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| 55 | <H3><A NAME="SECTION004362000000000000000"></A><A NAME="typesseq-strings"></A> |
| 56 | <BR> |
| 57 | 2.3.6.2 String Formatting Operations |
| 58 | </H3> |
| 59 | |
| 60 | <P> |
| 61 | <a id='l2h-211' xml:id='l2h-211'></a> |
| 62 | |
| 63 | <P> |
| 64 | String and Unicode objects have one unique built-in operation: the |
| 65 | <code>%</code> operator (modulo). This is also known as the string |
| 66 | <em>formatting</em> or <em>interpolation</em> operator. Given |
| 67 | <code><var>format</var> % <var>values</var></code> (where <var>format</var> is a string or |
| 68 | Unicode object), <code>%</code> conversion specifications in <var>format</var> |
| 69 | are replaced with zero or more elements of <var>values</var>. The effect |
| 70 | is similar to the using <tt class="cfunction">sprintf()</tt> in the C language. If |
| 71 | <var>format</var> is a Unicode object, or if any of the objects being |
| 72 | converted using the <code>%s</code> conversion are Unicode objects, the |
| 73 | result will also be a Unicode object. |
| 74 | |
| 75 | <P> |
| 76 | If <var>format</var> requires a single argument, <var>values</var> may be a |
| 77 | single non-tuple object.<A NAME="tex2html9" |
| 78 | HREF="#foot2430"><SUP>2.8</SUP></A> Otherwise, <var>values</var> must be a tuple with |
| 79 | exactly the number of items specified by the format string, or a |
| 80 | single mapping object (for example, a dictionary). |
| 81 | |
| 82 | <P> |
| 83 | A conversion specifier contains two or more characters and has the |
| 84 | following components, which must occur in this order: |
| 85 | |
| 86 | <P> |
| 87 | |
| 88 | <OL> |
| 89 | <LI>The "<tt class="character">%</tt>" character, which marks the start of the |
| 90 | specifier. |
| 91 | </LI> |
| 92 | <LI>Mapping key (optional), consisting of a parenthesised sequence |
| 93 | of characters (for example, <code>(somename)</code>). |
| 94 | </LI> |
| 95 | <LI>Conversion flags (optional), which affect the result of some |
| 96 | conversion types. |
| 97 | </LI> |
| 98 | <LI>Minimum field width (optional). If specified as an |
| 99 | "<tt class="character">*</tt>" (asterisk), the actual width is read from the |
| 100 | next element of the tuple in <var>values</var>, and the object to |
| 101 | convert comes after the minimum field width and optional |
| 102 | precision. |
| 103 | </LI> |
| 104 | <LI>Precision (optional), given as a "<tt class="character">.</tt>" (dot) followed |
| 105 | by the precision. If specified as "<tt class="character">*</tt>" (an |
| 106 | asterisk), the actual width is read from the next element of |
| 107 | the tuple in <var>values</var>, and the value to convert comes after |
| 108 | the precision. |
| 109 | </LI> |
| 110 | <LI>Length modifier (optional). |
| 111 | </LI> |
| 112 | <LI>Conversion type. |
| 113 | </LI> |
| 114 | </OL> |
| 115 | |
| 116 | <P> |
| 117 | When the right argument is a dictionary (or other mapping type), then |
| 118 | the formats in the string <em>must</em> include a parenthesised mapping key into |
| 119 | that dictionary inserted immediately after the "<tt class="character">%</tt>" |
| 120 | character. The mapping key selects the value to be formatted from the |
| 121 | mapping. For example: |
| 122 | |
| 123 | <P> |
| 124 | <div class="verbatim"><pre> |
| 125 | >>> print '%(language)s has %(#)03d quote types.' % \ |
| 126 | {'language': "Python", "#": 2} |
| 127 | Python has 002 quote types. |
| 128 | </pre></div> |
| 129 | |
| 130 | <P> |
| 131 | In this case no <code>*</code> specifiers may occur in a format (since they |
| 132 | require a sequential parameter list). |
| 133 | |
| 134 | <P> |
| 135 | The conversion flag characters are: |
| 136 | |
| 137 | <P> |
| 138 | <div class="center"><table class="realtable"> |
| 139 | <thead> |
| 140 | <tr> |
| 141 | <th class="center">Flag</th> |
| 142 | <th class="left" >Meaning</th> |
| 143 | </tr> |
| 144 | </thead> |
| 145 | <tbody> |
| 146 | <tr><td class="center" valign="baseline"><tt class="character">#</tt></td> |
| 147 | <td class="left" >The value conversion will use the ``alternate form'' |
| 148 | (where defined below).</td></tr> |
| 149 | <tr><td class="center" valign="baseline"><tt class="character">0</tt></td> |
| 150 | <td class="left" >The conversion will be zero padded for numeric values.</td></tr> |
| 151 | <tr><td class="center" valign="baseline"><tt class="character">-</tt></td> |
| 152 | <td class="left" >The converted value is left adjusted (overrides |
| 153 | the "<tt class="character">0</tt>" conversion if both are given).</td></tr> |
| 154 | <tr><td class="center" valign="baseline"><tt class="character"> </tt></td> |
| 155 | <td class="left" >(a space) A blank should be left before a positive number |
| 156 | (or empty string) produced by a signed conversion.</td></tr> |
| 157 | <tr><td class="center" valign="baseline"><tt class="character">+</tt></td> |
| 158 | <td class="left" >A sign character ("<tt class="character">+</tt>" or "<tt class="character">-</tt>") will |
| 159 | precede the conversion (overrides a "space" flag).</td></tr></tbody> |
| 160 | </table></div> |
| 161 | |
| 162 | <P> |
| 163 | The length modifier may be <code>h</code>, <code>l</code>, and <code>L</code> may be |
| 164 | present, but are ignored as they are not necessary for Python. |
| 165 | |
| 166 | <P> |
| 167 | The conversion types are: |
| 168 | |
| 169 | <P> |
| 170 | <div class="center"><table class="realtable"> |
| 171 | <thead> |
| 172 | <tr> |
| 173 | <th class="center">Conversion</th> |
| 174 | <th class="left" >Meaning</th> |
| 175 | <th class="center">Notes</th> |
| 176 | </tr> |
| 177 | </thead> |
| 178 | <tbody> |
| 179 | <tr><td class="center" valign="baseline"><tt class="character">d</tt></td> |
| 180 | <td class="left" >Signed integer decimal.</td> |
| 181 | <td class="center"></td></tr> |
| 182 | <tr><td class="center" valign="baseline"><tt class="character">i</tt></td> |
| 183 | <td class="left" >Signed integer decimal.</td> |
| 184 | <td class="center"></td></tr> |
| 185 | <tr><td class="center" valign="baseline"><tt class="character">o</tt></td> |
| 186 | <td class="left" >Unsigned octal.</td> |
| 187 | <td class="center">(1)</td></tr> |
| 188 | <tr><td class="center" valign="baseline"><tt class="character">u</tt></td> |
| 189 | <td class="left" >Unsigned decimal.</td> |
| 190 | <td class="center"></td></tr> |
| 191 | <tr><td class="center" valign="baseline"><tt class="character">x</tt></td> |
| 192 | <td class="left" >Unsigned hexadecimal (lowercase).</td> |
| 193 | <td class="center">(2)</td></tr> |
| 194 | <tr><td class="center" valign="baseline"><tt class="character">X</tt></td> |
| 195 | <td class="left" >Unsigned hexadecimal (uppercase).</td> |
| 196 | <td class="center">(2)</td></tr> |
| 197 | <tr><td class="center" valign="baseline"><tt class="character">e</tt></td> |
| 198 | <td class="left" >Floating point exponential format (lowercase).</td> |
| 199 | <td class="center"></td></tr> |
| 200 | <tr><td class="center" valign="baseline"><tt class="character">E</tt></td> |
| 201 | <td class="left" >Floating point exponential format (uppercase).</td> |
| 202 | <td class="center"></td></tr> |
| 203 | <tr><td class="center" valign="baseline"><tt class="character">f</tt></td> |
| 204 | <td class="left" >Floating point decimal format.</td> |
| 205 | <td class="center"></td></tr> |
| 206 | <tr><td class="center" valign="baseline"><tt class="character">F</tt></td> |
| 207 | <td class="left" >Floating point decimal format.</td> |
| 208 | <td class="center"></td></tr> |
| 209 | <tr><td class="center" valign="baseline"><tt class="character">g</tt></td> |
| 210 | <td class="left" >Same as "<tt class="character">e</tt>" if exponent is greater than -4 or |
| 211 | less than precision, "<tt class="character">f</tt>" otherwise.</td> |
| 212 | <td class="center"></td></tr> |
| 213 | <tr><td class="center" valign="baseline"><tt class="character">G</tt></td> |
| 214 | <td class="left" >Same as "<tt class="character">E</tt>" if exponent is greater than -4 or |
| 215 | less than precision, "<tt class="character">F</tt>" otherwise.</td> |
| 216 | <td class="center"></td></tr> |
| 217 | <tr><td class="center" valign="baseline"><tt class="character">c</tt></td> |
| 218 | <td class="left" >Single character (accepts integer or single character |
| 219 | string).</td> |
| 220 | <td class="center"></td></tr> |
| 221 | <tr><td class="center" valign="baseline"><tt class="character">r</tt></td> |
| 222 | <td class="left" >String (converts any python object using |
| 223 | <tt class="function">repr()</tt>).</td> |
| 224 | <td class="center">(3)</td></tr> |
| 225 | <tr><td class="center" valign="baseline"><tt class="character">s</tt></td> |
| 226 | <td class="left" >String (converts any python object using |
| 227 | <tt class="function">str()</tt>).</td> |
| 228 | <td class="center">(4)</td></tr> |
| 229 | <tr><td class="center" valign="baseline"><tt class="character">%</tt></td> |
| 230 | <td class="left" >No argument is converted, results in a "<tt class="character">%</tt>" |
| 231 | character in the result.</td> |
| 232 | <td class="center"></td></tr></tbody> |
| 233 | </table></div> |
| 234 | |
| 235 | <P> |
| 236 | Notes: |
| 237 | <DL> |
| 238 | <DT><STRONG>(1)</STRONG></DT> |
| 239 | <DD>The alternate form causes a leading zero ("<tt class="character">0</tt>") to be |
| 240 | inserted between left-hand padding and the formatting of the |
| 241 | number if the leading character of the result is not already a |
| 242 | zero. |
| 243 | |
| 244 | </DD> |
| 245 | <DT><STRONG>(2)</STRONG></DT> |
| 246 | <DD>The alternate form causes a leading <code>'0x'</code> or <code>'0X'</code> |
| 247 | (depending on whether the "<tt class="character">x</tt>" or "<tt class="character">X</tt>" format |
| 248 | was used) to be inserted between left-hand padding and the |
| 249 | formatting of the number if the leading character of the result is |
| 250 | not already a zero. |
| 251 | |
| 252 | </DD> |
| 253 | <DT><STRONG>(3)</STRONG></DT> |
| 254 | <DD>The <code>%r</code> conversion was added in Python 2.0. |
| 255 | |
| 256 | </DD> |
| 257 | <DT><STRONG>(4)</STRONG></DT> |
| 258 | <DD>If the object or format provided is a <tt class="class">unicode</tt> string, |
| 259 | the resulting string will also be <tt class="class">unicode</tt>. |
| 260 | </DD> |
| 261 | </DL> |
| 262 | |
| 263 | <P> |
| 264 | Since Python strings have an explicit length, <code>%s</code> conversions |
| 265 | do not assume that <code>'\0'</code> is the end of the string. |
| 266 | |
| 267 | <P> |
| 268 | For safety reasons, floating point precisions are clipped to 50; |
| 269 | <code>%f</code> conversions for numbers whose absolute value is over 1e25 |
| 270 | are replaced by <code>%g</code> conversions.<A NAME="tex2html10" |
| 271 | HREF="#foot2537"><SUP>2.9</SUP></A> All other errors raise exceptions. |
| 272 | |
| 273 | <P> |
| 274 | Additional string operations are defined in standard modules |
| 275 | <tt class="module"><a href="module-string.html">string</a></tt><a id='l2h-212' xml:id='l2h-212'></a> and |
| 276 | <tt class="module"><a href="module-re.html">re</a></tt>.<a id='l2h-213' xml:id='l2h-213'></a> |
| 277 | |
| 278 | <P> |
| 279 | <BR><HR><H4>Footnotes</H4> |
| 280 | <DL> |
| 281 | <DT><A NAME="foot2430">... object.</A><A |
| 282 | href="typesseq-strings.html#tex2html9"><SUP>2.8</SUP></A></DT> |
| 283 | <DD>To format only a tuple you |
| 284 | should therefore provide a singleton tuple whose only element |
| 285 | is the tuple to be formatted. |
| 286 | |
| 287 | </DD> |
| 288 | <DT><A NAME="foot2537">... conversions.</A><A |
| 289 | href="typesseq-strings.html#tex2html10"><SUP>2.9</SUP></A></DT> |
| 290 | <DD> |
| 291 | These numbers are fairly arbitrary. They are intended to |
| 292 | avoid printing endless strings of meaningless digits without hampering |
| 293 | correct use and without having to know the exact precision of floating |
| 294 | point values on a particular machine. |
| 295 | |
| 296 | |
| 297 | </DD> |
| 298 | </DL> |
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