More docs about Deferred<->Future interoperability. (#6734)

docs/topics/asyncio.rst

@@ -72,24 +72,32 @@ those imports happen.
 
 .. _asyncio-await-dfd:
 
-Awaiting on Deferreds
-=====================
+Integrating Deferred code and asyncio code
+==========================================
 
-When the asyncio reactor isn't installed, you can await on Deferreds in the
-coroutines directly. When it is installed, this is not possible anymore, due to
-specifics of the Scrapy coroutine integration (the coroutines are wrapped into
-:class:`asyncio.Future` objects, not into
-:class:`~twisted.internet.defer.Deferred` directly), and you need to wrap them into
-Futures. Scrapy provides two helpers for this:
+Coroutine functions can await on Deferreds by wrapping them into
+:class:`asyncio.Future` objects. Scrapy provides two helpers for this:
 
 .. autofunction:: scrapy.utils.defer.deferred_to_future
 .. autofunction:: scrapy.utils.defer.maybe_deferred_to_future
+
+.. tip:: If you don't need to support reactors other than the default
+         :class:`~twisted.internet.asyncioreactor.AsyncioSelectorReactor`, you
+         can use :func:`~scrapy.utils.defer.deferred_to_future`, otherwise you
+         should use :func:`~scrapy.utils.defer.maybe_deferred_to_future`.
+
 .. tip:: If you need to use these functions in code that aims to be compatible
          with lower versions of Scrapy that do not provide these functions,
          down to Scrapy 2.0 (earlier versions do not support
          :mod:`asyncio`), you can copy the implementation of these functions
          into your own code.
 
+Coroutines and futures can be wrapped into Deferreds (for example, when a
+Scrapy API requires passing a Deferred to it) using the following helpers:
+
+.. autofunction:: scrapy.utils.defer.deferred_from_coro
+.. autofunction:: scrapy.utils.defer.deferred_f_from_coro_f
+
 
 .. _enforce-asyncio-requirement:
 

docs/topics/coroutines.rst

@@ -9,6 +9,7 @@ Coroutines
 Scrapy has :ref:`partial support <coroutine-support>` for the
 :ref:`coroutine syntax <async>`.
 
+
 .. _coroutine-support:
 
 Supported callables
@@ -51,6 +52,29 @@ hence use coroutine syntax (e.g. ``await``, ``async for``, ``async with``):
 
     .. versionadded:: 2.7
 
+
+.. _coroutine-deferred-apis:
+
+Using Deferred-based APIs
+=========================
+
+In addition to native coroutine APIs Scrapy has some APIs that return a
+:class:`~twisted.internet.defer.Deferred` object or take a user-supplied
+function that returns a :class:`~twisted.internet.defer.Deferred` object. These
+APIs are also asynchronous but don't yet support native ``async def`` syntax.
+For example:
+
+-   The :meth:`ExecutionEngine.download` method returns a
+    :class:`~twisted.internet.defer.Deferred` object.
+-   A custom download handler needs to define a ``download_request()`` method that
+    returns a :class:`~twisted.internet.defer.Deferred` object.
+
+In most cases you can use these APIs in code that otherwise uses coroutines, by
+wrapping a :class:`~twisted.internet.defer.Deferred` object into a
+:class:`~asyncio.Future` object or vice versa. See :ref:`asyncio-await-dfd` for
+more information about this.
+
+
 General usage
 =============
 

scrapy/utils/defer.py

@@ -362,7 +362,8 @@ def deferred_from_coro(o: _T) -> _T: ...
 
 
 def deferred_from_coro(o: _T) -> Deferred | _T:
-    """Converts a coroutine into a Deferred, or returns the object as is if it isn't a coroutine"""
+    """Converts a coroutine or other awaitable object into a Deferred,
+    or returns the object as is if it isn't a coroutine."""
     if isinstance(o, Deferred):
         return o
     if asyncio.isfuture(o) or inspect.isawaitable(o):
@@ -442,12 +443,12 @@ def maybe_deferred_to_future(d: Deferred[_T]) -> Deferred[_T] | Future[_T]:
     What you can await in Scrapy callables defined as coroutines depends on the
     value of :setting:`TWISTED_REACTOR`:
 
-    -   When not using the asyncio reactor, you can only await on
-        :class:`~twisted.internet.defer.Deferred` objects.
-
     -   When :ref:`using the asyncio reactor <install-asyncio>`, you can only
         await on :class:`asyncio.Future` objects.
 
+    -   When not using the asyncio reactor, you can only await on
+        :class:`~twisted.internet.defer.Deferred` objects.
+
     If you want to write code that uses ``Deferred`` objects but works with any
     reactor, use this function on all ``Deferred`` objects::