gather

Learn to await multiple coroutines with asyncio.gather.

gather

asyncio.gather launches many coroutines at once and waits for all of them to finish. It returns their results as a list in the order given.

import asyncio

async def fetch(n):
    return n * 2

async def main():
    results = await asyncio.gather(fetch(1), fetch(2), fetch(3))
    return results  # [2, 4, 6]

gather starts each call in turn; it awaits until all complete. If one raises, gather raises by default; with return_exceptions=True errors come back in the result list.

return_exceptions and errors

If one of the coroutines passed to gather raises, by default gather raises too. This is good for fast failure.

But if you want to see all results (including errors), use return_exceptions=True. The error objects appear in the result list; you can continue processing.

results = await asyncio.gather(
    fetch(1), bad_fetch(),
    return_exceptions=True)

This option is useful when partial success matters.

Exercise Parallel fetch with gather

Start with import asyncio. The function async def fetch(n) returns n * 2. Inside async def main() call await asyncio.gather(fetch(1), fetch(2), fetch(3)) and return it. Add print(asyncio.run(main())).

Show example solution

import asyncio

async def fetch(n):
    return n * 2

async def main():
    return await asyncio.gather(fetch(1), fetch(2), fetch(3))

print(asyncio.run(main()))

Test yourself

In what order does asyncio.gather return results?
- By completion order
- By call order
- In random order
- It returns only the first result
Show answer

By call order — gather returns results by call order; completion order does not matter.

gather call

import asyncio

async def main():
    return await asyncio.____(fetch(1), fetch(2))

Show answer

import asyncio

async def main():
    return await asyncio.gather(fetch(1), fetch(2))

Match the gather concepts

gather(*coros)

Awaits all
call order

Result order
return_exceptions=True

Puts errors in list
await gather(...)

Waits for completion

Next lesson: wait

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