[Python-checkins] cpython: Add some simple examples to the PEP 380 section of What's New

[nick.coghlan]

http://hg.python.org/cpython/rev/538cbae3ec2d
changeset:   75451:538cbae3ec2d
user:        Nick Coghlan <ncoghlan at gmail.com>
date:        Tue Mar 06 22:31:12 2012 +1000
summary:
  Add some simple examples to the PEP 380 section of What's New

files:
  Doc/whatsnew/3.3.rst |  47 ++++++++++++++++++++++++++++++++
  1 files changed, 47 insertions(+), 0 deletions(-)


diff --git a/Doc/whatsnew/3.3.rst b/Doc/whatsnew/3.3.rst
--- a/Doc/whatsnew/3.3.rst
+++ b/Doc/whatsnew/3.3.rst
@@ -262,9 +262,56 @@
 containing 'yield' to be factored out and placed in another generator.
 Additionally, the subgenerator is allowed to return with a value, and the
 value is made available to the delegating generator.
+
 While designed primarily for use in delegating to a subgenerator, the ``yield
 from`` expression actually allows delegation to arbitrary subiterators.
 
+For simple iterators, ``yield from iterable`` is essentially just a shortened
+form of ``for item in iterable: yield item``::
+
+    >>> def g(x):
+    ...     yield from range(x, 0, -1)
+    ...     yield from range(x)
+    ...
+    >>> list(g(5))
+    [5, 4, 3, 2, 1, 0, 1, 2, 3, 4]
+
+However, unlike an ordinary loop, ``yield from`` allows subgenerators to
+receive sent and thrown values directly from the calling scope, and
+return a final value to the outer generator::
+
+    >>> def accumulate(start=0):
+    ...     tally = start
+    ...     while 1:
+    ...         next = yield
+    ...         if next is None:
+    ...             return tally
+    ...         tally += next
+    ...
+    >>> def gather_tallies(tallies, start=0):
+    ...     while 1:
+    ...         tally = yield from accumulate()
+    ...         tallies.append(tally)
+    ...
+    >>> tallies = []
+    >>> acc = gather_tallies(tallies)
+    >>> next(acc) # Ensure the accumulator is ready to accept values
+    >>> for i in range(10):
+    ...     acc.send(i)
+    ...
+    >>> acc.send(None) # Finish the first tally
+    >>> for i in range(5):
+    ...     acc.send(i)
+    ...
+    >>> acc.send(None) # Finish the second tally
+    >>> tallies
+    [45, 10]
+
+The main principle driving this change is to allow even generators that are
+designed to be used with the ``send`` and ``throw`` methods to be split into
+multiple subgenerators as easily as a single large function can be split into
+multiple subfunctions.
+
 (Implementation by Greg Ewing, integrated into 3.3 by Renaud Blanch, Ryan
 Kelly and Nick Coghlan, documentation by Zbigniew Jędrzejewski-Szmek and
 Nick Coghlan)

-- 
Repository URL: http://hg.python.org/cpython