PEP: 530 Title: Asynchronous Comprehensions Version:
492
and 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.
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]
We propose to allow using async for
inside list, set and dict comprehensions. Pending 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.
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.
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.
The proposal is fully backwards compatible.
530
was accepted by Guido, September 6, 20161.
The implementation is tracked in issue 280082. The reference implementation git repository is available at3.
I thank Guido van Rossum, Victor Stinner and Elvis Pranskevichus for their feedback, code reviews, and discussions around this PEP.
This document has been placed in the public domain.