How to use await in a python lambda

Question

I'm trying to do something like this:

mylist.sort(key=lambda x: await somefunction(x)) 

But I get this error:

SyntaxError: 'await' outside async function 

Which makes sense because the lambda is not async.

I tried to use async lambda x: ... but that throws a SyntaxError: invalid syntax.

Pep 492 states:

Syntax for asynchronous lambda functions could be provided, but this construct is outside of the scope of this PEP.

But I could not find out if that syntax was implemented in CPython.

Is there a way to declare an async lambda, or to use an async function for sorting a list?

Answer 1

You can't. There is no async lambda, and even if there were, you coudln't pass it in as key function to list.sort(), since a key function will be called as a synchronous function and not awaited. An easy work-around is to annotate your list yourself:

mylist_annotated = [(await some_function(x), x) for x in mylist] mylist_annotated.sort() mylist = [x for key, x in mylist_annotated] 

Note that await expressions in list comprehensions are only supported in Python 3.6+. If you're using 3.5, you can do the following:

mylist_annotated = [] for x in mylist:     mylist_annotated.append((await some_function(x), x))  mylist_annotated.sort() mylist = [x for key, x in mylist_annotated] 

Answer 2

An "async lambda" can be emulated by combining a lambda with an async generator:

key=lambda x: (await somefunction(x) for _ in '_').__anext__() 

It is possible to move the ( ).__anext__() to a helper, which likely makes the pattern clearer as well:

def head(async_iterator): return async_iterator.__anext__()  key=lambda x: head(await somefunction(x) for _ in '_') 

Note that the sort method/function in the standard library are not async. One needs an async version, such as asyncstdlib.sorted (disclaimer: I maintain this library):

import asyncstdlib as a  mylist = await a.sorted(mylist, key=lambda x: head(await somefunction(x) for _ in '_')) 

---

Understanding the lambda ...: (...).__anext__() pattern

An "async lambda" would be an anonymous asynchronous function, or in other words an anonymous function evaluating to an awaitable. This is in parallel to how async def defines a named function evaluating to an awaitable.
The task can be split into two parts: An anonymous function expression and a nested awaitable expression.

• An anonymous function expression is exactly what a lambda ...: ... is.

• An awaitable expression is only allowed inside a coroutine function; however:

  • An (asynchronous) generator expression implicitly creates a (coroutine) function. As an async generator only needs async to run, it can be defined in a sync function (since Python 3.7).
  • An asynchronous iterable can be used as an awaitable via its __anext__ method.

These three parts are directly used in the "async lambda" pattern:

#   | regular lambda for the callable and scope #   |         | async generator expression for an async scope #   v         v                                    v first item as an awaitable key=lambda x: (await somefunction(x) for _ in '_').__anext__() 

The for _ in '_' in the async generator is only to have exactly one iteration. Any variant with at least one iteration will do.