [await fun() for fun in funcs]
	result = {await fun() for fun in funcs}
	result = {fun: await fun() for fun in funcs}

	result = [await fun() for fun in funcs if await smth]
	result = {await fun() for fun in funcs if await smth}
	result = {fun: await fun() for fun in funcs if await smth}

	result = [await fun() async for fun in funcs]
	result = {await fun() async for fun in funcs}
	result = {fun: await fun() async for fun in funcs}

	result = [await fun() async for fun in funcs if await smth]
	result = {await fun() async for fun in funcs if await smth}
	result = {fun: await fun() async for fun in funcs if await smth}

	This is only valid in ``async def`` function body.


	Grammar Updates
	---------------

	The proposal requires one change on the grammar level: adding the
	optional "async" keyword to ``comp_for``::

	comp_for: [ASYNC] 'for' exprlist 'in' or_test [comp_iter]

	The ``comprehension`` AST node will have the new ``is_async`` argument.


	Backwards Compatibility
	-----------------------

	The proposal is fully backwards compatible.


	Acceptance
	==========

	:pep:`530` was accepted by Guido, September 6, 2016 [1]_.


	Implementation
	==============

	The implementation is tracked in issue 28008 [3]_. The reference
	implementation git repository is available at [2]_.


	References
	==========

	.. [1] https://mail.python.org/pipermail/python-ideas/2016-September/042141.html

	.. [2] https://github.com/1st1/cpython/tree/asyncomp

	.. [3] http://bugs.python.org/issue28008


	Acknowledgments
	===============

	I thank Guido van Rossum, Victor Stinner and Elvis Pranskevichus
	for their feedback, code reviews, and discussions around this
	PEP.

	Copyright
	=========

	This document has been placed in the public domain.

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