How to use await in a python lambda

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

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]

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