On Wed, Apr 22, 2015 at 10:44 PM, PJ Eby <pje@telecommunity.com> wrote:
On Tue, Apr 21, 2015 at 1:26 PM, Yury Selivanov <yselivanov.ml@gmail.com> wrote:
It is an error to pass a regular context manager without ``__aenter__`` and ``__aexit__`` methods to ``async with``. It is a ``SyntaxError`` to use ``async with`` outside of a coroutine.
I find this a little weird. Why not just have `with` and `for` inside a coroutine dynamically check the iterator or context manager, and either behave sync or async accordingly? Why must there be a *syntactic* difference?
IIRC Guido always like to have different syntax for calling regular functions and coroutines. That's why we need explicit syntax for asynchronous context managers and iterators.
Not only would this simplify the syntax, it would also allow dropping the need for `async` to be a true keyword, since functions could be defined via "def async foo():" rather than "async def foo():"
...which, incidentally, highlights one of the things that's been bothering me about all this "async foo" stuff: "async def" looks like it *defines the function* asynchronously (as with "async with" and "async for"), rather than defining an asynchronous function. ISTM it should be "def async bar():" or even "def bar() async:".
Also, even that seems suspect to me: if `await` looks for an __await__ method and simply returns the same object (synchronously) if the object doesn't have an await method, then your code sample that supposedly will fail if a function ceases to be a coroutine *will not actually fail*.
In my experience working with coroutine systems, making a system polymorphic (do something appropriate with what's given) and idempotent (don't do anything if what's wanted is already done) makes it more robust. In particular, it eliminates the issue of mixing coroutines and non-coroutines.
To sum up: I can see the use case for a new `await` distinguished from `yield`, but I don't see the need to create new syntax for everything; ISTM that adding the new asynchronous protocols and using them on demand is sufficient. Marking a function asynchronous so it can use asynchronous iteration and context management seems reasonably useful, but I don't think it's terribly important for the type of function result. Indeed, ISTM that the built-in `object` class could just implement `__await__` as a no-op returning self, and then *all* results are trivially asynchronous results and can be awaited idempotently, so that awaiting something that has already been waited for is a no-op. (Prior art: the Javascript Promise.resolve() method, which takes either a promise or a plain value and returns a promise, so that you can write code which is always-async in the presence of values that may already be known.)
Finally, if the async for and with operations have to be distinguished by syntax at the point of use (vs. just always being used in coroutines), then ISTM that they should be `with async foo:` and `for async x in bar:`, since the asynchronousness is just an aspect of how the main keyword is executed.
tl;dr: I like the overall ideas but hate the syntax and type segregation involved: declaring a function async at the top is OK to enable async with/for semantics and await expressions, but the rest seems unnecessary and bad for writing robust code. (e.g. note that requiring different syntax means a function must either duplicate code or restrict its input types more, and type changes in remote parts of the program will propagate syntax changes throughout.) _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/andrew.svetlov%40gmail.co...
-- Thanks, Andrew Svetlov