Add PEP 530: Asynchronous Comprehensions

pep-0530.txt

@@ -0,0 +1,135 @@
+PEP: 530
+Title: Asynchronous Comprehensions
+Version: $Revision$
+Last-Modified: $Date$
+Author: Yury Selivanov <yury@magic.io>
+Discussions-To: <python-dev@python.org>
+Status: Draft
+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() async for fun in funcs]
+    result = {await fun() async for fun in funcs}
+    result = {fun: await fun() async for fun in funcs}
+
+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.
+
+
+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: