[Python-checkins] r70100 - peps/trunk/pep-0372.txt
raymond.hettinger
python-checkins at python.org
Mon Mar 2 21:40:37 CET 2009
Author: raymond.hettinger
Date: Mon Mar 2 21:40:36 2009
New Revision: 70100
Log:
Mark the PEP as Accepted.
Replace the odict references with OrderedDict.
Specify the __repr__() format.
Update the __eq__() specification as discussed on python-dev.
Modified:
peps/trunk/pep-0372.txt
Modified: peps/trunk/pep-0372.txt
==============================================================================
--- peps/trunk/pep-0372.txt (original)
+++ peps/trunk/pep-0372.txt Mon Mar 2 21:40:36 2009
@@ -4,7 +4,7 @@
Last-Modified: $Date$
Author: Armin Ronacher <armin.ronacher at active-4.com>
Raymond Hettinger <python at rcn.com>
-Status: Draft
+Status: Accepted
Type: Standards Track
Content-Type: text/x-rst
Created: 15-Jun-2008
@@ -16,7 +16,7 @@
========
This PEP proposes an ordered dictionary as a new data structure for
-the ``collections`` module, called "odict" in this PEP for short. The
+the ``collections`` module, called "OrderedDict" in this PEP. The
proposed API incorporates the experiences gained from working with
similar implementations that exist in various real-world applications
and other programming languages.
@@ -44,13 +44,13 @@
The following example shows the behavior for simple assignments:
- >>> d = odict()
+ >>> d = OrderedDict()
>>> d['parrot'] = 'dead'
>>> d['penguin'] = 'exploded'
>>> d.items()
[('parrot', 'dead'), ('penguin', 'exploded')]
-That the ordering is preserved makes an odict useful for a couple of
+That the ordering is preserved makes an OrderedDict useful for a couple of
situations:
- XML/HTML processing libraries currently drop the ordering of
@@ -97,10 +97,10 @@
well as mappings like a dict does. Unlike a regular dictionary,
the insertion order is preserved.
- >>> d = odict([('a', 'b'), ('c', 'd')])
+ >>> d = OrderedDict([('a', 'b'), ('c', 'd')])
>>> d.update({'foo': 'bar'})
>>> d
- collections.odict([('a', 'b'), ('c', 'd'), ('foo', 'bar')])
+ collections.OrderedDict([('a', 'b'), ('c', 'd'), ('foo', 'bar')])
If ordered dicts are updated from regular dicts, the ordering of new
keys is of course undefined.
@@ -119,7 +119,7 @@
New methods not available on dict:
-``odict.__reversed__()``
+``OrderedDict.__reversed__()``
Supports reverse iteration by key.
@@ -138,8 +138,8 @@
former. This has the side-effect that the position of the first
key is used because only the value is actually overwritten::
- >>> odict([('a', 1), ('b', 2), ('a', 3)])
- collections.odict([('a', 3), ('b', 2)])
+ >>> OrderedDict([('a', 1), ('b', 2), ('a', 3)])
+ collections.OrderedDict([('a', 3), ('b', 2)])
This behavior is consistent with existing implementations in
Python, the PHP array and the hashmap in Ruby 1.9.
@@ -155,10 +155,10 @@
Do any limitations arise from subclassing dict?
Yes. Since the API for dicts is different in Py2.x and Py3.x, the
- odict API must also be different. So, the Py2.7 version will need
+ OrderedDict API must also be different. So, the Py2.7 version will need
to override iterkeys, itervalues, and iteritems.
-Does ``odict.popitem()`` return a particular key/value pair?
+Does ``OrderedDict.popitem()`` return a particular key/value pair?
Yes. It pops-off the most recently inserted new key and its
corresponding value. This corresponds to the usual LIFO behavior
@@ -167,23 +167,26 @@
The actual implementation is more efficient and pops directly
from a sorted list of keys.
-Does odict support indexing, slicing, and whatnot?
+Does OrderedDict support indexing, slicing, and whatnot?
- As a matter of fact, ``odict`` does not implement the ``Sequence``
+ As a matter of fact, ``OrderedDict`` does not implement the ``Sequence``
interface. Rather, it is a ``MutableMapping`` that remembers
the order of key insertion. The only sequence-like addition is
support for ``reversed``.
-Does odict support alternate sort orders such as alphabetical?
+ An further advantage of not allowing indexing is that it leaves open
+ the possibility of a fast C implementation using linked lists.
+
+Does OrderedDict support alternate sort orders such as alphabetical?
No. Those wanting different sort orders really need to be using another
- technique. The odict is all about recording insertion order. If any
+ technique. The OrderedDict is all about recording insertion order. If any
other order is of interest, then another structure (like an in-memory
dbm) is likely a better fit.
-How well does odict work with the json module, PyYAML, and ConfigParser?
+How well does OrderedDict work with the json module, PyYAML, and ConfigParser?
- For json, the good news is that json's encoder respects odict's iteration order::
+ For json, the good news is that json's encoder respects OrderedDict's iteration order::
>>> items = [('one', 1), ('two', 2), ('three',3), ('four',4), ('five',5)]
>>> json.dumps(OrderedDict(items))
@@ -221,20 +224,18 @@
>>> config.remove_option('Log', 'error')
>>> config.write(open('myconfig.ini', 'w'))
-How does odict handle equality testing?
+How does OrderedDict handle equality testing?
+
+ Comparing two ordered dictionaries implies that the test will be
+ order-sensitive so that list ``(od1.items())==list(od2.items())``.
- Being a dict, one might expect equality tests to not care about order. For
- an odict-to-dict comparison, this would be a necessity and it's probably
- not wise to silently switch comparison modes based on the input types.
- Also, some third-party tools that expect dict inputs may also expect the
- comparison to not care about order. Accordingly, we decided to punt and
- let the usual dict equality testing run without reference to internal
- ordering. This should be documented clearly since different people will
- have different expectations. If a use case does arise, it's not hard for
- a user explicitly craft an order based comparison::
+ When ordered dicts are compared with other Mappings, their order
+ insensitive comparison is used. This allows ordered dictionaries
+ to be substituted anywhere regular dictionaries are used.
- # Explict order-sensitive comparison
- >>> list(od1.items())==list(od2.items())
+How __repr__ format will maintain order during an repr/eval round-trip?
+
+ OrderedDict([('a', 1), ('b', 2)])
What are the trade-offs of the possible underlying data structures?
@@ -255,6 +256,7 @@
would keep the same big-oh performance as regular dictionaries. It is
the fastest and most space efficient.
+
Reference Implementation
========================
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