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| 54 | |
| 55 | <H2><A NAME="SECTION004360000000000000000"></A><A NAME="typesseq"></A> |
| 56 | <BR> |
| 57 | 2.3.6 Sequence Types -- |
| 58 | <tt class="class">str</tt>, <tt class="class">unicode</tt>, <tt class="class">list</tt>, |
| 59 | <tt class="class">tuple</tt>, <tt class="class">buffer</tt>, <tt class="class">xrange</tt> |
| 60 | |
| 61 | </H2> |
| 62 | |
| 63 | <P> |
| 64 | There are six sequence types: strings, Unicode strings, lists, |
| 65 | tuples, buffers, and xrange objects. |
| 66 | |
| 67 | <P> |
| 68 | String literals are written in single or double quotes: |
| 69 | <code>'xyzzy'</code>, <code>"frobozz"</code>. See chapter 2 of the |
| 70 | <em class="citetitle"><a |
| 71 | href="../ref/strings.html" |
| 72 | title="Python Reference Manual" |
| 73 | >Python Reference Manual</a></em> for more about |
| 74 | string literals. Unicode strings are much like strings, but are |
| 75 | specified in the syntax using a preceding "<tt class="character">u</tt>" character: |
| 76 | <code>u'abc'</code>, <code>u"def"</code>. Lists are constructed with square brackets, |
| 77 | separating items with commas: <code>[a, b, c]</code>. Tuples are |
| 78 | constructed by the comma operator (not within square brackets), with |
| 79 | or without enclosing parentheses, but an empty tuple must have the |
| 80 | enclosing parentheses, such as <code>a, b, c</code> or <code>()</code>. A single |
| 81 | item tuple must have a trailing comma, such as <code>(d,)</code>. |
| 82 | <a id='l2h-156' xml:id='l2h-156'></a><a id='l2h-157' xml:id='l2h-157'></a><a id='l2h-158' xml:id='l2h-158'></a><a id='l2h-159' xml:id='l2h-159'></a><a id='l2h-160' xml:id='l2h-160'></a> |
| 83 | <P> |
| 84 | Buffer objects are not directly supported by Python syntax, but can be |
| 85 | created by calling the builtin function |
| 86 | <tt class="function">buffer()</tt>.<a id='l2h-161' xml:id='l2h-161'></a> They don't support |
| 87 | concatenation or repetition. |
| 88 | <a id='l2h-162' xml:id='l2h-162'></a> |
| 89 | <P> |
| 90 | Xrange objects are similar to buffers in that there is no specific |
| 91 | syntax to create them, but they are created using the <tt class="function">xrange()</tt> |
| 92 | function.<a id='l2h-163' xml:id='l2h-163'></a> They don't support slicing, |
| 93 | concatenation or repetition, and using <code>in</code>, <code>not in</code>, |
| 94 | <tt class="function">min()</tt> or <tt class="function">max()</tt> on them is inefficient. |
| 95 | <a id='l2h-164' xml:id='l2h-164'></a> |
| 96 | <P> |
| 97 | Most sequence types support the following operations. The "<tt class="samp">in</tt>" and |
| 98 | "<tt class="samp">not in</tt>" operations have the same priorities as the comparison |
| 99 | operations. The "<tt class="samp">+</tt>" and "<tt class="samp">*</tt>" operations have the same |
| 100 | priority as the corresponding numeric operations.<A NAME="tex2html8" |
| 101 | HREF="#foot1961"><SUP>2.7</SUP></A> |
| 102 | <P> |
| 103 | This table lists the sequence operations sorted in ascending priority |
| 104 | (operations in the same box have the same priority). In the table, |
| 105 | <var>s</var> and <var>t</var> are sequences of the same type; <var>n</var>, <var>i</var> |
| 106 | and <var>j</var> are integers: |
| 107 | |
| 108 | <P> |
| 109 | <div class="center"><table class="realtable"> |
| 110 | <thead> |
| 111 | <tr> |
| 112 | <th class="center">Operation</th> |
| 113 | <th class="left" >Result</th> |
| 114 | <th class="center">Notes</th> |
| 115 | </tr> |
| 116 | </thead> |
| 117 | <tbody> |
| 118 | <tr><td class="center" valign="baseline"><code><var>x</var> in <var>s</var></code></td> |
| 119 | <td class="left" ><code>True</code> if an item of <var>s</var> is equal to <var>x</var>, else <code>False</code></td> |
| 120 | <td class="center">(1)</td></tr> |
| 121 | <tr><td class="center" valign="baseline"><code><var>x</var> not in <var>s</var></code></td> |
| 122 | <td class="left" ><code>False</code> if an item of <var>s</var> is |
| 123 | equal to <var>x</var>, else <code>True</code></td> |
| 124 | <td class="center">(1)</td></tr> |
| 125 | |
| 126 | <tr><td class="center" valign="baseline"><code><var>s</var> + <var>t</var></code></td> |
| 127 | <td class="left" >the concatenation of <var>s</var> and <var>t</var></td> |
| 128 | <td class="center">(6)</td></tr> |
| 129 | <tr><td class="center" valign="baseline"><code><var>s</var> * <var>n</var> , <var>n</var> * <var>s</var></code></td> |
| 130 | <td class="left" ><var>n</var> shallow copies of <var>s</var> concatenated</td> |
| 131 | <td class="center">(2)</td></tr> |
| 132 | |
| 133 | <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>]</code></td> |
| 134 | <td class="left" ><var>i</var>'th item of <var>s</var>, origin 0</td> |
| 135 | <td class="center">(3)</td></tr> |
| 136 | <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>:<var>j</var>]</code></td> |
| 137 | <td class="left" >slice of <var>s</var> from <var>i</var> to <var>j</var></td> |
| 138 | <td class="center">(3), (4)</td></tr> |
| 139 | <tr><td class="center" valign="baseline"><code><var>s</var>[<var>i</var>:<var>j</var>:<var>k</var>]</code></td> |
| 140 | <td class="left" >slice of <var>s</var> from <var>i</var> to <var>j</var> with step <var>k</var></td> |
| 141 | <td class="center">(3), (5)</td></tr> |
| 142 | |
| 143 | <tr><td class="center" valign="baseline"><code>len(<var>s</var>)</code></td> |
| 144 | <td class="left" >length of <var>s</var></td> |
| 145 | <td class="center"></td></tr> |
| 146 | <tr><td class="center" valign="baseline"><code>min(<var>s</var>)</code></td> |
| 147 | <td class="left" >smallest item of <var>s</var></td> |
| 148 | <td class="center"></td></tr> |
| 149 | <tr><td class="center" valign="baseline"><code>max(<var>s</var>)</code></td> |
| 150 | <td class="left" >largest item of <var>s</var></td> |
| 151 | <td class="center"></td></tr></tbody> |
| 152 | </table></div> |
| 153 | <a id='l2h-165' xml:id='l2h-165'></a><a id='l2h-166' xml:id='l2h-166'></a><a id='l2h-167' xml:id='l2h-167'></a><a id='l2h-168' xml:id='l2h-168'></a><a id='l2h-169' xml:id='l2h-169'></a><a id='l2h-170' xml:id='l2h-170'></a><a id='l2h-171' xml:id='l2h-171'></a><a id='l2h-172' xml:id='l2h-172'></a><a id='l2h-173' xml:id='l2h-173'></a><a id='l2h-174' xml:id='l2h-174'></a><a id='l2h-175' xml:id='l2h-175'></a> |
| 154 | <P> |
| 155 | Notes: |
| 156 | |
| 157 | <P> |
| 158 | <DL> |
| 159 | <DT><STRONG>(1)</STRONG></DT> |
| 160 | <DD>When <var>s</var> is a string or Unicode string object the |
| 161 | <code>in</code> and <code>not in</code> operations act like a substring test. In |
| 162 | Python versions before 2.3, <var>x</var> had to be a string of length 1. |
| 163 | In Python 2.3 and beyond, <var>x</var> may be a string of any length. |
| 164 | |
| 165 | <P> |
| 166 | </DD> |
| 167 | <DT><STRONG>(2)</STRONG></DT> |
| 168 | <DD>Values of <var>n</var> less than <code>0</code> are treated as |
| 169 | <code>0</code> (which yields an empty sequence of the same type as |
| 170 | <var>s</var>). Note also that the copies are shallow; nested structures |
| 171 | are not copied. This often haunts new Python programmers; consider: |
| 172 | |
| 173 | <P> |
| 174 | <div class="verbatim"><pre> |
| 175 | >>> lists = [[]] * 3 |
| 176 | >>> lists |
| 177 | [[], [], []] |
| 178 | >>> lists[0].append(3) |
| 179 | >>> lists |
| 180 | [[3], [3], [3]] |
| 181 | </pre></div> |
| 182 | |
| 183 | <P> |
| 184 | What has happened is that <code>[[]]</code> is a one-element list containing |
| 185 | an empty list, so all three elements of <code>[[]] * 3</code> are (pointers to) |
| 186 | this single empty list. Modifying any of the elements of <code>lists</code> |
| 187 | modifies this single list. You can create a list of different lists this |
| 188 | way: |
| 189 | |
| 190 | <P> |
| 191 | <div class="verbatim"><pre> |
| 192 | >>> lists = [[] for i in range(3)] |
| 193 | >>> lists[0].append(3) |
| 194 | >>> lists[1].append(5) |
| 195 | >>> lists[2].append(7) |
| 196 | >>> lists |
| 197 | [[3], [5], [7]] |
| 198 | </pre></div> |
| 199 | |
| 200 | <P> |
| 201 | </DD> |
| 202 | <DT><STRONG>(3)</STRONG></DT> |
| 203 | <DD>If <var>i</var> or <var>j</var> is negative, the index is relative to |
| 204 | the end of the string: <code>len(<var>s</var>) + <var>i</var></code> or |
| 205 | <code>len(<var>s</var>) + <var>j</var></code> is substituted. But note that <code>-0</code> is |
| 206 | still <code>0</code>. |
| 207 | |
| 208 | <P> |
| 209 | </DD> |
| 210 | <DT><STRONG>(4)</STRONG></DT> |
| 211 | <DD>The slice of <var>s</var> from <var>i</var> to <var>j</var> is defined as |
| 212 | the sequence of items with index <var>k</var> such that <code><var>i</var> <= |
| 213 | <var>k</var> < <var>j</var></code>. If <var>i</var> or <var>j</var> is greater than |
| 214 | <code>len(<var>s</var>)</code>, use <code>len(<var>s</var>)</code>. If <var>i</var> is omitted, |
| 215 | use <code>0</code>. If <var>j</var> is omitted, use <code>len(<var>s</var>)</code>. If |
| 216 | <var>i</var> is greater than or equal to <var>j</var>, the slice is empty. |
| 217 | |
| 218 | <P> |
| 219 | </DD> |
| 220 | <DT><STRONG>(5)</STRONG></DT> |
| 221 | <DD>The slice of <var>s</var> from <var>i</var> to <var>j</var> with step |
| 222 | <var>k</var> is defined as the sequence of items with index |
| 223 | <code><var>x</var> = <var>i</var> + <var>n</var>*<var>k</var></code> such that |
| 224 | <!-- MATH |
| 225 | $0 \leq n < \frac{j-i}{k}$ |
| 226 | --> |
| 227 | <SPAN CLASS="MATH"><IMG |
| 228 | WIDTH="88" HEIGHT="37" ALIGN="MIDDLE" BORDER="0" |
| 229 | SRC="img1.png" |
| 230 | ALT="$0 \leq n < \frac{j-i}{k}$"></SPAN>. In other words, the indices |
| 231 | are <code>i</code>, <code>i+k</code>, <code>i+2*k</code>, <code>i+3*k</code> and so on, stopping when |
| 232 | <var>j</var> is reached (but never including <var>j</var>). If <var>i</var> or <var>j</var> |
| 233 | is greater than <code>len(<var>s</var>)</code>, use <code>len(<var>s</var>)</code>. If |
| 234 | <var>i</var> or <var>j</var> are omitted then they become ``end'' values |
| 235 | (which end depends on the sign of <var>k</var>). Note, <var>k</var> cannot |
| 236 | be zero. |
| 237 | |
| 238 | <P> |
| 239 | </DD> |
| 240 | <DT><STRONG>(6)</STRONG></DT> |
| 241 | <DD>If <var>s</var> and <var>t</var> are both strings, some Python |
| 242 | implementations such as CPython can usually perform an in-place optimization |
| 243 | for assignments of the form <code><var>s</var>=<var>s</var>+<var>t</var></code> or |
| 244 | <code><var>s</var>+=<var>t</var></code>. When applicable, this optimization makes |
| 245 | quadratic run-time much less likely. This optimization is both version |
| 246 | and implementation dependent. For performance sensitive code, it is |
| 247 | preferable to use the <tt class="method">str.join()</tt> method which assures consistent |
| 248 | linear concatenation performance across versions and implementations. |
| 249 | |
| 250 | <span class="versionnote">Changed in version 2.4: |
| 251 | Formerly, string concatenation never occurred in-place.</span> |
| 252 | |
| 253 | <P> |
| 254 | </DD> |
| 255 | </DL> |
| 256 | |
| 257 | <P> |
| 258 | <BR><HR><H4>Footnotes</H4> |
| 259 | <DL> |
| 260 | <DT><A NAME="foot1961">... operations.</A><A |
| 261 | href="typesseq.html#tex2html8"><SUP>2.7</SUP></A></DT> |
| 262 | <DD>They must |
| 263 | have since the parser can't tell the type of the operands. |
| 264 | |
| 265 | </DD> |
| 266 | </DL> |
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| 268 | <div class='online-navigation'> |
| 269 | <!--Table of Child-Links--> |
| 270 | <A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a> |
| 271 | |
| 272 | <UL CLASS="ChildLinks"> |
| 273 | <LI><A href="string-methods.html">2.3.6.1 String Methods</a> |
| 274 | <LI><A href="typesseq-strings.html">2.3.6.2 String Formatting Operations</a> |
| 275 | <LI><A href="typesseq-xrange.html">2.3.6.3 XRange Type</a> |
| 276 | <LI><A href="typesseq-mutable.html">2.3.6.4 Mutable Sequence Types</a> |
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