Updates and fixes based on community comments. Some of the examples

has bugs in them.  Also, they should use Python 2.2's {}.iteritems()
where appropriate.

Renamed and reorg'd the Open Issues section, with an entry about the
shortcut syntax, and a question about nested for loop.

Added a section on Implementation, noting Python 2.2's new dictionary
sequence-of-2-element-sequences constructor.

pep-0274.txt

@@ -7,7 +7,7 @@ Status: Draft
 Type: Standards Track
 Created: 25-Oct-2001
 Python-Version: 2.3
-Post-History:
+Post-History: 29-Oct-2001
 
 
 Abstract
@@ -59,44 +59,76 @@ Examples
     >>> print {i : chr(65+i) for i in range(4)}
     {0 : 'A', 1 : 'B', 2 : 'C', 3 : 'D'}
 
-    >>> print {k : v for k, v in someDict.items()} == someDict.copy()
+    >>> print {k : v for k, v in someDict.iteritems()} == someDict.copy()
     1
 
     >>> print {x.lower() : 1 for x in list_of_email_addrs}
     {'barry@zope.com'   : 1, 'barry@python.org' : 1, 'guido@python.org' : 1}
 
     >>> def invert(d):
-    ...     return {v : k for k, v in d}
+    ...     return {v : k for k, v in d.iteritems()}
     ...
     >>> d = {0 : 'A', 1 : 'B', 2 : 'C', 3 : 'D'}
     >>> print invert(d)
-    {'A' : 0, 'B' : 1, 'C' : 2, 'D' : 4}
-
-    >>> print {k, v for k in range(4) for v in range(-4, 0, 1)}
-    {0 : -4, 1 : -3, 2 : -2, 3 : -1}
+    {'A' : 0, 'B' : 1, 'C' : 2, 'D' : 3}
 
 
-Optional Enhancements
+Open Issues
 
-    There is one further shortcut we could adopt.  Suppose we wanted
-    to create a set of items, such as in the "list_of_email_addrs"
-    example above.  Here, we're simply taking the target of the for
-    loop and turning that into the key for the dict comprehension.
-    The assertion is that this would be a common idiom, so the
-    shortcut below allows for an easy spelling of it, by allow us to
-    omit the "key :" part of the left hand clause:
+    - There is one further shortcut we could adopt.  Suppose we wanted
+      to create a set of items, such as in the "list_of_email_addrs"
+      example above.  Here, we're simply taking the target of the for
+      loop and turning that into the key for the dict comprehension.
+      The assertion is that this would be a common idiom, so the
+      shortcut below allows for an easy spelling of it, by allow us to
+      omit the "key :" part of the left hand clause:
 
-    >>> print {1 for x in list_of_email_addrs}
-    {'barry@zope.com'   : 1, 'barry@python.org' : 1, 'guido@python.org' : 1}
+      >>> print {1 for x in list_of_email_addrs}
+      {'barry@zope.com'   : 1, 'barry@python.org' : 1, 'guido@python.org' : 1}
 
-    Or say we wanted to map email addresses to the MX record handling
-    their mail:
+      Or say we wanted to map email addresses to the MX record handling
+      their mail:
 
-    >>> print {mx_for_addr(x) for x in list_of_email_addrs}
-    {'barry@zope.com'   : 'mail.zope.com',
-     'barry@python.org' : 'mail.python.org,
-     'guido@python.org' : 'mail.python.org,
-     }
+      >>> print {mx_for_addr(x) for x in list_of_email_addrs}
+      {'barry@zope.com'   : 'mail.zope.com',
+       'barry@python.org' : 'mail.python.org,
+       'guido@python.org' : 'mail.python.org,
+       }
+
+      Questions: what about nested loops?  Where does the key come
+      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.
 
 
 References