Raymond Hettinger's latest update, slightly reformatted.

pep-0279.txt

@@ -60,6 +60,11 @@ Rationale
     keywords.  These generator tools go into Python 2.3 when
     generators become final and are not imported from __future__.
 
+    SourceForge contains a working, pure Python simulation of every
+    feature proposed in this PEP [8].  SourceForge also has a separate
+    file with a simulation test suite and working source code for the
+    examples listed used in this PEP [9].
+
 
 Specification for new built-ins:
 
@@ -118,7 +123,7 @@ Specification for new built-ins:
                 raise StopIteration
             yield fun(*args)
 
-    def xzip( *collections ):
+    def xzip( *collections ):   ### Code from Python Cookbook [6]
         '''
         xzip(...)
         xzip(seq1 [, seq2 [...]]) -> [(seq1[0], seq2[0] ...), (...)]
@@ -146,6 +151,12 @@ Specification for new built-ins:
     presented and evaluated in the world prior to Python 2.2 which did
     not include generators.
 
+    Note B:  An alternate, simplified definition of indexed was proposed:
+
+    def indexed( collection, cnt=0, limit=sys.maxint ):
+        'Generates an indexed series:  (0,seqn[0]), (1,seqn[1]) ...'
+        return xzip( xrange(cnt,limit), collection )
+
 
 Specification for Generator Comprehensions:
 
@@ -177,6 +188,37 @@ Specification for Generator Comprehensions:
     indicates that brackets do not make a false suggestion and are
     in fact helpful.
 
+    Note B: An iterable instance is returned by the above code.  The
+    purpose is to allow the object to be re-started and looped-over
+    multiple times.  This accurately mimics the behavior of list
+    comprehensions.  As a result, the following code (provided by Oren
+    Tirosh) works equally well with or without 'yield':
+
+        letters = [yield chr(i) for i in xrange(ord('a'),ord('z')+1)]
+        digits = [yield str(i) for i in xrange(10)]
+        letdig = [yield l+d for l in letters for d in digits]
+
+    Note C: List comprehensions expose their looping variable and
+    leave the variable in the enclosing scope.  The code, [str(i) for
+    i in range(8)] leaves 'i' set to 7 in the scope where the
+    comprehension appears.  This behavior is by design and reflects an
+    intent to duplicate the result of coding a for-loop instead of a
+    list comprehension.  Further, the variable 'i' is in a defined and
+    potentially useful state on the line immediately following the
+    list comprehension.
+
+    In contrast, generator comprehensions do not expose the looping
+    variable to the enclosing scope.  The code, [yield str(i) for i in
+    range(8)] leaves 'i' untouched in the scope where the
+    comprehension appears.  This is also by design and reflects an
+    intent to duplicate the result of coding a generator directly
+    instead of a generator comprehension.  Further, the variable 'i'
+    is not in a defined state on the line immediately following the
+    list comprehension.  It does not come into existence until
+    iteration starts.  Since several generators may be running at
+    once, there are potentially multiple, unequal instances of 'i' at
+    any one time.
+
 
 Specification for Generator Parameter Passing:
 
@@ -259,14 +301,14 @@ Specification for Generator Parameter Passing:
     Example of a Complex Consumer
 
     Loop over the picture files in a directory, shrink them
-    one-at-a-time to thumbnail size using PIL, and send them to a lazy
-    consumer.  That consumer is responsible for creating a large blank
-    image, accepting thumbnails one-at-a-time and placing them in a
-    5x3 grid format onto the blank image.  Whenever the grid is full,
-    it writes-out the large image as an index print.  A FlushStream
-    exception indicates that no more thumbnails are available and that
-    the partial index print should be written out if there are one or
-    more thumbnails on it.
+    one-at-a-time to thumbnail size using PIL [7], and send them to a
+    lazy consumer.  That consumer is responsible for creating a large
+    blank image, accepting thumbnails one-at-a-time and placing them
+    in a 5x3 grid format onto the blank image.  Whenever the grid is
+    full, it writes-out the large image as an index print.  A
+    FlushStream exception indicates that no more thumbnails are
+    available and that the partial index print should be written out
+    if there are one or more thumbnails on it.
 
 
     Example of a Producer and Consumer Used Together in a Pipelike Fashion
@@ -348,6 +390,19 @@ References
     [5] Dr. David Mertz's draft column for Charming Python.
         http://gnosis.cx/publish/programming/charming_python_b5.txt
 
+    [6] The code fragment for xmap() was found at:
+        http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66448
+
+    [7] PIL, the Python Imaging Library can be found at:
+        http://www.pythonware.com/products/pil/
+
+    [8] A pure Python simulation of every feature in this PEP is at:
+        http://sourceforge.net/tracker/download.php?group_id=5470&atid=305470&file_id=17348&aid=513752
+
+    [9] The full, working source code for each of the examples in this PEP
+        along with other examples and tests is at:
+        http://sourceforge.net/tracker/download.php?group_id=5470&atid=305470&file_id=17412&aid=513756
+
 
 Copyright
 
@@ -360,5 +415,3 @@ mode: indented-text
 indent-tabs-mode: nil
 fill-column: 70
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-
-