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| 55 | <H2><A NAME="SECTION007320000000000000000"></A><A NAME="subscriptions"></A> |
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| 57 | <BR> |
| 58 | 5.3.2 Subscriptions |
| 59 | </H2> |
| 60 | |
| 61 | <P> |
| 62 | A subscription selects an item of a sequence (string, tuple or list) |
| 63 | or mapping (dictionary) object: |
| 64 | <a id='l2h-369' xml:id='l2h-369'></a><a id='l2h-370' xml:id='l2h-370'></a><a id='l2h-371' xml:id='l2h-371'></a><a id='l2h-372' xml:id='l2h-372'></a><a id='l2h-373' xml:id='l2h-373'></a><a id='l2h-374' xml:id='l2h-374'></a><a id='l2h-375' xml:id='l2h-375'></a> |
| 65 | <P> |
| 66 | <dl><dd class="grammar"> |
| 67 | <div class="productions"> |
| 68 | <table> |
| 69 | <tr> |
| 70 | <td><a id='tok-subscription' xml:id='tok-subscription'>subscription</a></td> |
| 71 | <td>::=</td> |
| 72 | <td><a class='grammartoken' href="primaries.html#tok-primary">primary</a> "[" <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a> "]"</td></tr> |
| 73 | </table> |
| 74 | </div> |
| 75 | <a class="grammar-footer" |
| 76 | href="grammar.txt" type="text/plain" |
| 77 | >Download entire grammar as text.</a> |
| 78 | </dd></dl> |
| 79 | |
| 80 | <P> |
| 81 | The primary must evaluate to an object of a sequence or mapping type. |
| 82 | |
| 83 | <P> |
| 84 | If the primary is a mapping, the expression list must evaluate to an |
| 85 | object whose value is one of the keys of the mapping, and the |
| 86 | subscription selects the value in the mapping that corresponds to that |
| 87 | key. (The expression list is a tuple except if it has exactly one |
| 88 | item.) |
| 89 | |
| 90 | <P> |
| 91 | If the primary is a sequence, the expression (list) must evaluate to a |
| 92 | plain integer. If this value is negative, the length of the sequence |
| 93 | is added to it (so that, e.g., <code>x[-1]</code> selects the last item of |
| 94 | <code>x</code>.) The resulting value must be a nonnegative integer less |
| 95 | than the number of items in the sequence, and the subscription selects |
| 96 | the item whose index is that value (counting from zero). |
| 97 | |
| 98 | <P> |
| 99 | A string's items are characters. A character is not a separate data |
| 100 | type but a string of exactly one character. |
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