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<H1><A NAME="SECTION0001200000000000000000">
11 Other Language Changes</A>
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<P>
Here are all of the changes that Python 2.4 makes to the core Python
language.

<P>

<UL>
<LI>Decorators for functions and methods were added (<a class="rfc" id='rfcref-1456' xml:id='rfcref-1456'
href="http://www.python.org/peps/pep-0318.html">PEP 318</a>).

<P>
</LI>
<LI>Built-in <tt class="function">set</tt> and <tt class="function">frozenset</tt> types were 
added (<a class="rfc" id='rfcref-1458' xml:id='rfcref-1458'
href="http://www.python.org/peps/pep-0218.html">PEP 218</a>).  Other new built-ins include the <tt class="function">reversed(<var>seq</var>)</tt> function (<a class="rfc" id='rfcref-1460' xml:id='rfcref-1460'
href="http://www.python.org/peps/pep-0322.html">PEP 322</a>).

<P>
</LI>
<LI>Generator expressions were added (<a class="rfc" id='rfcref-1462' xml:id='rfcref-1462'
href="http://www.python.org/peps/pep-0289.html">PEP 289</a>).

<P>
</LI>
<LI>Certain numeric expressions no longer return values restricted to 32 or 64 bits (<a class="rfc" id='rfcref-1464' xml:id='rfcref-1464'
href="http://www.python.org/peps/pep-0237.html">PEP 237</a>).

<P>
</LI>
<LI>You can now put parentheses around the list of names in a
<code>from <var>module</var> import <var>names</var></code> statement (<a class="rfc" id='rfcref-1466' xml:id='rfcref-1466'
href="http://www.python.org/peps/pep-0328.html">PEP 328</a>).

<P>
</LI>
<LI>The <tt class="method">dict.update()</tt> method now accepts the same
argument forms as the <tt class="class">dict</tt> constructor.  This includes any
mapping, any iterable of key/value pairs, and keyword arguments.
(Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>The string methods <tt class="method">ljust()</tt>, <tt class="method">rjust()</tt>, and
<tt class="method">center()</tt> now take an optional argument for specifying a
fill character other than a space.
(Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>Strings also gained an <tt class="method">rsplit()</tt> method that
works like the <tt class="method">split()</tt> method but splits from the end of
the string.  

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; 'www.python.org'.split('.', 1)
['www', 'python.org']
'www.python.org'.rsplit('.', 1)
['www.python', 'org']
</pre></div>      

<P>
</LI>
<LI>Three keyword parameters, <var>cmp</var>, <var>key</var>, and
<var>reverse</var>, were added to the <tt class="method">sort()</tt> method of lists.
These parameters make some common usages of <tt class="method">sort()</tt> simpler.
All of these parameters are optional.

<P>
For the <var>cmp</var> parameter, the value should be a comparison function
that takes two parameters and returns -1, 0, or +1 depending on how
the parameters compare.  This function will then be used to sort the
list.  Previously this was the only parameter that could be provided
to <tt class="method">sort()</tt>.

<P>
<var>key</var> should be a single-parameter function that takes a list
element and returns a comparison key for the element.  The list is
then sorted using the comparison keys.  The following example sorts a
list case-insensitively:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; L = ['A', 'b', 'c', 'D']
&gt;&gt;&gt; L.sort()                 # Case-sensitive sort
&gt;&gt;&gt; L
['A', 'D', 'b', 'c']
&gt;&gt;&gt; # Using 'key' parameter to sort list
&gt;&gt;&gt; L.sort(key=lambda x: x.lower())
&gt;&gt;&gt; L
['A', 'b', 'c', 'D']
&gt;&gt;&gt; # Old-fashioned way
&gt;&gt;&gt; L.sort(cmp=lambda x,y: cmp(x.lower(), y.lower()))
&gt;&gt;&gt; L
['A', 'b', 'c', 'D']
</pre></div>

<P>
The last example, which uses the <var>cmp</var> parameter, is the old way
to perform a case-insensitive sort.  It works but is slower than using
a <var>key</var> parameter.  Using <var>key</var> calls <tt class="method">lower()</tt> method
once for each element in the list while using <var>cmp</var> will call it
twice for each comparison, so using <var>key</var> saves on invocations of
the <tt class="method">lower()</tt> method.

<P>
For simple key functions and comparison functions, it is often
possible to avoid a <tt class="keyword">lambda</tt> expression by using an unbound
method instead.  For example, the above case-insensitive sort is best
written as:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; L.sort(key=str.lower)
&gt;&gt;&gt; L
['A', 'b', 'c', 'D']
</pre></div>       

<P>
Finally, the <var>reverse</var> parameter takes a Boolean value.  If the
value is true, the list will be sorted into reverse order.
Instead of <code>L.sort() ; L.reverse()</code>, you can now write
<code>L.sort(reverse=True)</code>.

<P>
The results of sorting are now guaranteed to be stable.  This means
that two entries with equal keys will be returned in the same order as
they were input.  For example, you can sort a list of people by name,
and then sort the list by age, resulting in a list sorted by age where
people with the same age are in name-sorted order.

<P>
(All changes to <tt class="method">sort()</tt> contributed by Raymond Hettinger.)

<P>
</LI>
<LI>There is a new built-in function
<tt class="function">sorted(<var>iterable</var>)</tt> that works like the in-place
<tt class="method">list.sort()</tt> method but can be used in
expressions.  The differences are:
  
<UL>
<LI>the input may be any iterable;
</LI>
<LI>a newly formed copy is sorted, leaving the original intact; and
</LI>
<LI>the expression returns the new sorted copy
  
</LI>
</UL>

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; L = [9,7,8,3,2,4,1,6,5]
&gt;&gt;&gt; [10+i for i in sorted(L)]       # usable in a list comprehension
[11, 12, 13, 14, 15, 16, 17, 18, 19]
&gt;&gt;&gt; L                               # original is left unchanged
[9,7,8,3,2,4,1,6,5]
&gt;&gt;&gt; sorted('Monty Python')          # any iterable may be an input
[' ', 'M', 'P', 'h', 'n', 'n', 'o', 'o', 't', 't', 'y', 'y']

&gt;&gt;&gt; # List the contents of a dict sorted by key values
&gt;&gt;&gt; colormap = dict(red=1, blue=2, green=3, black=4, yellow=5)
&gt;&gt;&gt; for k, v in sorted(colormap.iteritems()):
...     print k, v
...
black 4
blue 2
green 3
red 1
yellow 5
</pre></div>

<P>
(Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>Integer operations will no longer trigger an <tt class="exception">OverflowWarning</tt>.
The <tt class="exception">OverflowWarning</tt> warning will disappear in Python 2.5.

<P>
</LI>
<LI>The interpreter gained a new switch, <b class="programopt">-m</b>, that
takes a name, searches for the corresponding  module on <code>sys.path</code>,
and runs the module as a script.  For example, 
you can now run the Python profiler with <code>python -m profile</code>.
(Contributed by Nick Coghlan.)

<P>
</LI>
<LI>The <tt class="function">eval(<var>expr</var>, <var>globals</var>, <var>locals</var>)</tt>
and <tt class="function">execfile(<var>filename</var>, <var>globals</var>, <var>locals</var>)</tt>
functions and the <tt class="keyword">exec</tt> statement now accept any mapping type
for the <var>locals</var> parameter.  Previously this had to be a regular
Python dictionary.  (Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>The <tt class="function">zip()</tt> built-in function and <tt class="function">itertools.izip()</tt>
  now return an empty list if called with no arguments.
  Previously they raised a <tt class="exception">TypeError</tt>
  exception.  This makes them more
  suitable for use with variable length argument lists:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; def transpose(array):
...    return zip(*array)
...
&gt;&gt;&gt; transpose([(1,2,3), (4,5,6)])
[(1, 4), (2, 5), (3, 6)]
&gt;&gt;&gt; transpose([])
[]
</pre></div>
(Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>Encountering a failure while importing a module no longer leaves
a partially-initialized module object in <code>sys.modules</code>.  The
incomplete module object left behind would fool further imports of the
same module into succeeding, leading to confusing errors.  
(Fixed by Tim Peters.)

<P>
</LI>
<LI><tt class="constant">None</tt> is now a constant; code that binds a new value to 
the name "<tt class="samp">None</tt>" is now a syntax error.
(Contributed by Raymond Hettinger.)       

<P>
</LI>
</UL>

<P>

<H2><A NAME="SECTION0001210000000000000000">
11.1 Optimizations</A>
</H2>

<P>

<UL>
<LI>The inner loops for list and tuple slicing
 were optimized and now run about one-third faster.  The inner loops
 for dictionaries were also optimized, resulting in performance boosts for
 <tt class="method">keys()</tt>, <tt class="method">values()</tt>, <tt class="method">items()</tt>,
 <tt class="method">iterkeys()</tt>, <tt class="method">itervalues()</tt>, and <tt class="method">iteritems()</tt>.
 (Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>The machinery for growing and shrinking lists was optimized for
 speed and for space efficiency.  Appending and popping from lists now
 runs faster due to more efficient code paths and less frequent use of
 the underlying system <tt class="cfunction">realloc()</tt>.  List comprehensions
 also benefit.   <tt class="method">list.extend()</tt> was also optimized and no
 longer converts its argument into a temporary list before extending
 the base list.  (Contributed by Raymond Hettinger.)

<P>
</LI>
<LI><tt class="function">list()</tt>, <tt class="function">tuple()</tt>, <tt class="function">map()</tt>,
  <tt class="function">filter()</tt>, and <tt class="function">zip()</tt> now run several times
  faster with non-sequence arguments that supply a <tt class="method">__len__()</tt>
  method.  (Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>The methods <tt class="method">list.__getitem__()</tt>,
  <tt class="method">dict.__getitem__()</tt>, and <tt class="method">dict.__contains__()</tt> are
  are now implemented as <tt class="class">method_descriptor</tt> objects rather
  than <tt class="class">wrapper_descriptor</tt> objects.  This form of 
  access doubles their performance and makes them more suitable for
  use as arguments to functionals:
  "<tt class="samp">map(mydict.__getitem__, keylist)</tt>".
  (Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>Added a new opcode, <code>LIST_APPEND</code>, that simplifies
  the generated bytecode for list comprehensions and speeds them up
  by about a third.  (Contributed by Raymond Hettinger.)

<P>
</LI>
<LI>The peephole bytecode optimizer has been improved to 
produce shorter, faster bytecode; remarkably, the resulting bytecode is 
more readable.  (Enhanced by Raymond Hettinger.)

<P>
</LI>
<LI>String concatenations in statements of the form <code>s = s +
"abc"</code> and <code>s += "abc"</code> are now performed more efficiently in
certain circumstances.  This optimization won't be present in other
Python implementations such as Jython, so you shouldn't rely on it;
using the <tt class="method">join()</tt> method of strings is still recommended when
you want to efficiently glue a large number of strings together.
(Contributed by Armin Rigo.)       

<P>
</LI>
</UL>

<P>
The net result of the 2.4 optimizations is that Python 2.4 runs the
pystone benchmark around 5% faster than Python 2.3 and 35% faster
than Python 2.2.  (pystone is not a particularly good benchmark, but
it's the most commonly used measurement of Python's performance.  Your
own applications may show greater or smaller benefits from Python&nbsp;2.4.)

<P>

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	63	<H1><A NAME="SECTION0001200000000000000000">
	64	11 Other Language Changes</A>
65	</H1>
66
67	<P>
68	Here are all of the changes that Python 2.4 makes to the core Python
69	language.
70
71	<P>
72
73	<UL>
74	<LI>Decorators for functions and methods were added (<a class="rfc" id='rfcref-1456' xml:id='rfcref-1456'
75	href="http://www.python.org/peps/pep-0318.html">PEP 318</a>).
76
77	<P>
78	</LI>
79	<LI>Built-in <tt class="function">set</tt> and <tt class="function">frozenset</tt> types were
80	added (<a class="rfc" id='rfcref-1458' xml:id='rfcref-1458'
81	href="http://www.python.org/peps/pep-0218.html">PEP 218</a>). Other new built-ins include the <tt class="function">reversed(<var>seq</var>)</tt> function (<a class="rfc" id='rfcref-1460' xml:id='rfcref-1460'
82	href="http://www.python.org/peps/pep-0322.html">PEP 322</a>).
83
84	<P>
85	</LI>
86	<LI>Generator expressions were added (<a class="rfc" id='rfcref-1462' xml:id='rfcref-1462'
87	href="http://www.python.org/peps/pep-0289.html">PEP 289</a>).
88
89	<P>
90	</LI>
91	<LI>Certain numeric expressions no longer return values restricted to 32 or 64 bits (<a class="rfc" id='rfcref-1464' xml:id='rfcref-1464'
92	href="http://www.python.org/peps/pep-0237.html">PEP 237</a>).
93
94	<P>
95	</LI>
96	<LI>You can now put parentheses around the list of names in a
97	<code>from <var>module</var> import <var>names</var></code> statement (<a class="rfc" id='rfcref-1466' xml:id='rfcref-1466'
98	href="http://www.python.org/peps/pep-0328.html">PEP 328</a>).
99
100	<P>
101	</LI>
102	<LI>The <tt class="method">dict.update()</tt> method now accepts the same
103	argument forms as the <tt class="class">dict</tt> constructor. This includes any
104	mapping, any iterable of key/value pairs, and keyword arguments.
105	(Contributed by Raymond Hettinger.)
106
107	<P>
108	</LI>
109	<LI>The string methods <tt class="method">ljust()</tt>, <tt class="method">rjust()</tt>, and
110	<tt class="method">center()</tt> now take an optional argument for specifying a
111	fill character other than a space.
112	(Contributed by Raymond Hettinger.)
113
114	<P>
115	</LI>
116	<LI>Strings also gained an <tt class="method">rsplit()</tt> method that
117	works like the <tt class="method">split()</tt> method but splits from the end of
118	the string.
119
120	<P>
121	<div class="verbatim"><pre>
122	>>> 'www.python.org'.split('.', 1)
123	['www', 'python.org']
124	'www.python.org'.rsplit('.', 1)
125	['www.python', 'org']
126	</pre></div>
127
128	<P>
129	</LI>
130	<LI>Three keyword parameters, <var>cmp</var>, <var>key</var>, and
131	<var>reverse</var>, were added to the <tt class="method">sort()</tt> method of lists.
132	These parameters make some common usages of <tt class="method">sort()</tt> simpler.
133	All of these parameters are optional.
134
135	<P>
136	For the <var>cmp</var> parameter, the value should be a comparison function
137	that takes two parameters and returns -1, 0, or +1 depending on how
138	the parameters compare. This function will then be used to sort the
139	list. Previously this was the only parameter that could be provided
140	to <tt class="method">sort()</tt>.
141
142	<P>
143	<var>key</var> should be a single-parameter function that takes a list
144	element and returns a comparison key for the element. The list is
145	then sorted using the comparison keys. The following example sorts a
146	list case-insensitively:
147
148	<P>
149	<div class="verbatim"><pre>
150	>>> L = ['A', 'b', 'c', 'D']
151	>>> L.sort() # Case-sensitive sort
152	>>> L
153	['A', 'D', 'b', 'c']
154	>>> # Using 'key' parameter to sort list
155	>>> L.sort(key=lambda x: x.lower())
156	>>> L
157	['A', 'b', 'c', 'D']
158	>>> # Old-fashioned way
159	>>> L.sort(cmp=lambda x,y: cmp(x.lower(), y.lower()))
160	>>> L
161	['A', 'b', 'c', 'D']
162	</pre></div>
163
164	<P>
165	The last example, which uses the <var>cmp</var> parameter, is the old way
166	to perform a case-insensitive sort. It works but is slower than using
167	a <var>key</var> parameter. Using <var>key</var> calls <tt class="method">lower()</tt> method
168	once for each element in the list while using <var>cmp</var> will call it
169	twice for each comparison, so using <var>key</var> saves on invocations of
170	the <tt class="method">lower()</tt> method.
171
172	<P>
173	For simple key functions and comparison functions, it is often
174	possible to avoid a <tt class="keyword">lambda</tt> expression by using an unbound
175	method instead. For example, the above case-insensitive sort is best
176	written as:
177
178	<P>
179	<div class="verbatim"><pre>
180	>>> L.sort(key=str.lower)
181	>>> L
182	['A', 'b', 'c', 'D']
183	</pre></div>
184
185	<P>
186	Finally, the <var>reverse</var> parameter takes a Boolean value. If the
187	value is true, the list will be sorted into reverse order.
188	Instead of <code>L.sort() ; L.reverse()</code>, you can now write
189	<code>L.sort(reverse=True)</code>.
190
191	<P>
192	The results of sorting are now guaranteed to be stable. This means
193	that two entries with equal keys will be returned in the same order as
194	they were input. For example, you can sort a list of people by name,
195	and then sort the list by age, resulting in a list sorted by age where
196	people with the same age are in name-sorted order.
197
198	<P>
199	(All changes to <tt class="method">sort()</tt> contributed by Raymond Hettinger.)
200
201	<P>
202	</LI>
203	<LI>There is a new built-in function
204	<tt class="function">sorted(<var>iterable</var>)</tt> that works like the in-place
205	<tt class="method">list.sort()</tt> method but can be used in
206	expressions. The differences are:
207
208	<UL>
209	<LI>the input may be any iterable;
210	</LI>
211	<LI>a newly formed copy is sorted, leaving the original intact; and
212	</LI>
213	<LI>the expression returns the new sorted copy
214
215	</LI>
216	</UL>
217
218	<P>
219	<div class="verbatim"><pre>
220	>>> L = [9,7,8,3,2,4,1,6,5]
221	>>> [10+i for i in sorted(L)] # usable in a list comprehension
222	[11, 12, 13, 14, 15, 16, 17, 18, 19]
223	>>> L # original is left unchanged
224	[9,7,8,3,2,4,1,6,5]
225	>>> sorted('Monty Python') # any iterable may be an input
226	[' ', 'M', 'P', 'h', 'n', 'n', 'o', 'o', 't', 't', 'y', 'y']
227
228	>>> # List the contents of a dict sorted by key values
229	>>> colormap = dict(red=1, blue=2, green=3, black=4, yellow=5)
230	>>> for k, v in sorted(colormap.iteritems()):
231	... print k, v
232	...
233	black 4
234	blue 2
235	green 3
236	red 1
237	yellow 5
238	</pre></div>
239
240	<P>
241	(Contributed by Raymond Hettinger.)
242
243	<P>
244	</LI>
245	<LI>Integer operations will no longer trigger an <tt class="exception">OverflowWarning</tt>.
246	The <tt class="exception">OverflowWarning</tt> warning will disappear in Python 2.5.
247
248	<P>
249	</LI>
250	<LI>The interpreter gained a new switch, <b class="programopt">-m</b>, that
251	takes a name, searches for the corresponding module on <code>sys.path</code>,
252	and runs the module as a script. For example,
253	you can now run the Python profiler with <code>python -m profile</code>.
254	(Contributed by Nick Coghlan.)
255
256	<P>
257	</LI>
258	<LI>The <tt class="function">eval(<var>expr</var>, <var>globals</var>, <var>locals</var>)</tt>
259	and <tt class="function">execfile(<var>filename</var>, <var>globals</var>, <var>locals</var>)</tt>
260	functions and the <tt class="keyword">exec</tt> statement now accept any mapping type
261	for the <var>locals</var> parameter. Previously this had to be a regular
262	Python dictionary. (Contributed by Raymond Hettinger.)
263
264	<P>
265	</LI>
266	<LI>The <tt class="function">zip()</tt> built-in function and <tt class="function">itertools.izip()</tt>
267	now return an empty list if called with no arguments.
268	Previously they raised a <tt class="exception">TypeError</tt>
269	exception. This makes them more
270	suitable for use with variable length argument lists:
271
272	<P>
273	<div class="verbatim"><pre>
274	>>> def transpose(array):
275	... return zip(*array)
276	...
277	>>> transpose([(1,2,3), (4,5,6)])
278	[(1, 4), (2, 5), (3, 6)]
279	>>> transpose([])
280	[]
281	</pre></div>
282	(Contributed by Raymond Hettinger.)
283
284	<P>
285	</LI>
286	<LI>Encountering a failure while importing a module no longer leaves
287	a partially-initialized module object in <code>sys.modules</code>. The
288	incomplete module object left behind would fool further imports of the
289	same module into succeeding, leading to confusing errors.
290	(Fixed by Tim Peters.)
291
292	<P>
293	</LI>
294	<LI><tt class="constant">None</tt> is now a constant; code that binds a new value to
295	the name "<tt class="samp">None</tt>" is now a syntax error.
296	(Contributed by Raymond Hettinger.)
297
298	<P>
299	</LI>
300	</UL>
301
302	<P>
303
304	<H2><A NAME="SECTION0001210000000000000000">
305	11.1 Optimizations</A>
306	</H2>
307
308	<P>
309
310	<UL>
311	<LI>The inner loops for list and tuple slicing
312	were optimized and now run about one-third faster. The inner loops
313	for dictionaries were also optimized, resulting in performance boosts for
314	<tt class="method">keys()</tt>, <tt class="method">values()</tt>, <tt class="method">items()</tt>,
315	<tt class="method">iterkeys()</tt>, <tt class="method">itervalues()</tt>, and <tt class="method">iteritems()</tt>.
316	(Contributed by Raymond Hettinger.)
317
318	<P>
319	</LI>
320	<LI>The machinery for growing and shrinking lists was optimized for
321	speed and for space efficiency. Appending and popping from lists now
322	runs faster due to more efficient code paths and less frequent use of
323	the underlying system <tt class="cfunction">realloc()</tt>. List comprehensions
324	also benefit. <tt class="method">list.extend()</tt> was also optimized and no
325	longer converts its argument into a temporary list before extending
326	the base list. (Contributed by Raymond Hettinger.)
327
328	<P>
329	</LI>
330	<LI><tt class="function">list()</tt>, <tt class="function">tuple()</tt>, <tt class="function">map()</tt>,
331	<tt class="function">filter()</tt>, and <tt class="function">zip()</tt> now run several times
332	faster with non-sequence arguments that supply a <tt class="method">__len__()</tt>
333	method. (Contributed by Raymond Hettinger.)
334
335	<P>
336	</LI>
337	<LI>The methods <tt class="method">list.__getitem__()</tt>,
338	<tt class="method">dict.__getitem__()</tt>, and <tt class="method">dict.__contains__()</tt> are
339	are now implemented as <tt class="class">method_descriptor</tt> objects rather
340	than <tt class="class">wrapper_descriptor</tt> objects. This form of
341	access doubles their performance and makes them more suitable for
342	use as arguments to functionals:
343	"<tt class="samp">map(mydict.__getitem__, keylist)</tt>".
344	(Contributed by Raymond Hettinger.)
345
346	<P>
347	</LI>
348	<LI>Added a new opcode, <code>LIST_APPEND</code>, that simplifies
349	the generated bytecode for list comprehensions and speeds them up
350	by about a third. (Contributed by Raymond Hettinger.)
351
352	<P>
353	</LI>
354	<LI>The peephole bytecode optimizer has been improved to
355	produce shorter, faster bytecode; remarkably, the resulting bytecode is
356	more readable. (Enhanced by Raymond Hettinger.)
357
358	<P>
359	</LI>
360	<LI>String concatenations in statements of the form <code>s = s +
361	"abc"</code> and <code>s += "abc"</code> are now performed more efficiently in
362	certain circumstances. This optimization won't be present in other
363	Python implementations such as Jython, so you shouldn't rely on it;
364	using the <tt class="method">join()</tt> method of strings is still recommended when
365	you want to efficiently glue a large number of strings together.
366	(Contributed by Armin Rigo.)
367
368	<P>
369	</LI>
370	</UL>
371
372	<P>
373	The net result of the 2.4 optimizations is that Python 2.4 runs the
374	pystone benchmark around 5% faster than Python 2.3 and 35% faster
375	than Python 2.2. (pystone is not a particularly good benchmark, but
376	it's the most commonly used measurement of Python's performance. Your
377	own applications may show greater or smaller benefits from Python 2.4.)
378
379	<P>
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