PEP: 530 Title: Asynchronous Comprehensions Version: $Revision$ Last-Modified: $Date$ Author: Yury Selivanov 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. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: