Asynchronous Comprehensions

We propose to allow usingasyncfor inside list, set and dictcomprehensions. PendingPEP 525 approval, we can also allow creationof asynchronous generator expressions.

Examples:

• set comprehension:{iasyncforiinagen()};
• list comprehension:[iasyncforiinagen()];
• dict comprehension:{i:i**2asyncforiinagen()};
• generator expression:(i**2asyncforiinagen()).

It is allowed to useasyncfor along withif andforclauses in asynchronous comprehensions and generator expressions:

dataset={dataforlineinaiter()asyncfordatainlineifcheck(data)}

Asynchronous comprehensions are only allowed inside anasyncdeffunction.

In principle, asynchronous generator expressions are allowed inany context. However, in Python 3.6, due toasync andawaitsoft-keyword status, asynchronous generator expressions are onlyallowed in anasyncdef function. Onceasync andawaitbecome reserved keywords in Python 3.7, this restriction will beremoved.

await in Comprehensions

We propose to allow the use ofawait expressions in bothasynchronous and synchronous comprehensions:

result=[awaitfun()forfuninfuncs]result={awaitfun()forfuninfuncs}result={fun:awaitfun()forfuninfuncs}result=[awaitfun()forfuninfuncsifawaitsmth]result={awaitfun()forfuninfuncsifawaitsmth}result={fun:awaitfun()forfuninfuncsifawaitsmth}result=[awaitfun()asyncforfuninfuncs]result={awaitfun()asyncforfuninfuncs}result={fun:awaitfun()asyncforfuninfuncs}result=[awaitfun()asyncforfuninfuncsifawaitsmth]result={awaitfun()asyncforfuninfuncsifawaitsmth}result={fun:awaitfun()asyncforfuninfuncsifawaitsmth}

This is only valid inasyncdef function body.

Grammar Updates

The proposal requires one change on the grammar level: adding theoptional “async” keyword tocomp_for:

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

Thecomprehension AST node will have the newis_async argument.

Backwards Compatibility

The proposal is fully backwards compatible.