eliminate redundancy between yield stmt and yield expr docs (closes #12704)

benjaminp · benjaminp · commit d1c85fd2835b · 2014-01-26T22:52:08.000-05:00
Patch by Nikolaus Rath.
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
@@ -319,27 +319,25 @@ Yield expressions
    yield_atom: "(" `yield_expression` ")"
    yield_expression: "yield" [`expression_list` | "from" `expression`]
 
-The :keyword:`yield` expression is only used when defining a :term:`generator`
-function,
-and can only be used in the body of a function definition.  Using a
-:keyword:`yield` expression in a function definition is sufficient to cause that
-definition to create a generator function instead of a normal function.
+The yield expression is only used when defining a :term:`generator` function and
+thus can only be used in the body of a function definition.  Using a yield
+expression in a function's body causes that function to be a generator.
 
 When a generator function is called, it returns an iterator known as a
 generator.  That generator then controls the execution of a generator function.
 The execution starts when one of the generator's methods is called.  At that
-time, the execution proceeds to the first :keyword:`yield` expression, where it
-is suspended again, returning the value of :token:`expression_list` to
-generator's caller.  By suspended we mean that all local state is retained,
-including the current bindings of local variables, the instruction pointer, and
-the internal evaluation stack.  When the execution is resumed by calling one of
-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. If :meth:`~generator.__next__` is used (typically via either a
-:keyword:`for` or the :func:`next` builtin) then the result is :const:`None`,
-otherwise, if :meth:`~generator.send` is used, then the result will be the
-value passed in to that method.
+time, the execution proceeds to the first yield expression, where it is
+suspended again, returning the value of :token:`expression_list` to generator's
+caller.  By suspended, we mean that all local state is retained, including the
+current bindings of local variables, the instruction pointer, and the internal
+evaluation stack.  When the execution is resumed by calling one of the
+generator's methods, the function can proceed exactly as if the yield expression
+was just another external call.  The value of the yield expression after
+resuming depends on the method which resumed the execution.  If
+:meth:`~generator.__next__` is used (typically via either a :keyword:`for` or
+the :func:`next` builtin) then the result is :const:`None`.  Otherwise, if
+:meth:`~generator.send` is used, then the result will be the value passed in to
+that method.
 
 .. index:: single: coroutine
 
@@ -349,11 +347,11 @@ suspended.  The only difference is that a generator function cannot control
 where should the execution continue after it yields; the control is always
 transferred to the generator's caller.
 
-: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:`~generator.close` method
-will be called, allowing any pending :keyword:`finally` clauses to execute.
+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:`~generator.close` method will be called,
+allowing any pending :keyword:`finally` clauses to execute.
 
 When ``yield from <expr>`` is used, it treats the supplied expression as
 a subiterator. All values produced by that subiterator are passed directly
@@ -373,11 +371,23 @@ the yield expression. It can be either set explicitly when raising
    .. versionchanged:: 3.3
       Added ``yield from <expr>`` to delegate control flow to a subiterator
 
-The parentheses can be omitted when the :keyword:`yield` expression is the
-sole expression on the right hand side of an assignment statement.
+The parentheses may be omitted when the yield expression is the sole expression
+on the right hand side of an assignment statement.
 
-.. index:: object: generator
+.. seealso::
+
+   :pep:`0255` - Simple Generators
+      The proposal for adding generators and the :keyword:`yield` statement to Python.
+
+   :pep:`0342` - Coroutines via Enhanced Generators
+      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.
+
+.. index:: object: generator
 
 Generator-iterator methods
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -395,13 +405,12 @@ is already executing raises a :exc:`ValueError` exception.
 .. method:: generator.__next__()
 
    Starts the execution of a generator function or resumes it at the last
-   executed :keyword:`yield` expression.  When a generator function is resumed
-   with a :meth:`~generator.__next__` method, the current :keyword:`yield`
-   expression always evaluates to :const:`None`.  The execution then continues
-   to the next :keyword:`yield` expression, where the generator is suspended
-   again, and the value of the :token:`expression_list` is returned to
-   :meth:`next`'s caller.
-   If the generator exits without yielding another value, a :exc:`StopIteration`
+   executed yield expression.  When a generator function is resumed with a
+   :meth:`~generator.__next__` method, the current yield expression always
+   evaluates to :const:`None`.  The execution then continues to the next yield
+   expression, where the generator is suspended again, and the value of the
+   :token:`expression_list` is returned to :meth:`next`'s caller.  If the
+   generator exits without yielding another value, a :exc:`StopIteration`
    exception is raised.
 
    This method is normally called implicitly, e.g. by a :keyword:`for` loop, or
@@ -411,12 +420,12 @@ is already executing raises a :exc:`ValueError` exception.
 .. method:: generator.send(value)
 
    Resumes the execution and "sends" a value into the generator function.  The
-   ``value`` argument becomes the result of the current :keyword:`yield`
-   expression.  The :meth:`send` method returns the next value yielded by the
-   generator, or raises :exc:`StopIteration` if the generator exits without
-   yielding another value.  When :meth:`send` is called to start the generator,
-   it must be called with :const:`None` as the argument, because there is no
-   :keyword:`yield` expression that could receive the value.
+   *value* argument becomes the result of the current yield expression.  The
+   :meth:`send` method returns the next value yielded by the generator, or
+   raises :exc:`StopIteration` if the generator exits without yielding another
+   value.  When :meth:`send` is called to start the generator, it must be called
+   with :const:`None` as the argument, because there is no yield expression that
+   could receive the value.
 
 
 .. method:: generator.throw(type[, value[, traceback]])
@@ -478,20 +487,6 @@ For examples using ``yield from``, see :ref:`pep-380` in "What's New in
 Python."
 
 
-.. seealso::
-
-   :pep:`0255` - Simple Generators
-      The proposal for adding generators and the :keyword:`yield` statement to Python.
-
-   :pep:`0342` - Coroutines via Enhanced Generators
-      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:
 
 Primaries
diff --git a/Doc/reference/simple_stmts.rst b/Doc/reference/simple_stmts.rst
@@ -445,53 +445,26 @@ The :keyword:`yield` statement
 .. productionlist::
    yield_stmt: `yield_expression`
 
-The :keyword:`yield` statement is only used when defining a generator function,
-and is only used in the body of the generator function. Using a :keyword:`yield`
-statement in a function definition is sufficient to cause that definition to
-create a generator function instead of a normal function.
-
-When a generator function is called, it returns an iterator known as a generator
-iterator, or more commonly, a generator.  The body of the generator function is
-executed by calling the :func:`next` function on the generator repeatedly until
-it raises an exception.
-
-When a :keyword:`yield` statement is executed, the state of the generator is
-frozen and the value of :token:`expression_list` is returned to :meth:`next`'s
-caller.  By "frozen" we mean that all local state is retained, including the
-current bindings of local variables, the instruction pointer, and the internal
-evaluation stack: enough information is saved so that the next time :func:`next`
-is invoked, the function can proceed exactly as if the :keyword:`yield`
-statement were just another external call.
-
-The :keyword:`yield` statement is 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 <expr>`` is used, it treats the supplied expression as
-a subiterator, producing values from it until the underlying iterator is
-exhausted.
-
-   .. versionchanged:: 3.3
-      Added ``yield from <expr>`` to delegate control flow to a subiterator
-
-For full details of :keyword:`yield` semantics, refer to the :ref:`yieldexpr`
-section.
+A :keyword:`yield` statement is semantically equivalent to a :ref:`yield
+expression <yieldexpr>`. The yield statement can be used to omit the parentheses
+that would otherwise be required in the equivalent yield expression
+statement. For example, the yield statements ::
 
-.. seealso::
+  yield <expr>
+  yield from <expr>
 
-   :pep:`0255` - Simple Generators
-      The proposal for adding generators and the :keyword:`yield` statement to Python.
+are equivalent to the yield expression statements ::
 
-   :pep:`0342` - Coroutines via Enhanced Generators
-      The proposal to enhance the API and syntax of generators, making them
-      usable as simple coroutines.
+  (yield <expr>)
+  (yield from <expr>)
 
-   :pep:`0380` - Syntax for Delegating to a Subgenerator
-      The proposal to introduce the :token:`yield_from` syntax, making delegation
-      to sub-generators easy.
+Yield expressions and statements are only used when defining a :term:`generator`
+function, and are only used in the body of the generator function.  Using yield
+in a function definition is sufficient to cause that definition to create a
+generator function instead of a normal function.
 
+For full details of :keyword:`yield` semantics, refer to the
+:ref:`yieldexpr` section.
 
 .. _raise:
 
