[OpenSPARC-T2-SAM] / sam-t2 / devtools / v9 / html / python / ref / assignment.html

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<H1><A NAME="SECTION008300000000000000000"></A><A NAME="assignment"></A>
<BR>
6.3 Assignment statements 
</H1>

<P>
Assignment statements<a id='l2h-465' xml:id='l2h-465'></a> are used to
(re)bind names to values and to modify attributes or items of mutable
objects:
<a id='l2h-466' xml:id='l2h-466'></a><a id='l2h-467' xml:id='l2h-467'></a><a id='l2h-468' xml:id='l2h-468'></a><a id='l2h-469' xml:id='l2h-469'></a>
<P>
<dl><dd class="grammar">
<div class="productions">
<table>
<tr>
    <td><a id='tok-assignment_stmt' xml:id='tok-assignment_stmt'>assignment_stmt</a></td>
    <td>::=</td>
    <td>(<a class='grammartoken' href="assignment.html#tok-target_list">target_list</a> "=")+ <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a></td></tr>
  <tr>
    <td><a id='tok-target_list' xml:id='tok-target_list'>target_list</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="assignment.html#tok-target">target</a> ("," <a class='grammartoken' href="assignment.html#tok-target">target</a>)* [","]</td></tr>
  <tr>
    <td><a id='tok-target' xml:id='tok-target'>target</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| "(" <a class='grammartoken' href="assignment.html#tok-target_list">target_list</a> ")"</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| "[" <a class='grammartoken' href="assignment.html#tok-target_list">target_list</a> "]"</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| <a class='grammartoken' href="attribute-references.html#tok-attributeref">attributeref</a></code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| <a class='grammartoken' href="subscriptions.html#tok-subscription">subscription</a></code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| <a class='grammartoken' href="slicings.html#tok-slicing">slicing</a></code></td></tr>
</table>
</div>
<a class="grammar-footer"
  href="grammar.txt" type="text/plain"
  >Download entire grammar as text.</a>
</dd></dl>

<P>
(See section&nbsp;<A href="primaries.html#primaries">5.3</A> for the syntax definitions for the last
three symbols.)

<P>
An assignment statement evaluates the expression list (remember that
this can be a single expression or a comma-separated list, the latter
yielding a tuple) and assigns the single resulting object to each of
the target lists, from left to right.
<a id='l2h-470' xml:id='l2h-470'></a>
<P>
Assignment is defined recursively depending on the form of the target
(list).  When a target is part of a mutable object (an attribute
reference, subscription or slicing), the mutable object must
ultimately perform the assignment and decide about its validity, and
may raise an exception if the assignment is unacceptable.  The rules
observed by various types and the exceptions raised are given with the
definition of the object types (see section&nbsp;<A href="types.html#types">3.2</A>).
<a id='l2h-482' xml:id='l2h-482'></a>
<a id='l2h-471' xml:id='l2h-471'></a>
<P>
Assignment of an object to a target list is recursively defined as
follows.
<a id='l2h-472' xml:id='l2h-472'></a>
<P>

<UL>
<LI>If the target list is a single target: The object is assigned to that
target.

<P>
</LI>
<LI>If the target list is a comma-separated list of targets: The object
must be a sequence with the same number of items as there are
targets in the target list, and the items are assigned, from left to
right, to the corresponding targets.  (This rule is relaxed as of
Python 1.5; in earlier versions, the object had to be a tuple.  Since
strings are sequences, an assignment like "<tt class="samp">a, b = "xy"</tt>" is
now legal as long as the string has the right length.)

<P>
</LI>
</UL>

<P>
Assignment of an object to a single target is recursively defined as
follows.

<P>

 
<UL>
<LI>If the target is an identifier (name):

<P>

<UL>
<LI>If the name does not occur in a <tt class="keyword">global</tt> statement in the current
code block: the name is bound to the object in the current local
namespace.
<a id='l2h-473' xml:id='l2h-473'></a>
<P>
</LI>
<LI>Otherwise: the name is bound to the object in the current global
namespace.

<P>
</LI>
</UL> 
<P>
The name is rebound if it was already bound.  This may cause the
reference count for the object previously bound to the name to reach
zero, causing the object to be deallocated and its
destructor<a id='l2h-483' xml:id='l2h-483'></a> (if it has one) to be called.

<P>
</LI>
<LI>If the target is a target list enclosed in parentheses or in square
brackets: The object must be a sequence with the same number of items
as there are targets in the target list, and its items are assigned,
from left to right, to the corresponding targets.

<P>
</LI>
<LI>If the target is an attribute reference: The primary expression in the
reference is evaluated.  It should yield an object with assignable
attributes; if this is not the case, <tt class="exception">TypeError</tt> is raised.  That
object is then asked to assign the assigned object to the given
attribute; if it cannot perform the assignment, it raises an exception
(usually but not necessarily <tt class="exception">AttributeError</tt>).
<a id='l2h-474' xml:id='l2h-474'></a>
<P>
</LI>
<LI>If the target is a subscription: The primary expression in the
reference is evaluated.  It should yield either a mutable sequence
object (such as a list) or a mapping object (such as a dictionary). Next,
the subscript expression is evaluated.
<a id='l2h-475' xml:id='l2h-475'></a><a id='l2h-476' xml:id='l2h-476'></a>
<P>
If the primary is a mutable sequence object (such as a list), the subscript
must yield a plain integer.  If it is negative, the sequence's length
is added to it.  The resulting value must be a nonnegative integer
less than the sequence's length, and the sequence is asked to assign
the assigned object to its item with that index.  If the index is out
of range, <tt class="exception">IndexError</tt> is raised (assignment to a subscripted
sequence cannot add new items to a list).
<a id='l2h-477' xml:id='l2h-477'></a><a id='l2h-478' xml:id='l2h-478'></a>
<P>
If the primary is a mapping object (such as a dictionary), the subscript must
have a type compatible with the mapping's key type, and the mapping is
then asked to create a key/datum pair which maps the subscript to
the assigned object.  This can either replace an existing key/value
pair with the same key value, or insert a new key/value pair (if no
key with the same value existed).
<a id='l2h-479' xml:id='l2h-479'></a><a id='l2h-480' xml:id='l2h-480'></a>
<P>
</LI>
<LI>If the target is a slicing: The primary expression in the reference is
evaluated.  It should yield a mutable sequence object (such as a list).  The
assigned object should be a sequence object of the same type.  Next,
the lower and upper bound expressions are evaluated, insofar they are
present; defaults are zero and the sequence's length.  The bounds
should evaluate to (small) integers.  If either bound is negative, the
sequence's length is added to it.  The resulting bounds are clipped to
lie between zero and the sequence's length, inclusive.  Finally, the
sequence object is asked to replace the slice with the items of the
assigned sequence.  The length of the slice may be different from the
length of the assigned sequence, thus changing the length of the
target sequence, if the object allows it.
<a id='l2h-481' xml:id='l2h-481'></a>
<P>
</LI>
</UL>

<P>
(In the current implementation, the syntax for targets is taken
to be the same as for expressions, and invalid syntax is rejected
during the code generation phase, causing less detailed error
messages.)

<P>
WARNING: Although the definition of assignment implies that overlaps
between the left-hand side and the right-hand side are `safe' (for example
"<tt class="samp">a, b = b, a</tt>" swaps two variables), overlaps <em>within</em> the
collection of assigned-to variables are not safe!  For instance, the
following program prints "<tt class="samp">[0, 2]</tt>":

<P>
<div class="verbatim"><pre>
x = [0, 1]
i = 0
i, x[i] = 1, 2
print x
</pre></div>

<P>

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	54
	55	<H1><A NAME="SECTION008300000000000000000"></A><A NAME="assignment"></A>
	56	<BR>
	57	6.3 Assignment statements
	58	</H1>
	59
	60	<P>
	61	Assignment statements<a id='l2h-465' xml:id='l2h-465'></a> are used to
	62	(re)bind names to values and to modify attributes or items of mutable
	63	objects:
	64	<a id='l2h-466' xml:id='l2h-466'></a><a id='l2h-467' xml:id='l2h-467'></a><a id='l2h-468' xml:id='l2h-468'></a><a id='l2h-469' xml:id='l2h-469'></a>
65	<P>
66	<dl><dd class="grammar">
67	<div class="productions">
68	<table>
69	<tr>
70	<td><a id='tok-assignment_stmt' xml:id='tok-assignment_stmt'>assignment_stmt</a></td>
71	<td>::=</td>
72	<td>(<a class='grammartoken' href="assignment.html#tok-target_list">target_list</a> "=")+ <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a></td></tr>
73	<tr>
74	<td><a id='tok-target_list' xml:id='tok-target_list'>target_list</a></td>
75	<td>::=</td>
76	<td><a class='grammartoken' href="assignment.html#tok-target">target</a> ("," <a class='grammartoken' href="assignment.html#tok-target">target</a>)* [","]</td></tr>
77	<tr>
78	<td><a id='tok-target' xml:id='tok-target'>target</a></td>
79	<td>::=</td>
80	<td><a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a></td></tr>
81	<tr>
82	<td></td>
83	<td></td>
84	<td><code>\| "(" <a class='grammartoken' href="assignment.html#tok-target_list">target_list</a> ")"</code></td></tr>
85	<tr>
86	<td></td>
87	<td></td>
88	<td><code>\| "[" <a class='grammartoken' href="assignment.html#tok-target_list">target_list</a> "]"</code></td></tr>
89	<tr>
90	<td></td>
91	<td></td>
92	<td><code>\| <a class='grammartoken' href="attribute-references.html#tok-attributeref">attributeref</a></code></td></tr>
93	<tr>
94	<td></td>
95	<td></td>
96	<td><code>\| <a class='grammartoken' href="subscriptions.html#tok-subscription">subscription</a></code></td></tr>
97	<tr>
98	<td></td>
99	<td></td>
100	<td><code>\| <a class='grammartoken' href="slicings.html#tok-slicing">slicing</a></code></td></tr>
101	</table>
102	</div>
103	<a class="grammar-footer"
104	href="grammar.txt" type="text/plain"
105	>Download entire grammar as text.</a>
106	</dd></dl>
107
108	<P>
109	(See section <A href="primaries.html#primaries">5.3</A> for the syntax definitions for the last
110	three symbols.)
111
112	<P>
113	An assignment statement evaluates the expression list (remember that
114	this can be a single expression or a comma-separated list, the latter
115	yielding a tuple) and assigns the single resulting object to each of
116	the target lists, from left to right.
117	<a id='l2h-470' xml:id='l2h-470'></a>
118	<P>
119	Assignment is defined recursively depending on the form of the target
120	(list). When a target is part of a mutable object (an attribute
121	reference, subscription or slicing), the mutable object must
122	ultimately perform the assignment and decide about its validity, and
123	may raise an exception if the assignment is unacceptable. The rules
124	observed by various types and the exceptions raised are given with the
125	definition of the object types (see section <A href="types.html#types">3.2</A>).
126	<a id='l2h-482' xml:id='l2h-482'></a>
127	<a id='l2h-471' xml:id='l2h-471'></a>
128	<P>
129	Assignment of an object to a target list is recursively defined as
130	follows.
131	<a id='l2h-472' xml:id='l2h-472'></a>
132	<P>
133
134	<UL>
135	<LI>If the target list is a single target: The object is assigned to that
136	target.
137
138	<P>
139	</LI>
140	<LI>If the target list is a comma-separated list of targets: The object
141	must be a sequence with the same number of items as there are
142	targets in the target list, and the items are assigned, from left to
143	right, to the corresponding targets. (This rule is relaxed as of
144	Python 1.5; in earlier versions, the object had to be a tuple. Since
145	strings are sequences, an assignment like "<tt class="samp">a, b = "xy"</tt>" is
146	now legal as long as the string has the right length.)
147
148	<P>
149	</LI>
150	</UL>
151
152	<P>
153	Assignment of an object to a single target is recursively defined as
154	follows.
155
156	<P>
157
158
159	<UL>
160	<LI>If the target is an identifier (name):
161
162	<P>
163
164	<UL>
165	<LI>If the name does not occur in a <tt class="keyword">global</tt> statement in the current
166	code block: the name is bound to the object in the current local
167	namespace.
168	<a id='l2h-473' xml:id='l2h-473'></a>
169	<P>
170	</LI>
171	<LI>Otherwise: the name is bound to the object in the current global
172	namespace.
173
174	<P>
175	</LI>
176	</UL>
177	<P>
178	The name is rebound if it was already bound. This may cause the
179	reference count for the object previously bound to the name to reach
180	zero, causing the object to be deallocated and its
181	destructor<a id='l2h-483' xml:id='l2h-483'></a> (if it has one) to be called.
182
183	<P>
184	</LI>
185	<LI>If the target is a target list enclosed in parentheses or in square
186	brackets: The object must be a sequence with the same number of items
187	as there are targets in the target list, and its items are assigned,
188	from left to right, to the corresponding targets.
189
190	<P>
191	</LI>
192	<LI>If the target is an attribute reference: The primary expression in the
193	reference is evaluated. It should yield an object with assignable
194	attributes; if this is not the case, <tt class="exception">TypeError</tt> is raised. That
195	object is then asked to assign the assigned object to the given
196	attribute; if it cannot perform the assignment, it raises an exception
197	(usually but not necessarily <tt class="exception">AttributeError</tt>).
198	<a id='l2h-474' xml:id='l2h-474'></a>
199	<P>
200	</LI>
201	<LI>If the target is a subscription: The primary expression in the
202	reference is evaluated. It should yield either a mutable sequence
203	object (such as a list) or a mapping object (such as a dictionary). Next,
204	the subscript expression is evaluated.
205	<a id='l2h-475' xml:id='l2h-475'></a><a id='l2h-476' xml:id='l2h-476'></a>
206	<P>
207	If the primary is a mutable sequence object (such as a list), the subscript
208	must yield a plain integer. If it is negative, the sequence's length
209	is added to it. The resulting value must be a nonnegative integer
210	less than the sequence's length, and the sequence is asked to assign
211	the assigned object to its item with that index. If the index is out
212	of range, <tt class="exception">IndexError</tt> is raised (assignment to a subscripted
213	sequence cannot add new items to a list).
214	<a id='l2h-477' xml:id='l2h-477'></a><a id='l2h-478' xml:id='l2h-478'></a>
215	<P>
216	If the primary is a mapping object (such as a dictionary), the subscript must
217	have a type compatible with the mapping's key type, and the mapping is
218	then asked to create a key/datum pair which maps the subscript to
219	the assigned object. This can either replace an existing key/value
220	pair with the same key value, or insert a new key/value pair (if no
221	key with the same value existed).
222	<a id='l2h-479' xml:id='l2h-479'></a><a id='l2h-480' xml:id='l2h-480'></a>
223	<P>
224	</LI>
225	<LI>If the target is a slicing: The primary expression in the reference is
226	evaluated. It should yield a mutable sequence object (such as a list). The
227	assigned object should be a sequence object of the same type. Next,
228	the lower and upper bound expressions are evaluated, insofar they are
229	present; defaults are zero and the sequence's length. The bounds
230	should evaluate to (small) integers. If either bound is negative, the
231	sequence's length is added to it. The resulting bounds are clipped to
232	lie between zero and the sequence's length, inclusive. Finally, the
233	sequence object is asked to replace the slice with the items of the
234	assigned sequence. The length of the slice may be different from the
235	length of the assigned sequence, thus changing the length of the
236	target sequence, if the object allows it.
237	<a id='l2h-481' xml:id='l2h-481'></a>
238	<P>
239	</LI>
240	</UL>
241
242	<P>
243	(In the current implementation, the syntax for targets is taken
244	to be the same as for expressions, and invalid syntax is rejected
245	during the code generation phase, causing less detailed error
246	messages.)
247
248	<P>
249	WARNING: Although the definition of assignment implies that overlaps
250	between the left-hand side and the right-hand side are `safe' (for example
251	"<tt class="samp">a, b = b, a</tt>" swaps two variables), overlaps <em>within</em> the
252	collection of assigned-to variables are not safe! For instance, the
253	following program prints "<tt class="samp">[0, 2]</tt>":
254
255	<P>
256	<div class="verbatim"><pre>
257	x = [0, 1]
258	i = 0
259	i, x[i] = 1, 2
260	print x
261	</pre></div>
262
263	<P>
264
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