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Diffstat (limited to 'Doc')
| -rw-r--r-- | Doc/reference/expressions.rst | 99 | ||||
| -rw-r--r-- | Doc/reference/simple_stmts.rst | 57 |
2 files changed, 62 insertions, 94 deletions
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst index 66358c8..06baba0 100644 --- 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 index b9ebaaa..40bbc39 100644 --- 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: |