async def

Learn to define coroutines with async def.

async def

async def declares a coroutine function. It takes parameters and returns a value like a normal def; but calling it returns a coroutine instead of a result.

async def add(a, b):
    return a + b

Inside, you can await other coroutines with await; that is what sets it apart from a normal function. await is not required; the coroutine can be defined without it.

In async def functions, the return value goes to the awaiter; without a return, None comes back.

Coroutine and return

Inside an async def, writing return value sends that value to the awaiter. Example: async def fetch(): ...; return data. The caller does data = await fetch().

A coroutine can have a single return; multiple returns can be used for branching but only the first reached runs.

Without a return value, None comes back; just like in normal functions.

Exercise Write async def add

The function async def add(a, b) does return a + b. Start with import asyncio. Add print(asyncio.run(add(3, 4))).

Show example solution

import asyncio

async def add(a, b):
    return a + b

print(asyncio.run(add(3, 4)))

Test yourself

Match the async def parts

async

Marker keyword
def

Function declaration
return

Returns a value
await

Can wait
What happens if there is no return inside async def?
- It raises an error
- None is returned
- An empty coroutine is returned
- True is returned
Show answer

None is returned — Without return in async def, None comes back; like a normal function.

async def return

async def add(a, b):
    ____ a + b

Show answer

async def add(a, b):
    return a + b

Next lesson: await asyncio.sleep

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