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| 55 | <H3><A NAME="SECTION005323000000000000000"></A><A NAME="descriptor-invocation"></A> | |
| 56 | <BR> | |
| 57 | 3.3.2.3 Invoking Descriptors | |
| 58 | </H3> | |
| 59 | ||
| 60 | <P> | |
| 61 | In general, a descriptor is an object attribute with ``binding behavior'', | |
| 62 | one whose attribute access has been overridden by methods in the descriptor | |
| 63 | protocol: <tt class="method">__get__()</tt>, <tt class="method">__set__()</tt>, and <tt class="method">__delete__()</tt>. | |
| 64 | If any of those methods are defined for an object, it is said to be a | |
| 65 | descriptor. | |
| 66 | ||
| 67 | <P> | |
| 68 | The default behavior for attribute access is to get, set, or delete the | |
| 69 | attribute from an object's dictionary. For instance, <code>a.x</code> has a | |
| 70 | lookup chain starting with <code>a.__dict__['x']</code>, then | |
| 71 | <code>type(a).__dict__['x']</code>, and continuing | |
| 72 | through the base classes of <code>type(a)</code> excluding metaclasses. | |
| 73 | ||
| 74 | <P> | |
| 75 | However, if the looked-up value is an object defining one of the descriptor | |
| 76 | methods, then Python may override the default behavior and invoke the | |
| 77 | descriptor method instead. Where this occurs in the precedence chain depends | |
| 78 | on which descriptor methods were defined and how they were called. Note that | |
| 79 | descriptors are only invoked for new style objects or classes | |
| 80 | (ones that subclass <tt class="class">object()</tt> or <tt class="class">type()</tt>). | |
| 81 | ||
| 82 | <P> | |
| 83 | The starting point for descriptor invocation is a binding, <code>a.x</code>. | |
| 84 | How the arguments are assembled depends on <code>a</code>: | |
| 85 | ||
| 86 | <P> | |
| 87 | <DL COMPACT> | |
| 88 | <DT>Direct Call</DT> | |
| 89 | <DD>The simplest and least common call is when user code | |
| 90 | directly invokes a descriptor method: <code>x.__get__(a)</code>. | |
| 91 | ||
| 92 | <P> | |
| 93 | </DD> | |
| 94 | <DT>Instance Binding</DT> | |
| 95 | <DD>If binding to a new-style object instance, | |
| 96 | <code>a.x</code> is transformed into the call: | |
| 97 | <code>type(a).__dict__['x'].__get__(a, type(a))</code>. | |
| 98 | ||
| 99 | <P> | |
| 100 | </DD> | |
| 101 | <DT>Class Binding</DT> | |
| 102 | <DD>If binding to a new-style class, <code>A.x</code> | |
| 103 | is transformed into the call: <code>A.__dict__['x'].__get__(None, A)</code>. | |
| 104 | ||
| 105 | <P> | |
| 106 | </DD> | |
| 107 | <DT>Super Binding</DT> | |
| 108 | <DD>If <code>a</code> is an instance of <tt class="class">super</tt>, | |
| 109 | then the binding <code>super(B, obj).m()</code> searches | |
| 110 | <code>obj.__class__.__mro__</code> for the base class <code>A</code> immediately | |
| 111 | preceding <code>B</code> and then invokes the descriptor with the call: | |
| 112 | <code>A.__dict__['m'].__get__(obj, A)</code>. | |
| 113 | ||
| 114 | <P> | |
| 115 | </DD> | |
| 116 | </DL> | |
| 117 | ||
| 118 | <P> | |
| 119 | For instance bindings, the precedence of descriptor invocation depends | |
| 120 | on the which descriptor methods are defined. Data descriptors define | |
| 121 | both <tt class="method">__get__()</tt> and <tt class="method">__set__()</tt>. Non-data descriptors have | |
| 122 | just the <tt class="method">__get__()</tt> method. Data descriptors always override | |
| 123 | a redefinition in an instance dictionary. In contrast, non-data | |
| 124 | descriptors can be overridden by instances. | |
| 125 | ||
| 126 | <P> | |
| 127 | Python methods (including <tt class="function">staticmethod()</tt> and <tt class="function">classmethod()</tt>) | |
| 128 | are implemented as non-data descriptors. Accordingly, instances can | |
| 129 | redefine and override methods. This allows individual instances to acquire | |
| 130 | behaviors that differ from other instances of the same class. | |
| 131 | ||
| 132 | <P> | |
| 133 | The <tt class="function">property()</tt> function is implemented as a data descriptor. | |
| 134 | Accordingly, instances cannot override the behavior of a property. | |
| 135 | ||
| 136 | <P> | |
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