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| 17 | <title>7. Compound statements</title> | |
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| 54 | ||
| 55 | <H1><A NAME="SECTION009000000000000000000"></A><A NAME="compound"></A> | |
| 56 | <BR> | |
| 57 | 7. Compound statements | |
| 58 | </H1> | |
| 59 | <a id='l2h-568' xml:id='l2h-568'></a> | |
| 60 | <P> | |
| 61 | Compound statements contain (groups of) other statements; they affect | |
| 62 | or control the execution of those other statements in some way. In | |
| 63 | general, compound statements span multiple lines, although in simple | |
| 64 | incarnations a whole compound statement may be contained in one line. | |
| 65 | ||
| 66 | <P> | |
| 67 | The <tt class="keyword">if</tt>, <tt class="keyword">while</tt> and <tt class="keyword">for</tt> statements implement | |
| 68 | traditional control flow constructs. <tt class="keyword">try</tt> specifies exception | |
| 69 | handlers and/or cleanup code for a group of statements. Function and | |
| 70 | class definitions are also syntactically compound statements. | |
| 71 | ||
| 72 | <P> | |
| 73 | Compound statements consist of one or more `clauses.' A clause | |
| 74 | consists of a header and a `suite.' The clause headers of a | |
| 75 | particular compound statement are all at the same indentation level. | |
| 76 | Each clause header begins with a uniquely identifying keyword and ends | |
| 77 | with a colon. A suite is a group of statements controlled by a | |
| 78 | clause. A suite can be one or more semicolon-separated simple | |
| 79 | statements on the same line as the header, following the header's | |
| 80 | colon, or it can be one or more indented statements on subsequent | |
| 81 | lines. Only the latter form of suite can contain nested compound | |
| 82 | statements; the following is illegal, mostly because it wouldn't be | |
| 83 | clear to which <tt class="keyword">if</tt> clause a following <tt class="keyword">else</tt> clause would | |
| 84 | belong: | |
| 85 | <a id='l2h-570' xml:id='l2h-570'></a> | |
| 86 | ||
| 87 | <P> | |
| 88 | <div class="verbatim"><pre> | |
| 89 | if test1: if test2: print x | |
| 90 | </pre></div> | |
| 91 | ||
| 92 | <P> | |
| 93 | Also note that the semicolon binds tighter than the colon in this | |
| 94 | context, so that in the following example, either all or none of the | |
| 95 | <tt class="keyword">print</tt> statements are executed: | |
| 96 | ||
| 97 | <P> | |
| 98 | <div class="verbatim"><pre> | |
| 99 | if x < y < z: print x; print y; print z | |
| 100 | </pre></div> | |
| 101 | ||
| 102 | <P> | |
| 103 | Summarizing: | |
| 104 | ||
| 105 | <P> | |
| 106 | <dl><dd class="grammar"> | |
| 107 | <div class="productions"> | |
| 108 | <table> | |
| 109 | <tr> | |
| 110 | <td><a id='tok-compound_stmt' xml:id='tok-compound_stmt'>compound_stmt</a></td> | |
| 111 | <td>::=</td> | |
| 112 | <td><a class='grammartoken' href="if.html#tok-if_stmt">if_stmt</a></td></tr> | |
| 113 | <tr> | |
| 114 | <td></td> | |
| 115 | <td></td> | |
| 116 | <td><code>| <a class='grammartoken' href="while.html#tok-while_stmt">while_stmt</a></code></td></tr> | |
| 117 | <tr> | |
| 118 | <td></td> | |
| 119 | <td></td> | |
| 120 | <td><code>| <a class='grammartoken' href="for.html#tok-for_stmt">for_stmt</a></code></td></tr> | |
| 121 | <tr> | |
| 122 | <td></td> | |
| 123 | <td></td> | |
| 124 | <td><code>| <a class='grammartoken' href="try.html#tok-try_stmt">try_stmt</a></code></td></tr> | |
| 125 | <tr> | |
| 126 | <td></td> | |
| 127 | <td></td> | |
| 128 | <td><code>| <a class='grammartoken' href="function.html#tok-funcdef">funcdef</a></code></td></tr> | |
| 129 | <tr> | |
| 130 | <td></td> | |
| 131 | <td></td> | |
| 132 | <td><code>| <a class='grammartoken' href="class.html#tok-classdef">classdef</a></code></td></tr> | |
| 133 | <tr> | |
| 134 | <td><a id='tok-suite' xml:id='tok-suite'>suite</a></td> | |
| 135 | <td>::=</td> | |
| 136 | <td><a class='grammartoken' href="compound.html#tok-stmt_list">stmt_list</a> NEWLINE | |
| 137 | | NEWLINE INDENT <a class='grammartoken' href="compound.html#tok-statement">statement</a>+ DEDENT</td></tr> | |
| 138 | <tr> | |
| 139 | <td><a id='tok-statement' xml:id='tok-statement'>statement</a></td> | |
| 140 | <td>::=</td> | |
| 141 | <td><a class='grammartoken' href="compound.html#tok-stmt_list">stmt_list</a> NEWLINE | <a class='grammartoken' href="compound.html#tok-compound_stmt">compound_stmt</a></td></tr> | |
| 142 | <tr> | |
| 143 | <td><a id='tok-stmt_list' xml:id='tok-stmt_list'>stmt_list</a></td> | |
| 144 | <td>::=</td> | |
| 145 | <td><a class='grammartoken' href="simple.html#tok-simple_stmt">simple_stmt</a> (";" <a class='grammartoken' href="simple.html#tok-simple_stmt">simple_stmt</a>)* [";"]</td></tr> | |
| 146 | </table> | |
| 147 | </div> | |
| 148 | <a class="grammar-footer" | |
| 149 | href="grammar.txt" type="text/plain" | |
| 150 | >Download entire grammar as text.</a> | |
| 151 | </dd></dl> | |
| 152 | ||
| 153 | <P> | |
| 154 | Note that statements always end in a | |
| 155 | <code>NEWLINE</code><a id='l2h-571' xml:id='l2h-571'></a> possibly followed by a | |
| 156 | <code>DEDENT</code>.<a id='l2h-572' xml:id='l2h-572'></a> Also note that optional | |
| 157 | continuation clauses always begin with a keyword that cannot start a | |
| 158 | statement, thus there are no ambiguities (the `dangling | |
| 159 | <tt class="keyword">else</tt>' problem is solved in Python by requiring nested | |
| 160 | <tt class="keyword">if</tt> statements to be indented). | |
| 161 | <a id='l2h-569' xml:id='l2h-569'></a> | |
| 162 | <P> | |
| 163 | The formatting of the grammar rules in the following sections places | |
| 164 | each clause on a separate line for clarity. | |
| 165 | ||
| 166 | <P> | |
| 167 | ||
| 168 | <p><br /></p><hr class='online-navigation' /> | |
| 169 | <div class='online-navigation'> | |
| 170 | <!--Table of Child-Links--> | |
| 171 | <A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a> | |
| 172 | ||
| 173 | <UL CLASS="ChildLinks"> | |
| 174 | <LI><A href="if.html">7.1 The <tt class="keyword">if</tt> statement</a> | |
| 175 | <LI><A href="while.html">7.2 The <tt class="keyword">while</tt> statement</a> | |
| 176 | <LI><A href="for.html">7.3 The <tt class="keyword">for</tt> statement</a> | |
| 177 | <LI><A href="try.html">7.4 The <tt class="keyword">try</tt> statement</a> | |
| 178 | <LI><A href="function.html">7.5 Function definitions</a> | |
| 179 | <LI><A href="class.html">7.6 Class definitions</a> | |
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| 215 | </div> | |
| 216 | </div> | |
| 217 | <hr /> | |
| 218 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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