[OpenSPARC-T2-SAM] / sam-t2 / devtools / v8plus / html / python / ref / slicings.html

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<H2><A NAME="SECTION007330000000000000000"></A><A NAME="slicings"></A>
<a id='l2h-378' xml:id='l2h-378'></a>
<BR>
5.3.3 Slicings
</H2>

<P>
A slicing selects a range of items in a sequence object (e.g., a
string, tuple or list).  Slicings may be used as expressions or as
targets in assignment or del statements.  The syntax for a slicing:
<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>
<P>
<dl><dd class="grammar">
<div class="productions">
<table>
<tr>
    <td><a id='tok-slicing' xml:id='tok-slicing'>slicing</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-simple_slicing' xml:id='tok-simple_slicing'>simple_slicing</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-extended_slicing' xml:id='tok-extended_slicing'>extended_slicing</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-slice_list' xml:id='tok-slice_list'>slice_list</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-slice_item' xml:id='tok-slice_item'>slice_item</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-proper_slice' xml:id='tok-proper_slice'>proper_slice</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-short_slice' xml:id='tok-short_slice'>short_slice</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-long_slice' xml:id='tok-long_slice'>long_slice</a></td>
    <td>::=</td>
    <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>
  <tr>
    <td><a id='tok-lower_bound' xml:id='tok-lower_bound'>lower_bound</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a></td></tr>
  <tr>
    <td><a id='tok-upper_bound' xml:id='tok-upper_bound'>upper_bound</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a></td></tr>
  <tr>
    <td><a id='tok-stride' xml:id='tok-stride'>stride</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a></td></tr>
  <tr>
    <td><a id='tok-ellipsis' xml:id='tok-ellipsis'>ellipsis</a></td>
    <td>::=</td>
    <td>"..."</td></tr>
</table>
</div>
<a class="grammar-footer"
  href="grammar.txt" type="text/plain"
  >Download entire grammar as text.</a>
</dd></dl>

<P>
There is ambiguity in the formal syntax here: anything that looks like
an expression list also looks like a slice list, so any subscription
can be interpreted as a slicing.  Rather than further complicating the
syntax, this is disambiguated by defining that in this case the
interpretation as a subscription takes priority over the
interpretation as a slicing (this is the case if the slice list
contains no proper slice nor ellipses).  Similarly, when the slice
list has exactly one short slice and no trailing comma, the
interpretation as a simple slicing takes priority over that as an
extended slicing.<a id='l2h-383' xml:id='l2h-383'></a>
<P>
The semantics for a simple slicing are as follows.  The primary must
evaluate to a sequence object.  The lower and upper bound expressions,
if present, must evaluate to plain integers; defaults are zero and the
<code>sys.maxint</code>, respectively.  If either bound is negative, the
sequence's length is added to it.  The slicing now selects all items
with index <var>k</var> such that
<code><var>i</var> &lt;= <var>k</var> &lt; <var>j</var></code> where <var>i</var>
and <var>j</var> are the specified lower and upper bounds.  This may be an
empty sequence.  It is not an error if <var>i</var> or <var>j</var> lie outside the
range of valid indexes (such items don't exist so they aren't
selected).

<P>
The semantics for an extended slicing are as follows.  The primary
must evaluate to a mapping object, and it is indexed with a key that
is constructed from the slice list, as follows.  If the slice list
contains at least one comma, the key is a tuple containing the
conversion of the slice items; otherwise, the conversion of the lone
slice item is the key.  The conversion of a slice item that is an
expression is that expression.  The conversion of an ellipsis slice
item is the built-in <code>Ellipsis</code> object.  The conversion of a
proper slice is a slice object (see section <A href="types.html#types">3.2</A>) whose
<tt class="member">start</tt>, <tt class="member">stop</tt> and <tt class="member">step</tt> attributes are the
values of the expressions given as lower bound, upper bound and
stride, respectively, substituting <code>None</code> for missing
expressions.
<a id='l2h-385' xml:id='l2h-385'></a>
<P>

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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.
163	<a id='l2h-385' xml:id='l2h-385'></a>
164	<P>
165
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