Implement PEP 380 - 'yield from' (closes #11682)
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 655ebde..7da54a2 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -318,7 +318,7 @@
.. productionlist::
yield_atom: "(" `yield_expression` ")"
- yield_expression: "yield" [`expression_list`]
+ yield_expression: "yield" [`expression_list` | "from" `expression`]
The :keyword:`yield` expression is only used when defining a generator function,
and can only be used in the body of a function definition. Using a
@@ -336,7 +336,10 @@
the generator's methods, the function can proceed exactly as if the
:keyword:`yield` expression was just another external call. The value of the
:keyword:`yield` expression after resuming depends on the method which resumed
-the execution.
+the execution. If :meth:`__next__` is used (typically via either a
+:keyword:`for` or the :func:`next` builtin) then the result is :const:`None`,
+otherwise, if :meth:`send` is used, then the result will be the value passed
+in to that method.
.. index:: single: coroutine
@@ -346,12 +349,29 @@
where should the execution continue after it yields; the control is always
transferred to the generator's caller.
-The :keyword:`yield` statement is allowed in the :keyword:`try` clause of a
+:keyword:`yield` expressions are allowed in the :keyword:`try` clause of a
:keyword:`try` ... :keyword:`finally` construct. If the generator is not
resumed before it is finalized (by reaching a zero reference count or by being
garbage collected), the generator-iterator's :meth:`close` method will be
called, allowing any pending :keyword:`finally` clauses to execute.
+When ``yield from expression`` is used, it treats the supplied expression as
+a subiterator. All values produced by that subiterator are passed directly
+to the caller of the current generator's methods. Any values passed in with
+:meth:`send` and any exceptions passed in with :meth:`throw` are passed to
+the underlying iterator if it has the appropriate methods. If this is not the
+case, then :meth:`send` will raise :exc:`AttributeError` or :exc:`TypeError`,
+while :meth:`throw` will just raise the passed in exception immediately.
+
+When the underlying iterator is complete, the :attr:`~StopIteration.value`
+attribute of the raised :exc:`StopIteration` instance becomes the value of
+the yield expression. It can be either set explicitly when raising
+:exc:`StopIteration`, or automatically when the sub-iterator is a generator
+(by returning a value from the sub-generator).
+
+The parentheses can be omitted when the :keyword:`yield` expression is the
+sole expression on the right hand side of an assignment statement.
+
.. index:: object: generator
The following generator's methods can be used to control the execution of a
@@ -444,6 +464,10 @@
The proposal to enhance the API and syntax of generators, making them
usable as simple coroutines.
+ :pep:`0380` - Syntax for Delegating to a Subgenerator
+ The proposal to introduce the :token:`yield_from` syntax, making delegation
+ to sub-generators easy.
+
.. _primaries: