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54 | <A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a> | |
55 | ||
56 | <UL CLASS="ChildLinks"> | |
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61 | <HR> | |
62 | ||
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 | ||
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