Compose `async` with `returns`

async_return.py

@future.future_safe                                                                     
async def run_process(proc: asyncio.subprocess.Process) -> typing.Tuple[bytes, bytes]:  
    """Run the async process and check the return code."""                              
    stdout, stderr = await proc.communicate()                                           
    if proc.returncode:                                                                 
        logger.error('process exited with returncode {0}'.format(proc.returncode))      
        raise ValueError('process returned non-zero code')                              
    return stdout, stderr                                                               
                                                                                        
                                                                                        
def a_shell_cmd(cmd: str) -> FutureProc:                                                
    """Run a shell command in subprocess asynchronously.                                
                                                                                        
    NOTE: The API for this function is slightly different than the sync one.            
    """                                                                                 
    ctx = context.RequiresContextFutureResultE.ask()                                    
    logger.debug('running async {cmd}'.format(cmd=cmd))                                 
    return ctx.bind_future_result(                                                      
        lambda runner:                                                                  
            future.future_safe(runner.create_subprocess_exec)(                          
                *shlex.split(cmd),                                                      
                stdout=asyncio.subprocess.PIPE,                                         
                stderr=asyncio.subprocess.PIPE,                                         
            ).bind_future_result(run_process),                                          
    )