SyntaxError: 'await' outside function (tested)

SyntaxError: 'await' outside function (or outside async function) means await was used somewhere that isn't an async def. We measured each variant on Python 3.13.5, the fix, and the tempting non-fix.

1. await at the top level of a script

import asyncio

await asyncio.sleep(0)

Output:

  File "/tmp/aw/b1.py", line 3
    await asyncio.sleep(0)
    ^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: 'await' outside function

Measured on Python 3.13.5: a plain script can't use await outside a function. (Jupyter and python3 -m asyncio allow top-level await; we piped the same await asyncio.sleep(0, result=...) into python3 -m asyncio and it printed the result. A .py file run normally does not.)

2. await inside a normal def

import asyncio

def fetch():
    await asyncio.sleep(0)
    return "data"

Output:

  File "/tmp/aw/b2.py", line 4
    await asyncio.sleep(0)
    ^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: 'await' outside async function

Measured: inside a regular def the wording changes to 'await' outside async function. The function needs to be async def, and then its callers must await it too.

3. Lambdas can't be async; comprehensions can

import asyncio

async def main():
    data = [await asyncio.sleep(0, result=i) for i in range(3)]
    print(data)
    squares = lambda x: await asyncio.sleep(0, result=x * x)

Output:

  File "/tmp/aw/b3.py", line 6
    squares = lambda x: await asyncio.sleep(0, result=x * x)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: 'await' outside async function

Measured: the error points at the lambda on line 6, not the list comprehension on line 4. Inside an async def, await in a comprehension is allowed; a lambda is always a sync function, so await in it never works. Use a small async def instead.

4. Fix: put the code in async def main() and call asyncio.run()

import asyncio

async def fetch():
    await asyncio.sleep(0)
    return "data"

async def main():
    print(await fetch())

asyncio.run(main())

Output:

data

asyncio.run() is the one bridge from sync code into async code: call it once at the entry point, and use await everywhere inside.

5. Don't “fix” it by deleting await

import asyncio

async def fetch():
    await asyncio.sleep(0)
    return "data"

result = fetch()          # "fixing" the SyntaxError by dropping await
print(result == "data", type(result).__name__)

Output:

False coroutine
<sys>:0: RuntimeWarning: coroutine 'fetch' was never awaited

Measured: the SyntaxError goes away, but calling an async def without await doesn't run it. You get a coroutine object (not "data") and a RuntimeWarning: coroutine 'fetch' was never awaited at exit, which is easy to miss.

Written with AI assistance. Measured on 2026-10-09 with Python 3.13.5 on Linux: top-level await in a script and in python3 -m asyncio, await in a regular def, in a lambda and in a comprehension inside async def, the asyncio.run fix, and calling an async function without await. Every block was run verbatim and its output pasted from that run. PlainHeaders is our extension; everything else here is built-in Python.