带有生成器表达式的 asyncio.gather

Question

Why doesn't asyncio.gather work with a generator expression?

import asyncio

async def func():
    await asyncio.sleep(2)

# Works
async def call3():
    x = (func() for x in range(3))
    await asyncio.gather(*x)

# Doesn't work
async def call3():
    await asyncio.gather(func() for x in range(3))

# Works
async def call3():
    await asyncio.gather(*[func() for x in range(3)])

asyncio.run(call3())

The second variant gives:

[...]
  File "test.py", line 13, in <genexpr>
    await asyncio.gather(func() for x in range(3))
RuntimeError: Task got bad yield: <coroutine object func at 0x10421dc20>

Is this expected behavior?

Answer 1

await asyncio.gather(func() for x in range(3))

This doesn't work because this is passing the generator object as argument to gather . gather doesn't expect an iterable , it expects coroutines as individual arguments. Which means you need to unpack the generator.

Answer 2

Unpack the generator:

await asyncio.gather(*(func() for i in range(10)))  # star expands generator

We must expand it because asyncio.gather expects a list of arguments (ie asyncio.gather(coroutine0, coroutine1, coroutine2, coroutine3) ), not an iterable

Answer 3

Python uses * / ** for both 'un-packing' and just 'packing' based on whether it's used for variable assignment or not.

def foo(*args,**kwargs):...

In this case, all non-keyworded args are getting put into a tuple args and all kwargs are getting packed into a new dictionary. A single variable passed in still gets packed into a tuple( * ) or dict( ** ).

This is kind of a hybrid

first,*i_take_the_rest,last = range(10)
>>> first=0,i_take_the_rest=[1,2,3,4,5,6,7,8],last=9
*a,b = range(1)
>>> a=[],b=0

But here it unpacks:

combined_iterables = [*range(10),*range(3)]
merged_dict = {**first_dict,**second_dict}

So basically if it's on the left side of the equals or if it's used in a function/method definition like *foo it's packing stuff into a list or tuple (respectively). In comprehensions, however, it has the unpacking behavior.