Asynchronous in REPL: Autoawait

Starting with IPython 7.0, and when user Python 3.6 and above, IPython offer the ability to run asynchronous code from the REPL. Constructs which are :exc:`SyntaxError` s in the Python REPL can be used seamlessly in IPython.

The example given here are for terminal IPython, running async code in a notebook interface or any other frontend using the Jupyter protocol will need to use a newer version of IPykernel. The details of how async code runs in IPykernel will differ between IPython, IPykernel and their versions.

When a supported library is used, IPython will automatically await Futures and Coroutines in the REPL. This will happen if an :ref:`await <await>` is use at top level scope, or if any structure valid only in async def function context are present. For example, the following being a syntax error in the Python REPL:

Python 3.6.0
[GCC 4.2.1]
Type "help", "copyright", "credits" or "license" for more information.
>>> import aiohttp
>>> result = aiohttp.get('https://api.github.com')
>>> response = await result
  File "<stdin>", line 1
    response = await result
                          ^
SyntaxError: invalid syntax

Should behave as expected in the IPython REPL:

Python 3.6.0
Type 'copyright', 'credits' or 'license' for more information
IPython 7.0.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import aiohttp
   ...: result = aiohttp.get('https://api.github.com')

In [2]: response = await result
<pause for a few 100s ms>

In [3]: await response.json()
Out[3]:
{'authorizations_url': 'https://api.github.com/authorizations',
 'code_search_url': 'https://api.github.com/search/code?q={query}...',
...
}

You can use the c.InteractiveShell.autoawait configuration option and set it to :any:`False` to deactivate automatic wrapping of asynchronous code. You can also use the :magic:`%autoawait` magic to toggle the behavior at runtime:

In [1]: %autoawait False

In [2]: %autoawait
IPython autoawait is `Off`, and set to use `IPython.core.interactiveshell._asyncio_runner`

By default IPython will assume integration with Python's provided :mod:`asyncio`, but integration with other libraries is provided. In particular we provide experimental integration with the curio and trio library, the later one being necessary if you require the ability to do nested call of IPython's embed() functionality.

You can switch current integration by using the c.InteractiveShell.loop_runner option or the autoawait <name integration> magic.

For example:

In [1]: %autoawait trio

In [2]: import trio

In [3]: async def child(i):
   ...:     print("   child %s goes to sleep"%i)
   ...:     await trio.sleep(2)
   ...:     print("   child %s wakes up"%i)

In [4]: print('parent start')
   ...: async with trio.open_nursery() as n:
   ...:     for i in range(5):
   ...:         n.spawn(child, i)
   ...: print('parent end')
parent start
   child 2 goes to sleep
   child 0 goes to sleep
   child 3 goes to sleep
   child 1 goes to sleep
   child 4 goes to sleep
   <about 2 seconds pause>
   child 2 wakes up
   child 1 wakes up
   child 0 wakes up
   child 3 wakes up
   child 4 wakes up
parent end

In the above example, async with at top level scope is a syntax error in Python.

Using this mode can have unexpected consequences if used in interaction with other features of IPython and various registered extensions. In particular if you are a direct or indirect user of the AST transformers, these may not apply to your code.

When Using command line IPython, the default loop (or runner) does not process in the background, so top level asynchronous code must finish for the REPL to allow you to enter more code. As with usual Python semantic, the awaitables are started only when awaited for the first time. That is to say, in first example, no network request is done between In[1] and In[2].

Internals

As running asynchronous code is not supported in interactive REPL (as of Python 3.7) we have to rely to a number of complex workaround to allow this to happen. It is interesting to understand how this works in order to comprehend potential bugs, or provide a custom runner.

Among the many approaches that are at our disposition, we find only one that suited out need. Under the hood we use the code object from a async-def function and run it in global namespace after modifying it to not create a new locals() scope:

async def inner_async():
    locals().update(**global_namespace)
    #
    # here is user code
    #
    return last_user_statement
codeobj = modify(inner_async.__code__)
coroutine = eval(codeobj, user_ns)
display(loop_runner(coroutine))

The first thing you'll notice is that unlike classical exec, there is only one namespace. Second, user code runs in a function scope, and not a module scope.

On top of the above there are significant modification to the AST of function, and loop_runner can be arbitrary complex. So there is a significant overhead to this kind of code.

By default the generated coroutine function will be consumed by Asyncio's loop_runner = asyncio.get_evenloop().run_until_complete() method. It is though possible to provide your own.

A loop runner is a synchronous function responsible from running a coroutine object.

The runner is responsible from ensuring that coroutine run to completion, and should return the result of executing the coroutine. Let's write a runner for trio that print a message when used as an exercise, trio is special as it usually prefer to run a function object and make a coroutine by itself, we can get around this limitation by wrapping it in an async-def without parameters and passing this value to trio:

In [1]: import trio
   ...: from types import CoroutineType
   ...:
   ...: def trio_runner(coro:CoroutineType):
   ...:     print('running asynchronous code')
   ...:     async def corowrap(coro):
   ...:         return await coro
   ...:     return trio.run(corowrap, coro)

We can set it up by passing it to %autoawait:

In [2]: %autoawait trio_runner

In [3]: async def async_hello(name):
   ...:     await trio.sleep(1)
   ...:     print(f'Hello {name} world !')
   ...:     await trio.sleep(1)

In [4]: await async_hello('async')
running asynchronous code
Hello async world !

Asynchronous programming in python (and in particular in the REPL) is still a relatively young subject. We expect some code to not behave as you expect, so feel free to contribute improvements to this codebase and give us feedback.