projects / OpenSPARC-T2-DV / blame
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| 17 | <title>5.2.5 Generator expressions</title> | |
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| 54 | ||
| 55 | <H2><A NAME="SECTION007250000000000000000"></A><A NAME="genexpr"></A> | |
| 56 | <BR> | |
| 57 | 5.2.5 Generator expressions | |
| 58 | </H2> | |
| 59 | <a id='l2h-349' xml:id='l2h-349'></a> | |
| 60 | <P> | |
| 61 | A generator expression is a compact generator notation in parentheses: | |
| 62 | ||
| 63 | <P> | |
| 64 | <dl><dd class="grammar"> | |
| 65 | <div class="productions"> | |
| 66 | <table> | |
| 67 | <tr> | |
| 68 | <td><a id='tok-generator_expression' xml:id='tok-generator_expression'>generator_expression</a></td> | |
| 69 | <td>::=</td> | |
| 70 | <td>"(" <a class='grammartoken' href="lists.html#tok-test">test</a> <a class='grammartoken' href="genexpr.html#tok-genexpr_for">genexpr_for</a> ")"</td></tr> | |
| 71 | <tr> | |
| 72 | <td><a id='tok-genexpr_for' xml:id='tok-genexpr_for'>genexpr_for</a></td> | |
| 73 | <td>::=</td> | |
| 74 | <td>"for" <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a> "in" <a class='grammartoken' href="lists.html#tok-test">test</a> | |
| 75 | [<a class='grammartoken' href="genexpr.html#tok-genexpr_iter">genexpr_iter</a>]</td></tr> | |
| 76 | <tr> | |
| 77 | <td><a id='tok-genexpr_iter' xml:id='tok-genexpr_iter'>genexpr_iter</a></td> | |
| 78 | <td>::=</td> | |
| 79 | <td><a class='grammartoken' href="genexpr.html#tok-genexpr_for">genexpr_for</a> | <a class='grammartoken' href="genexpr.html#tok-genexpr_if">genexpr_if</a></td></tr> | |
| 80 | <tr> | |
| 81 | <td><a id='tok-genexpr_if' xml:id='tok-genexpr_if'>genexpr_if</a></td> | |
| 82 | <td>::=</td> | |
| 83 | <td>"if" <a class='grammartoken' href="lists.html#tok-test">test</a> [<a class='grammartoken' href="genexpr.html#tok-genexpr_iter">genexpr_iter</a>]</td></tr> | |
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| 86 | <a class="grammar-footer" | |
| 87 | href="grammar.txt" type="text/plain" | |
| 88 | >Download entire grammar as text.</a> | |
| 89 | </dd></dl> | |
| 90 | ||
| 91 | <P> | |
| 92 | A generator expression yields a new generator object. | |
| 93 | <a id='l2h-350' xml:id='l2h-350'></a><a id='l2h-351' xml:id='l2h-351'></a>It consists of a single expression followed by at least one | |
| 94 | <tt class="keyword">for</tt> clause and zero or more <tt class="keyword">for</tt> or <tt class="keyword">if</tt> | |
| 95 | clauses. The iterating values of the new generator are those that | |
| 96 | would be produced by considering each of the <tt class="keyword">for</tt> or | |
| 97 | <tt class="keyword">if</tt> clauses a block, nesting from left to right, and | |
| 98 | evaluating the expression to yield a value that is reached the | |
| 99 | innermost block for each iteration. | |
| 100 | ||
| 101 | <P> | |
| 102 | Variables used in the generator expression are evaluated lazily | |
| 103 | when the <tt class="method">next()</tt> method is called for generator object | |
| 104 | (in the same fashion as normal generators). However, the leftmost | |
| 105 | <tt class="keyword">for</tt> clause is immediately evaluated so that error produced | |
| 106 | by it can be seen before any other possible error in the code that | |
| 107 | handles the generator expression. | |
| 108 | Subsequent <tt class="keyword">for</tt> clauses cannot be evaluated immediately since | |
| 109 | they may depend on the previous <tt class="keyword">for</tt> loop. | |
| 110 | For example: "<tt class="samp">(x*y for x in range(10) for y in bar(x))</tt>". | |
| 111 | ||
| 112 | <P> | |
| 113 | The parentheses can be omitted on calls with only one argument. | |
| 114 | See section <A href="calls.html#calls">5.3.4</A> for the detail. | |
| 115 | ||
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| 152 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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