174 lines
4.4 KiB
Plaintext
174 lines
4.4 KiB
Plaintext
PEP: 530
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Title: Asynchronous Comprehensions
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Version: $Revision$
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Last-Modified: $Date$
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Author: Yury Selivanov <yury@edgedb.com>
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Discussions-To: python-dev@python.org
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Status: Final
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 03-Sep-2016
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Python-Version: 3.6
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Post-History: 03-Sep-2016
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Abstract
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========
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:pep:`492` and :pep:`525` introduce support for native coroutines and
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asynchronous generators using ``async`` / ``await`` syntax. This PEP
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proposes to add asynchronous versions of list, set, dict comprehensions
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and generator expressions.
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Rationale and Goals
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===================
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Python has extensive support for synchronous comprehensions, allowing
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to produce lists, dicts, and sets with a simple and concise syntax. We
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propose implementing similar syntactic constructions for the
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asynchronous code.
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To illustrate the readability improvement, consider the following
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example::
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result = []
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async for i in aiter():
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if i % 2:
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result.append(i)
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With the proposed asynchronous comprehensions syntax, the above code
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becomes as short as::
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result = [i async for i in aiter() if i % 2]
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The PEP also makes it possible to use the ``await`` expressions in
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all kinds of comprehensions::
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result = [await fun() for fun in funcs]
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Specification
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=============
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Asynchronous Comprehensions
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---------------------------
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We propose to allow using ``async for`` inside list, set and dict
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comprehensions. Pending :pep:`525` approval, we can also allow creation
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of asynchronous generator expressions.
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Examples:
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* set comprehension: ``{i async for i in agen()}``;
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* list comprehension: ``[i async for i in agen()]``;
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* dict comprehension: ``{i: i ** 2 async for i in agen()}``;
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* generator expression: ``(i ** 2 async for i in agen())``.
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It is allowed to use ``async for`` along with ``if`` and ``for``
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clauses in asynchronous comprehensions and generator expressions::
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dataset = {data for line in aiter()
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async for data in line
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if check(data)}
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Asynchronous comprehensions are only allowed inside an ``async def``
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function.
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In principle, asynchronous generator expressions are allowed in
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any context. However, in Python 3.6, due to ``async`` and ``await``
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soft-keyword status, asynchronous generator expressions are only
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allowed in an ``async def`` function. Once ``async`` and ``await``
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become reserved keywords in Python 3.7, this restriction will be
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removed.
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``await`` in Comprehensions
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---------------------------
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We propose to allow the use of ``await`` expressions in both
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asynchronous and synchronous comprehensions::
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result = [await fun() for fun in funcs]
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result = {await fun() for fun in funcs}
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result = {fun: await fun() for fun in funcs}
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result = [await fun() for fun in funcs if await smth]
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result = {await fun() for fun in funcs if await smth}
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result = {fun: await fun() for fun in funcs if await smth}
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result = [await fun() async for fun in funcs]
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result = {await fun() async for fun in funcs}
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result = {fun: await fun() async for fun in funcs}
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result = [await fun() async for fun in funcs if await smth]
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result = {await fun() async for fun in funcs if await smth}
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result = {fun: await fun() async for fun in funcs if await smth}
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This is only valid in ``async def`` function body.
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Grammar Updates
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---------------
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The proposal requires one change on the grammar level: adding the
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optional "async" keyword to ``comp_for``::
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comp_for: [ASYNC] 'for' exprlist 'in' or_test [comp_iter]
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The ``comprehension`` AST node will have the new ``is_async`` argument.
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Backwards Compatibility
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-----------------------
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The proposal is fully backwards compatible.
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Acceptance
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==========
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:pep:`530` was accepted by Guido, September 6, 2016 [1]_.
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Implementation
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==============
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The implementation is tracked in issue 28008 [3]_. The reference
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implementation git repository is available at [2]_.
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References
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==========
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.. [1] https://mail.python.org/pipermail/python-ideas/2016-September/042141.html
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.. [2] https://github.com/1st1/cpython/tree/asyncomp
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.. [3] http://bugs.python.org/issue28008
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Acknowledgments
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===============
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I thank Guido van Rossum, Victor Stinner and Elvis Pranskevichus
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for their feedback, code reviews, and discussions around this
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PEP.
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Copyright
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=========
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This document has been placed in the public domain.
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..
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coding: utf-8
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End:
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