await 可以简单的理解为异步等待。
await 只能等待异步对象包括
等待协程对象
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| import asyncio
import time
async def run():
now = time.time()
await asyncio.sleep(2)
print(time.time() - now)
asyncio.run(run())
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输出 2.004728078842163
等待非法对象
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| import asyncio
import time
async def run():
now = time.time()
await time.sleep(2)
print(time.time() - now)
asyncio.run(run())
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上面会报错
TypeError: object NoneType can’t be used in ‘await’ expression
调用
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| import asyncio
import time
async def run1():
time.sleep(2)
return "success"
async def run2():
response = await run1()
print(response)
asyncio.run(run2())
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输出 success
同步 && 异步？
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| import asyncio
import time
async def run1():
await asyncio.sleep(2)
return "success"
async def run2():
now = time.time()
response1 = await run1()
print(time.time() - now)
respons2 = await run1()
print(time.time() - now)
asyncio.run(run2())
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输出
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| 2.004086971282959
4.008858680725098
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乍一看，上面的代码加上 await 不就是同步代码吗？其实不是，当遇到 await 的 IO 阻塞后，这个任务会被挂起，然后，事件驱动转而会执行其他任务，当 IO 值返回，才继续向下。
如果把代码换成下面这样子
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| import asyncio
import time
async def run1():
time.sleep(2)
return "success"
async def run2():
now = time.time()
response1 = await run1()
print(time.time() - now)
respons2 = await run1()
print(time.time() - now)
asyncio.run(run2())
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也是一样的输出
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| 2.004086971282959
4.008858680725098
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但是，如果在异步中使用了非异步的 IO 会怎么办？
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| import asyncio
import time
async def run0():
time.sleep(1)
print("run0")
async def run1():
time.sleep(2)
return "run1"
async def run2():
response = await run1()
print(response)
async def main():
await asyncio.wait([asyncio.create_task(run2()), asyncio.create_task(run0())])
now = time.time()
asyncio.run(main())
print(time.time() - now)
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输出
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| run1
run0
3.0104901790618896
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可见，如果异步中使用非异步的 IO 是没有任何作用的。代码改成
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| import asyncio
import time
async def run0():
await asyncio.sleep(1)
print("run0")
async def run1():
await asyncio.sleep(2)
return "run1"
async def run2():
response = await run1()
print(response)
async def main():
await asyncio.wait([asyncio.create_task(run2()), asyncio.create_task(run0())])
now = time.time()
asyncio.run(main())
print(time.time() - now)
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会变成下面这样。
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| run0
run1
2.0059680938720703
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