Signal Signal - 22 days ago 8
Python Question

@asyncio.coroutine vs async def

With the

asyncio
library I've seen,

@asyncio.coroutine
def function():
...


and

async def function():
...


used interchangeably.

Is there any functional difference between the two?

Answer

Yes, there are functional differences between native coroutines using async def sytnax and generator-based coroutines using the asyncio.coroutine decorator.

According to PEP 492, which introduces the async def syntax:

  1. Native coroutine objects do not implement __iter__ and __next__ methods. Therefore, they cannot be iterated over or passed to iter(), list(), tuple() and other built-ins. They also cannot be used in a for..in loop.

    An attempt to use __iter__ or __next__ on a native coroutine object will result in a TypeError .

  2. Plain generators cannot yield from native coroutines: doing so will result in a TypeError .

  3. generator-based coroutines (for asyncio code must be decorated with @asyncio.coroutine) can yield from native coroutine objects.

  4. inspect.isgenerator() and inspect.isgeneratorfunction() return False for native coroutine objects and native coroutine functions.

Point 1 above means that while coroutine functions defined using the @asyncio.coroutine decorator syntax can behave as traditional generator functions, those defined with the async def syntax cannot.

Here are two minimal, ostensibly equivalent coroutine functions defined with the two syntaxes:

import asyncio

@asyncio.coroutine
def decorated(x):
    yield from x 

async def native(x):
    await x 

Although the bytecode for these two functions is almost identical:

>>> import dis
>>> dis.dis(decorated)
  5           0 LOAD_FAST                0 (x)
              3 GET_YIELD_FROM_ITER
              4 LOAD_CONST               0 (None)
              7 YIELD_FROM
              8 POP_TOP
              9 LOAD_CONST               0 (None)
             12 RETURN_VALUE
>>> dis.dis(native)
  8           0 LOAD_FAST                0 (x)
              3 GET_AWAITABLE
              4 LOAD_CONST               0 (None)
              7 YIELD_FROM
              8 POP_TOP
              9 LOAD_CONST               0 (None)
             12 RETURN_VALUE

... the only difference being GET_YIELD_FROM_ITER vs GET_AWAITABLE, they behave completely differently when an attempt is made to iterate over the objects they return:

>>> list(decorated('foo'))
['f', 'o', 'o']

>>> list(native('foo'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'coroutine' object is not iterable

Obviously 'foo' is not an awaitable, so the attempt to call native() with it doesn't make much sense, but the point is hopefully clear that the coroutine object it returns is not iterable, regardless of its argument.

A more detailed investigation of the async/await syntax by Brett Cannon: How the heck does async/await work in Python 3.5? covers this difference in more depth.

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