Async compatibility

Synchronous and asynchronous code are not directly compatible in that the functions must be called differently depending on the type. This limits what can be done, for example in how Quart interacts with Flask extensions and any effort to make Flask directly asynchronous.

In my opinion it is much easier to start with an asynchronous codebase that calls synchronous code than vice versa in Python. I will try and reason why below.

Calling sync code from async functions

This is mostly easy in that you can either call, or via a simple wrapper await a synchronous function,

async def example():
    await asyncio.coroutine(sync_call)()

whilst this doesn’t actually change the nature, the call is synchronous, it does work.

Calling async code from sync functions

This is where things get difficult, as it is only possible to create a single event loop. Hence this can only be used once,

def example():
    loop = asyncio.get_event_loop()

therefore if you are not at the very outer scope it isn’t really possible to call asynchronous code from a synchronous function.

This is problematic when dealing with Flask extensions as for example the extension may have something like,

def route():
    data = request.form
    return render_template_string("{{ name }}", name=data['name'])

whilst the route function can be wrapped with the asyncio.coroutine function and hence awaited, there is no (easy?) way to insert the await before the request.form and render_template calls.

It is for this reason that a proxy object, FlaskRequestProxy, and render, render_template() functions are created for the Flask extensions. The former adding synchronous request methods and the other providing synchronous functions.

Quart monkey patches a sync_wait method onto the base event loop allowing for definitions such as,

from quart.templating import render_template as quart_render_template

def render_template(*args):
    return asyncio.sync_wait(quart_render_template(*args))