projects / OpenSPARC-T2-SAM / blame
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| 54 | ||
| 55 | <H1><A NAME="SECTION0071000000000000000000"></A><A NAME="Booleans"></A> | |
| 56 | <BR> | |
| 57 | 5.10 Boolean operations | |
| 58 | </H1> | |
| 59 | <a id='l2h-439' xml:id='l2h-439'></a> | |
| 60 | <P> | |
| 61 | Boolean operations have the lowest priority of all Python operations: | |
| 62 | ||
| 63 | <P> | |
| 64 | <dl><dd class="grammar"> | |
| 65 | <div class="productions"> | |
| 66 | <table> | |
| 67 | <tr> | |
| 68 | <td><a id='tok-expression' xml:id='tok-expression'>expression</a></td> | |
| 69 | <td>::=</td> | |
| 70 | <td><a class='grammartoken' href="Booleans.html#tok-or_test">or_test</a> | <a class='grammartoken' href="lambdas.html#tok-lambda_form">lambda_form</a></td></tr> | |
| 71 | <tr> | |
| 72 | <td><a id='tok-or_test' xml:id='tok-or_test'>or_test</a></td> | |
| 73 | <td>::=</td> | |
| 74 | <td><a class='grammartoken' href="Booleans.html#tok-and_test">and_test</a> | <a class='grammartoken' href="Booleans.html#tok-or_test">or_test</a> "or" <a class='grammartoken' href="Booleans.html#tok-and_test">and_test</a></td></tr> | |
| 75 | <tr> | |
| 76 | <td><a id='tok-and_test' xml:id='tok-and_test'>and_test</a></td> | |
| 77 | <td>::=</td> | |
| 78 | <td><a class='grammartoken' href="Booleans.html#tok-not_test">not_test</a> | <a class='grammartoken' href="Booleans.html#tok-and_test">and_test</a> "and" <a class='grammartoken' href="Booleans.html#tok-not_test">not_test</a></td></tr> | |
| 79 | <tr> | |
| 80 | <td><a id='tok-not_test' xml:id='tok-not_test'>not_test</a></td> | |
| 81 | <td>::=</td> | |
| 82 | <td><a class='grammartoken' href="comparisons.html#tok-comparison">comparison</a> | "not" <a class='grammartoken' href="Booleans.html#tok-not_test">not_test</a></td></tr> | |
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| 85 | <a class="grammar-footer" | |
| 86 | href="grammar.txt" type="text/plain" | |
| 87 | >Download entire grammar as text.</a> | |
| 88 | </dd></dl> | |
| 89 | ||
| 90 | <P> | |
| 91 | In the context of Boolean operations, and also when expressions are | |
| 92 | used by control flow statements, the following values are interpreted | |
| 93 | as false: <code>False</code>, <code>None</code>, numeric zero of all types, and empty | |
| 94 | strings and containers (including strings, tuples, lists, dictionaries, | |
| 95 | sets and frozensets). All other values are interpreted as true. | |
| 96 | ||
| 97 | <P> | |
| 98 | The operator <tt class="keyword">not</tt> yields <code>True</code> if its argument is false, | |
| 99 | <code>False</code> otherwise. | |
| 100 | <a id='l2h-440' xml:id='l2h-440'></a> | |
| 101 | <P> | |
| 102 | The expression <code><var>x</var> and <var>y</var></code> first evaluates <var>x</var>; if | |
| 103 | <var>x</var> is false, its value is returned; otherwise, <var>y</var> is | |
| 104 | evaluated and the resulting value is returned. | |
| 105 | <a id='l2h-441' xml:id='l2h-441'></a> | |
| 106 | <P> | |
| 107 | The expression <code><var>x</var> or <var>y</var></code> first evaluates <var>x</var>; if | |
| 108 | <var>x</var> is true, its value is returned; otherwise, <var>y</var> is | |
| 109 | evaluated and the resulting value is returned. | |
| 110 | <a id='l2h-442' xml:id='l2h-442'></a> | |
| 111 | <P> | |
| 112 | (Note that neither <tt class="keyword">and</tt> nor <tt class="keyword">or</tt> restrict the value | |
| 113 | and type they return to <code>False</code> and <code>True</code>, but rather return the | |
| 114 | last evaluated argument. | |
| 115 | This is sometimes useful, e.g., if <code>s</code> is a string that should be | |
| 116 | replaced by a default value if it is empty, the expression | |
| 117 | <code>s or 'foo'</code> yields the desired value. Because <tt class="keyword">not</tt> has to | |
| 118 | invent a value anyway, it does not bother to return a value of the | |
| 119 | same type as its argument, so e.g., <code>not 'foo'</code> yields <code>False</code>, | |
| 120 | not <code>''</code>.) | |
| 121 | ||
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| 158 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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