如何在python lambda中使用await [英] How to use await in a python lambda

问题描述

我正在尝试做这样的事情:

I'm trying to do something like this:

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

但是我得到这个错误:

SyntaxError: 'await' outside async function

这是有道理的，因为lambda不是异步的.

Which makes sense because the lambda is not async.

我尝试使用 async lambda x:... ，但这会引发 SyntaxError:无效语法.

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

Pep 492 状态:

可以提供异步lambda函数的语法，但是这种构造超出了本PEP的范围.

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

但是我无法确定该语法是否在CPython中实现.

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

是否可以声明异步lambda或使用异步函数对列表进行排序?

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

推荐答案

您不能.没有异步lambda ，即使存在，您也不会将其作为键函数传递给 list.sort()，因为会调用键函数作为同步功能，并且没有等待.一个简单的解决方法是自己注释列表:

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]

请注意，列表推导中的 await 表达式仅在Python 3.6+中受支持.如果您使用的是3.5，则可以执行以下操作:

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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