From d00e0d7e3ec2fa904f3cec5f3bac5472a7c55326 Mon Sep 17 00:00:00 2001
From: Barry Warsaw <barry@python.org>
Date: Thu, 25 Oct 2001 20:28:05 +0000
Subject: [PATCH] PEP 274, Dict Comprehensions, Warsaw

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+PEP: 274
+Title: Dict Comprehensions
+Version: $Revision$
+Last-Modified: $Date$
+Author: barry@zope.com (Barry A. Warsaw)
+Status: Draft
+Type: Standards Track
+Created: 25-Oct-2001
+Python-Version: 2.3
+Post-History:
+
+
+Abstract
+
+    PEP 202 introduces a syntactical extension to Python called the
+    "list comprehension"[1].  This PEP proposes a similar syntactical
+    extension called the "dictionary comprehension" or "dict
+    comprehension" for short.  You can use dict comprehensions in ways
+    very similar to list comprehensions, except that they produce
+    Python dictionary objects instead of list objects.
+
+
+Proposed Solution
+
+    Dict comprehensions are just like list comprehensions, except that
+    you group the expression using curly braces instead of square
+    braces.  Also, the left part before the `for' keyword expresses
+    both a key and a value, separated by a colon.  (There is an
+    optional part of this PEP that allows you to use a shortcut to
+    express just the value.)  The notation is specifically designed to
+    remind you of list comprehensions as applied to dictionaries.
+
+
+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.
+
+    However, the act of turning some data into a sequence of length-2
+    sequences can be inconvenient or inefficient from a memory or
+    performance standpoint.  Also, for some common operations, such as
+    turning a list of things into a set of things for quick duplicate
+    removal or set inclusion tests, a better syntax can help code
+    clarity.
+
+    As with list comprehensions, an explicit for loop can always be
+    used (and in fact was the only way to do it in earlier versions of
+    Python).  But as with list comprehensions, dict comprehensions can
+    provide a more syntactically succinct idiom that the traditional
+    for loop.
+
+
+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()
+    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}
+    ...
+    >>> 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}
+
+
+Optional Enhancements
+
+    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}
+
+    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,
+     }
+
+
+References
+
+    [1] PEP 202, List Comprehensions
+        http://www.python.org/peps/pep-0202.html
+
+
+Copyright
+
+    This document has been placed in the public domain.
+
+
+
+Local Variables:
+mode: indented-text
+indent-tabs-mode: nil
+fill-column: 70
+End: