I have written a library `cotoolz` that provides the primitive pieces needed for this. It implements a `comap` type which is like `map` but properly forwards `send`, `throw`, and `close` to the underlying coroutine. There is no special syntax needed, just write:

```

yield from comap(f, inner_coroutine())
```

This also supports `cozip` which lets you zip together multiple coroutines, for example:

```

yield from cozip(inner_coroutine_a(), inner_corouting_b(), inner_coroutine_c(), ...)
```

This will fan out the sends to all of the coroutines and collect the results into a single tuple to yield. Just like `zip`, this will be exchausted when the first coroutine is exhausted.

The library is available as free software on pypi or on here: https://github.com/llllllllll/cotoolz

On Tue, Apr 12, 2016 at 5:40 PM, Bar Harel <bzvi7919@gmail.com> wrote:

I asked a question in stackoverflow regarding a way of modifying yield from's return value.
There doesn't seem to be any way of modifying the data yielded by the yield from expression without breaking the yield from "pipe". By "pipe" I mean the fact that .send() and .throw() pass to the inner generator. Useful cases are parsers as I've demonstrated in the question, transcoders, encoding and decoding the yielded values without interrupting .send() or .throw() and generally coroutines.

I believe it would be beneficial to many aspects and libraries in Python, most notably asyncio.

I couldn't think of a good syntax though, other than creating a wrapping function, lets say in itertools, that creates a class overriding send, throw and __next__ and receives the generator and associated modification functions (like suggested in one of the answers).

What do you think? Is it useful? Any suggested syntax?

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