[Python-checkins] CVS: python/nondist/peps pep-0274.txt,1.1,1.2

2001年10月29日 10:47:01 -0800

Update of /cvsroot/python/python/nondist/peps
In directory usw-pr-cvs1:/tmp/cvs-serv28265
Modified Files:
	pep-0274.txt 
Log Message:
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.
Index: pep-0274.txt
===================================================================
RCS file: /cvsroot/python/python/nondist/peps/pep-0274.txt,v
retrieving revision 1.1
retrieving revision 1.2
diff -C2 -d -r1.1 -r1.2
*** pep-0274.txt	2001年10月25日 20:28:05	1.1
--- pep-0274.txt	2001年10月29日 18:46:59	1.2
***************
*** 8,12 ****
 Created: 25-Oct-2001
 Python-Version: 2.3
! Post-History:

--- 8,12 ----
 Created: 25-Oct-2001
 Python-Version: 2.3
! Post-History: 29-Oct-2001

***************
*** 60,64 ****
 {0 : 'A', 1 : 'B', 2 : 'C', 3 : 'D'}

! >>> print {k : v for k, v in someDict.items()} == someDict.copy()
 1

--- 60,64 ----
 {0 : 'A', 1 : 'B', 2 : 'C', 3 : 'D'}

! >>> print {k : v for k, v in someDict.iteritems()} == someDict.copy()
 1

***************
*** 67,101 ****

 >>> 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,
! }

--- 67,133 ----

 >>> def invert(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' : 3}

+ 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:

! >>> 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,
! }

! 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.