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17 | <title>5.3.3 Slicings</title> | |
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54 | ||
55 | <H2><A NAME="SECTION007330000000000000000"></A><A NAME="slicings"></A> | |
56 | <a id='l2h-378' xml:id='l2h-378'></a> | |
57 | <BR> | |
58 | 5.3.3 Slicings | |
59 | </H2> | |
60 | ||
61 | <P> | |
62 | A slicing selects a range of items in a sequence object (e.g., a | |
63 | string, tuple or list). Slicings may be used as expressions or as | |
64 | targets in assignment or del statements. The syntax for a slicing: | |
65 | <a id='l2h-379' xml:id='l2h-379'></a><a id='l2h-380' xml:id='l2h-380'></a><a id='l2h-381' xml:id='l2h-381'></a><a id='l2h-382' xml:id='l2h-382'></a> | |
66 | <P> | |
67 | <dl><dd class="grammar"> | |
68 | <div class="productions"> | |
69 | <table> | |
70 | <tr> | |
71 | <td><a id='tok-slicing' xml:id='tok-slicing'>slicing</a></td> | |
72 | <td>::=</td> | |
73 | <td><a class='grammartoken' href="slicings.html#tok-simple_slicing">simple_slicing</a> | <a class='grammartoken' href="slicings.html#tok-extended_slicing">extended_slicing</a></td></tr> | |
74 | <tr> | |
75 | <td><a id='tok-simple_slicing' xml:id='tok-simple_slicing'>simple_slicing</a></td> | |
76 | <td>::=</td> | |
77 | <td><a class='grammartoken' href="primaries.html#tok-primary">primary</a> "[" <a class='grammartoken' href="slicings.html#tok-short_slice">short_slice</a> "]"</td></tr> | |
78 | <tr> | |
79 | <td><a id='tok-extended_slicing' xml:id='tok-extended_slicing'>extended_slicing</a></td> | |
80 | <td>::=</td> | |
81 | <td><a class='grammartoken' href="primaries.html#tok-primary">primary</a> "[" <a class='grammartoken' href="slicings.html#tok-slice_list">slice_list</a> "]" </td></tr> | |
82 | <tr> | |
83 | <td><a id='tok-slice_list' xml:id='tok-slice_list'>slice_list</a></td> | |
84 | <td>::=</td> | |
85 | <td><a class='grammartoken' href="slicings.html#tok-slice_item">slice_item</a> ("," <a class='grammartoken' href="slicings.html#tok-slice_item">slice_item</a>)* [","]</td></tr> | |
86 | <tr> | |
87 | <td><a id='tok-slice_item' xml:id='tok-slice_item'>slice_item</a></td> | |
88 | <td>::=</td> | |
89 | <td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a> | <a class='grammartoken' href="slicings.html#tok-proper_slice">proper_slice</a> | <a class='grammartoken' href="slicings.html#tok-ellipsis">ellipsis</a></td></tr> | |
90 | <tr> | |
91 | <td><a id='tok-proper_slice' xml:id='tok-proper_slice'>proper_slice</a></td> | |
92 | <td>::=</td> | |
93 | <td><a class='grammartoken' href="slicings.html#tok-short_slice">short_slice</a> | <a class='grammartoken' href="slicings.html#tok-long_slice">long_slice</a></td></tr> | |
94 | <tr> | |
95 | <td><a id='tok-short_slice' xml:id='tok-short_slice'>short_slice</a></td> | |
96 | <td>::=</td> | |
97 | <td>[<a class='grammartoken' href="slicings.html#tok-lower_bound">lower_bound</a>] ":" [<a class='grammartoken' href="slicings.html#tok-upper_bound">upper_bound</a>]</td></tr> | |
98 | <tr> | |
99 | <td><a id='tok-long_slice' xml:id='tok-long_slice'>long_slice</a></td> | |
100 | <td>::=</td> | |
101 | <td><a class='grammartoken' href="slicings.html#tok-short_slice">short_slice</a> ":" [<a class='grammartoken' href="slicings.html#tok-stride">stride</a>]</td></tr> | |
102 | <tr> | |
103 | <td><a id='tok-lower_bound' xml:id='tok-lower_bound'>lower_bound</a></td> | |
104 | <td>::=</td> | |
105 | <td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a></td></tr> | |
106 | <tr> | |
107 | <td><a id='tok-upper_bound' xml:id='tok-upper_bound'>upper_bound</a></td> | |
108 | <td>::=</td> | |
109 | <td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a></td></tr> | |
110 | <tr> | |
111 | <td><a id='tok-stride' xml:id='tok-stride'>stride</a></td> | |
112 | <td>::=</td> | |
113 | <td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a></td></tr> | |
114 | <tr> | |
115 | <td><a id='tok-ellipsis' xml:id='tok-ellipsis'>ellipsis</a></td> | |
116 | <td>::=</td> | |
117 | <td>"..."</td></tr> | |
118 | </table> | |
119 | </div> | |
120 | <a class="grammar-footer" | |
121 | href="grammar.txt" type="text/plain" | |
122 | >Download entire grammar as text.</a> | |
123 | </dd></dl> | |
124 | ||
125 | <P> | |
126 | There is ambiguity in the formal syntax here: anything that looks like | |
127 | an expression list also looks like a slice list, so any subscription | |
128 | can be interpreted as a slicing. Rather than further complicating the | |
129 | syntax, this is disambiguated by defining that in this case the | |
130 | interpretation as a subscription takes priority over the | |
131 | interpretation as a slicing (this is the case if the slice list | |
132 | contains no proper slice nor ellipses). Similarly, when the slice | |
133 | list has exactly one short slice and no trailing comma, the | |
134 | interpretation as a simple slicing takes priority over that as an | |
135 | extended slicing.<a id='l2h-383' xml:id='l2h-383'></a> | |
136 | <P> | |
137 | The semantics for a simple slicing are as follows. The primary must | |
138 | evaluate to a sequence object. The lower and upper bound expressions, | |
139 | if present, must evaluate to plain integers; defaults are zero and the | |
140 | <code>sys.maxint</code>, respectively. If either bound is negative, the | |
141 | sequence's length is added to it. The slicing now selects all items | |
142 | with index <var>k</var> such that | |
143 | <code><var>i</var> <= <var>k</var> < <var>j</var></code> where <var>i</var> | |
144 | and <var>j</var> are the specified lower and upper bounds. This may be an | |
145 | empty sequence. It is not an error if <var>i</var> or <var>j</var> lie outside the | |
146 | range of valid indexes (such items don't exist so they aren't | |
147 | selected). | |
148 | ||
149 | <P> | |
150 | The semantics for an extended slicing are as follows. The primary | |
151 | must evaluate to a mapping object, and it is indexed with a key that | |
152 | is constructed from the slice list, as follows. If the slice list | |
153 | contains at least one comma, the key is a tuple containing the | |
154 | conversion of the slice items; otherwise, the conversion of the lone | |
155 | slice item is the key. The conversion of a slice item that is an | |
156 | expression is that expression. The conversion of an ellipsis slice | |
157 | item is the built-in <code>Ellipsis</code> object. The conversion of a | |
158 | proper slice is a slice object (see section <A href="types.html#types">3.2</A>) whose | |
159 | <tt class="member">start</tt>, <tt class="member">stop</tt> and <tt class="member">step</tt> attributes are the | |
160 | values of the expressions given as lower bound, upper bound and | |
161 | stride, respectively, substituting <code>None</code> for missing | |
162 | expressions. | |
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200 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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