Some rearrangements and fixes based on recent comments.

pep-0274.txt

@@ -35,10 +35,9 @@ Rationale
 
     There are times when you have some data arranged as a sequences of
     length-2 sequences, and you want to turn that into a dictionary.
-    In Python 2.2, the dictionary() constructor will take an optional
-    keyword argument that indicates specifically to interpret a
-    sequences of length-2 sequences as key/value pairs, and turn them
-    into a dictionary.
+    In Python 2.2, the dictionary() constructor accepts an argument
+    that is a sequence of length-2 sequences, used as (key, value)
+    pairs to initialize a new dictionary object.
 
     However, the act of turning some data into a sequence of length-2
     sequences can be inconvenient or inefficient from a memory or
@@ -54,6 +53,24 @@ Rationale
     for loop.
 
 
+Semantics
+
+    The semantics of dictionary comprehensions can actually be
+    demonstrated in stock Python 2.2, by passing a list comprehension
+    to the builtin dictionary constructor:
+
+    >>> dictionary([(i, chr(65+i)) for i in range(4)])
+
+    is semantically equivalent to
+
+    >>> {i : chr(65+i) for i in range(4)}
+
+    The dictionary constructor approach has two dictinct disadvantages
+    from the proposed syntax though.  First, it isn't as legible as a
+    dict comprehension.  Second, it forces the programmer to create an
+    in-core list object first, which could be expensive.
+
+
 Examples
 
     >>> print {i : chr(65+i) for i in range(4)}
@@ -72,6 +89,12 @@ Examples
     >>> print invert(d)
     {'A' : 0, 'B' : 1, 'C' : 2, 'D' : 3}
 
+    >>> {(k, v): k+v for k in range(4) for v in range(4)}
+    ... {(3, 3): 6, (3, 2): 5, (3, 1): 4, (0, 1): 1, (2, 1): 3,
+         (0, 2): 2, (3, 0): 3, (0, 3): 3, (1, 1): 2, (1, 0): 1,
+         (0, 0): 0, (1, 2): 3, (2, 0): 2, (1, 3): 4, (2, 2): 4, (
+         2, 3): 5}
+
 
 Open Issues
 
@@ -99,36 +122,10 @@ Open Issues
       from?  The shortcut probably doesn't save much typing, and comes
       at the expense of legibility, so it's of dubious value.
 
-    - Should nested for loops be allowed?  The following example,
-      taken from an earlier revision of this PEP illustrates the
-      problem:
-
-      >>> print {k, v for k in range(4) for v in range(-4, 0, 1)}
-
-      The intent of this example was to produce a mapping from a
-      number to its negative, but this code doesn't work because -- as
-      in list comprehensions -- the for loops are nested, not in
-      parallel!  So the value of this expression is actually
-
-      {0: -1, 1: -1, 2: -1, 3: -1}
-
-      which seems of dubious value.  For symmetry with list
-      comprehensions, perhaps this should be allowed, but it might be
-      better to disallow this syntax.
-
 
 Implementation
 
-    The semantics of dictionary comprehensions can actually be modeled
-    in stock Python 2.2, by passing a list comprehension to the
-    builtin dictionary constructor:
-
-    >>> dictionary([(i, chr(65+i)) for i in range(4)])
-
-    This has two dictinct disadvantages from the proposed syntax
-    though.  First, it's isn't as legible as a dict comprehension.
-    Second, it forces the programmer to create an in-core list object
-    first, which could be expensive.
+    TBD
 
 
 References