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| 55 | <H2><A NAME="SECTION006120000000000000000"> |
| 56 | 4.1.2 Template strings</A> |
| 57 | </H2> |
| 58 | |
| 59 | <P> |
| 60 | Templates provide simpler string substitutions as described in <a class="rfc" id='rfcref-86823' xml:id='rfcref-86823' |
| 61 | href="http://www.python.org/peps/pep-0292.html">PEP 292</a>. |
| 62 | Instead of the normal "<tt class="samp">%</tt>"-based substitutions, Templates support |
| 63 | "<tt class="samp">$</tt>"-based substitutions, using the following rules: |
| 64 | |
| 65 | <P> |
| 66 | |
| 67 | <UL> |
| 68 | <LI>"<tt class="samp">$$</tt>" is an escape; it is replaced with a single "<tt class="samp">$</tt>". |
| 69 | |
| 70 | <P> |
| 71 | </LI> |
| 72 | <LI>"<tt class="samp">$identifier</tt>" names a substitution placeholder matching a mapping |
| 73 | key of "identifier". By default, "identifier" must spell a Python |
| 74 | identifier. The first non-identifier character after the "<tt class="samp">$</tt>" character terminates this placeholder specification. |
| 75 | |
| 76 | <P> |
| 77 | </LI> |
| 78 | <LI>"<tt class="samp">${identifier}</tt>" is equivalent to "<tt class="samp">$identifier</tt>". It is |
| 79 | required when valid identifier characters follow the placeholder but are |
| 80 | not part of the placeholder, such as "${noun}ification". |
| 81 | </LI> |
| 82 | </UL> |
| 83 | |
| 84 | <P> |
| 85 | Any other appearance of "<tt class="samp">$</tt>" in the string will result in a |
| 86 | <tt class="exception">ValueError</tt> being raised. |
| 87 | |
| 88 | <P> |
| 89 | |
| 90 | <span class="versionnote">New in version 2.4.</span> |
| 91 | |
| 92 | <P> |
| 93 | The <tt class="module">string</tt> module provides a <tt class="class">Template</tt> class that implements |
| 94 | these rules. The methods of <tt class="class">Template</tt> are: |
| 95 | |
| 96 | <P> |
| 97 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 98 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-830' xml:id='l2h-830' class="class">Template</tt></b>(</nobr></td> |
| 99 | <td><var>template</var>)</td></tr></table></dt> |
| 100 | <dd> |
| 101 | The constructor takes a single argument which is the template string. |
| 102 | </dl> |
| 103 | |
| 104 | <P> |
| 105 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 106 | <td><nobr><b><tt id='l2h-831' xml:id='l2h-831' class="method">substitute</tt></b>(</nobr></td> |
| 107 | <td><var>mapping</var><big>[</big><var>, **kws</var><big>]</big><var></var>)</td></tr></table></dt> |
| 108 | <dd> |
| 109 | Performs the template substitution, returning a new string. <var>mapping</var> is |
| 110 | any dictionary-like object with keys that match the placeholders in the |
| 111 | template. Alternatively, you can provide keyword arguments, where the |
| 112 | keywords are the placeholders. When both <var>mapping</var> and <var>kws</var> are |
| 113 | given and there are duplicates, the placeholders from <var>kws</var> take |
| 114 | precedence. |
| 115 | </dl> |
| 116 | |
| 117 | <P> |
| 118 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 119 | <td><nobr><b><tt id='l2h-832' xml:id='l2h-832' class="method">safe_substitute</tt></b>(</nobr></td> |
| 120 | <td><var>mapping</var><big>[</big><var>, **kws</var><big>]</big><var></var>)</td></tr></table></dt> |
| 121 | <dd> |
| 122 | Like <tt class="method">substitute()</tt>, except that if placeholders are missing from |
| 123 | <var>mapping</var> and <var>kws</var>, instead of raising a <tt class="exception">KeyError</tt> |
| 124 | exception, the original placeholder will appear in the resulting string |
| 125 | intact. Also, unlike with <tt class="method">substitute()</tt>, any other appearances of the |
| 126 | "<tt class="samp">$</tt>" will simply return "<tt class="samp">$</tt>" instead of raising |
| 127 | <tt class="exception">ValueError</tt>. |
| 128 | |
| 129 | <P> |
| 130 | While other exceptions may still occur, this method is called ``safe'' because |
| 131 | substitutions always tries to return a usable string instead of raising an |
| 132 | exception. In another sense, <tt class="method">safe_substitute()</tt> may be anything other |
| 133 | than safe, since it will silently ignore malformed templates containing |
| 134 | dangling delimiters, unmatched braces, or placeholders that are not valid |
| 135 | Python identifiers. |
| 136 | </dl> |
| 137 | |
| 138 | <P> |
| 139 | <tt class="class">Template</tt> instances also provide one public data attribute: |
| 140 | |
| 141 | <P> |
| 142 | <dl><dt><b><tt id='l2h-833' xml:id='l2h-833' class="member">template</tt></b></dt> |
| 143 | <dd> |
| 144 | This is the object passed to the constructor's <var>template</var> argument. In |
| 145 | general, you shouldn't change it, but read-only access is not enforced. |
| 146 | </dl> |
| 147 | |
| 148 | <P> |
| 149 | Here is an example of how to use a Template: |
| 150 | |
| 151 | <P> |
| 152 | <div class="verbatim"><pre> |
| 153 | >>> from string import Template |
| 154 | >>> s = Template('$who likes $what') |
| 155 | >>> s.substitute(who='tim', what='kung pao') |
| 156 | 'tim likes kung pao' |
| 157 | >>> d = dict(who='tim') |
| 158 | >>> Template('Give $who $100').substitute(d) |
| 159 | Traceback (most recent call last): |
| 160 | [...] |
| 161 | ValueError: Invalid placeholder in string: line 1, col 10 |
| 162 | >>> Template('$who likes $what').substitute(d) |
| 163 | Traceback (most recent call last): |
| 164 | [...] |
| 165 | KeyError: 'what' |
| 166 | >>> Template('$who likes $what').safe_substitute(d) |
| 167 | 'tim likes $what' |
| 168 | </pre></div> |
| 169 | |
| 170 | <P> |
| 171 | Advanced usage: you can derive subclasses of <tt class="class">Template</tt> to customize the |
| 172 | placeholder syntax, delimiter character, or the entire regular expression used |
| 173 | to parse template strings. To do this, you can override these class |
| 174 | attributes: |
| 175 | |
| 176 | <P> |
| 177 | |
| 178 | <UL> |
| 179 | <LI><var>delimiter</var> - This is the literal string describing a placeholder |
| 180 | introducing delimiter. The default value "<tt class="samp">$</tt>". Note that this |
| 181 | should <em>not</em> be a regular expression, as the implementation will |
| 182 | call <tt class="method">re.escape()</tt> on this string as needed. |
| 183 | </LI> |
| 184 | <LI><var>idpattern</var> - This is the regular expression describing the pattern |
| 185 | for non-braced placeholders (the braces will be added automatically as |
| 186 | appropriate). The default value is the regular expression |
| 187 | "<tt class="samp">[_a-z][_a-z0-9]*</tt>". |
| 188 | </LI> |
| 189 | </UL> |
| 190 | |
| 191 | <P> |
| 192 | Alternatively, you can provide the entire regular expression pattern by |
| 193 | overriding the class attribute <var>pattern</var>. If you do this, the value must |
| 194 | be a regular expression object with four named capturing groups. The |
| 195 | capturing groups correspond to the rules given above, along with the invalid |
| 196 | placeholder rule: |
| 197 | |
| 198 | <P> |
| 199 | |
| 200 | <UL> |
| 201 | <LI><var>escaped</var> - This group matches the escape sequence, |
| 202 | e.g. "<tt class="samp">$$</tt>", in the default pattern. |
| 203 | </LI> |
| 204 | <LI><var>named</var> - This group matches the unbraced placeholder name; it |
| 205 | should not include the delimiter in capturing group. |
| 206 | </LI> |
| 207 | <LI><var>braced</var> - This group matches the brace enclosed placeholder name; |
| 208 | it should not include either the delimiter or braces in the capturing |
| 209 | group. |
| 210 | </LI> |
| 211 | <LI><var>invalid</var> - This group matches any other delimiter pattern (usually |
| 212 | a single delimiter), and it should appear last in the regular |
| 213 | expression. |
| 214 | </LI> |
| 215 | </UL> |
| 216 | |
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