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| 54 | <H2><A NAME="SECTION008310000000000000000"></A><A NAME="augassign"></A> | |
| 55 | <BR> | |
| 56 | 6.3.1 Augmented assignment statements | |
| 57 | </H2> | |
| 58 | ||
| 59 | <P> | |
| 60 | Augmented assignment is the combination, in a single statement, of a binary | |
| 61 | operation and an assignment statement: | |
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| 69 | <td><a id='tok-augmented_assignment_stmt' xml:id='tok-augmented_assignment_stmt'>augmented_assignment_stmt</a></td> | |
| 70 | <td>::=</td> | |
| 71 | <td><a class='grammartoken' href="assignment.html#tok-target">target</a> <a class='grammartoken' href="augassign.html#tok-augop">augop</a> <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a></td></tr> | |
| 72 | <tr> | |
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| 86 | ||
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| 88 | (See section <A href="primaries.html#primaries">5.3</A> for the syntax definitions for the last | |
| 89 | three symbols.) | |
| 90 | ||
| 91 | <P> | |
| 92 | An augmented assignment evaluates the target (which, unlike normal | |
| 93 | assignment statements, cannot be an unpacking) and the expression | |
| 94 | list, performs the binary operation specific to the type of assignment | |
| 95 | on the two operands, and assigns the result to the original | |
| 96 | target. The target is only evaluated once. | |
| 97 | ||
| 98 | <P> | |
| 99 | An augmented assignment expression like <code>x += 1</code> can be rewritten as | |
| 100 | <code>x = x + 1</code> to achieve a similar, but not exactly equal effect. In the | |
| 101 | augmented version, <code>x</code> is only evaluated once. Also, when possible, the | |
| 102 | actual operation is performed <em>in-place</em>, meaning that rather than | |
| 103 | creating a new object and assigning that to the target, the old object is | |
| 104 | modified instead. | |
| 105 | ||
| 106 | <P> | |
| 107 | With the exception of assigning to tuples and multiple targets in a single | |
| 108 | statement, the assignment done by augmented assignment statements is handled | |
| 109 | the same way as normal assignments. Similarly, with the exception of the | |
| 110 | possible <em>in-place</em> behavior, the binary operation performed by | |
| 111 | augmented assignment is the same as the normal binary operations. | |
| 112 | ||
| 113 | <P> | |
| 114 | For targets which are attribute references, the initial value is | |
| 115 | retrieved with a <tt class="method">getattr()</tt> and the result is assigned with a | |
| 116 | <tt class="method">setattr()</tt>. Notice that the two methods do not necessarily | |
| 117 | refer to the same variable. When <tt class="method">getattr()</tt> refers to a class | |
| 118 | variable, <tt class="method">setattr()</tt> still writes to an instance variable. | |
| 119 | For example: | |
| 120 | ||
| 121 | <P> | |
| 122 | <div class="verbatim"><pre> | |
| 123 | class A: | |
| 124 | x = 3 # class variable | |
| 125 | a = A() | |
| 126 | a.x += 1 # writes a.x as 4 leaving A.x as 3 | |
| 127 | </pre></div> | |
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