On 7 September 2016 at 04:54, Sven R. Kunze <srkunze@mail.de> wrote:
On 06.09.2016 20:37, Nick Coghlan wrote:
As Anthony already noted, the "async" keyword switches to the asynchronous version of the iterator protocol - you use this when your *iterator* needs to interact with the event loop, just as you do when deciding whether or not to mark a for loop as asynchronous.
Of course "async" switches to async mode. But that was not the question. I asked WHY that's necessary not what it does. I already noted that Python can detect when to make the switch without a marker. And you fail to explain where the issue with this point of view is.
The Python *runtime* can tell whether the result of an expression is a normal iterator or an asynchronous iterator, but the Python *compiler* can't. So at compile time, it has to decide what code to emit for the four different scenarios Anthony listed: # Normal comprehension, 100% synchronous and blocking squares = [i**i for i in range(10)] # Blocking/sync iterator producing awaitables which suspend before producing a "normal" value results = [await result for result in submit_requests()] # Non-blocking/async iterator that suspends before producing "normal" values results = [result async for submit_requests_and_wait_for_results()] # Non-blocking/async iterator that suspends before producing awaitables which suspend again before producing a "normal" value results = [await result async for submit_requests_asynchronously()] And hence needs the "async" keyword to detect when it has the latter two cases rather than the first two. Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia