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| 55 | <H2><A NAME="SECTION005320000000000000000"></A><A NAME="attribute-access"></A> |
| 56 | <BR> |
| 57 | 3.3.2 Customizing attribute access |
| 58 | </H2> |
| 59 | |
| 60 | <P> |
| 61 | The following methods can be defined to customize the meaning of |
| 62 | attribute access (use of, assignment to, or deletion of <code>x.name</code>) |
| 63 | for class instances. |
| 64 | |
| 65 | <P> |
| 66 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 67 | <td><nobr><b><tt id='l2h-206' xml:id='l2h-206' class="method">__getattr__</tt></b>(</nobr></td> |
| 68 | <td><var>self, name</var>)</td></tr></table></dt> |
| 69 | <dd> |
| 70 | Called when an attribute lookup has not found the attribute in the |
| 71 | usual places (i.e. it is not an instance attribute nor is it found in |
| 72 | the class tree for <code>self</code>). <code>name</code> is the attribute name. |
| 73 | This method should return the (computed) attribute value or raise an |
| 74 | <tt class="exception">AttributeError</tt> exception. |
| 75 | |
| 76 | <P> |
| 77 | Note that if the attribute is found through the normal mechanism, |
| 78 | <tt class="method">__getattr__()</tt> is not called. (This is an intentional |
| 79 | asymmetry between <tt class="method">__getattr__()</tt> and <tt class="method">__setattr__()</tt>.) |
| 80 | This is done both for efficiency reasons and because otherwise |
| 81 | <tt class="method">__setattr__()</tt> would have no way to access other attributes of |
| 82 | the instance. Note that at least for instance variables, you can fake |
| 83 | total control by not inserting any values in the instance attribute |
| 84 | dictionary (but instead inserting them in another object). See the |
| 85 | <tt class="method">__getattribute__()</tt> method below for a way to actually get |
| 86 | total control in new-style classes. |
| 87 | <a id='l2h-208' xml:id='l2h-208'></a></dl> |
| 88 | |
| 89 | <P> |
| 90 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 91 | <td><nobr><b><tt id='l2h-209' xml:id='l2h-209' class="method">__setattr__</tt></b>(</nobr></td> |
| 92 | <td><var>self, name, value</var>)</td></tr></table></dt> |
| 93 | <dd> |
| 94 | Called when an attribute assignment is attempted. This is called |
| 95 | instead of the normal mechanism (i.e. store the value in the instance |
| 96 | dictionary). <var>name</var> is the attribute name, <var>value</var> is the |
| 97 | value to be assigned to it. |
| 98 | |
| 99 | <P> |
| 100 | If <tt class="method">__setattr__()</tt> wants to assign to an instance attribute, it |
| 101 | should not simply execute "<tt class="samp">self.<var>name</var> = value</tt>" -- this |
| 102 | would cause a recursive call to itself. Instead, it should insert the |
| 103 | value in the dictionary of instance attributes, e.g., |
| 104 | "<tt class="samp">self.__dict__[<var>name</var>] = value</tt>". For new-style classes, |
| 105 | rather than accessing the instance dictionary, it should call the base |
| 106 | class method with the same name, for example, |
| 107 | "<tt class="samp">object.__setattr__(self, name, value)</tt>". |
| 108 | <a id='l2h-211' xml:id='l2h-211'></a></dl> |
| 109 | |
| 110 | <P> |
| 111 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 112 | <td><nobr><b><tt id='l2h-212' xml:id='l2h-212' class="method">__delattr__</tt></b>(</nobr></td> |
| 113 | <td><var>self, name</var>)</td></tr></table></dt> |
| 114 | <dd> |
| 115 | Like <tt class="method">__setattr__()</tt> but for attribute deletion instead of |
| 116 | assignment. This should only be implemented if "<tt class="samp">del |
| 117 | obj.<var>name</var></tt>" is meaningful for the object. |
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| 128 | <LI><A href="new-style-attribute-access.html">3.3.2.1 More attribute access for new-style classes</a> |
| 129 | <LI><A href="descriptors.html">3.3.2.2 Implementing Descriptors</a> |
| 130 | <LI><A href="descriptor-invocation.html">3.3.2.3 Invoking Descriptors</a> |
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