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<H3><A NAME="SECTION005323000000000000000"></A><A NAME="descriptor-invocation"></A>
<BR>
3.3.2.3 Invoking Descriptors
</H3>

<P>
In general, a descriptor is an object attribute with ``binding behavior'',
one whose attribute access has been overridden by methods in the descriptor
protocol:  <tt class="method">__get__()</tt>, <tt class="method">__set__()</tt>, and <tt class="method">__delete__()</tt>.
If any of those methods are defined for an object, it is said to be a
descriptor.

<P>
The default behavior for attribute access is to get, set, or delete the
attribute from an object's dictionary. For instance, <code>a.x</code> has a
lookup chain starting with <code>a.__dict__['x']</code>, then
<code>type(a).__dict__['x']</code>, and continuing
through the base classes of <code>type(a)</code> excluding metaclasses.

<P>
However, if the looked-up value is an object defining one of the descriptor
methods, then Python may override the default behavior and invoke the
descriptor method instead.  Where this occurs in the precedence chain depends
on which descriptor methods were defined and how they were called.  Note that
descriptors are only invoked for new style objects or classes
(ones that subclass <tt class="class">object()</tt> or <tt class="class">type()</tt>).

<P>
The starting point for descriptor invocation is a binding, <code>a.x</code>.
How the arguments are assembled depends on <code>a</code>:

<P>
<DL COMPACT>
<DT>Direct Call</DT>
<DD>The simplest and least common call is when user code
    directly invokes a descriptor method:    <code>x.__get__(a)</code>.

<P>
</DD>
<DT>Instance Binding</DT>
<DD>If binding to a new-style object instance,
    <code>a.x</code> is transformed into the call:
    <code>type(a).__dict__['x'].__get__(a, type(a))</code>.

<P>
</DD>
<DT>Class Binding</DT>
<DD>If binding to a new-style class, <code>A.x</code>
    is transformed into the call: <code>A.__dict__['x'].__get__(None, A)</code>.

<P>
</DD>
<DT>Super Binding</DT>
<DD>If <code>a</code> is an instance of <tt class="class">super</tt>,
    then the binding <code>super(B, obj).m()</code> searches
    <code>obj.__class__.__mro__</code> for the base class <code>A</code> immediately
    preceding <code>B</code> and then invokes the descriptor with the call:
    <code>A.__dict__['m'].__get__(obj, A)</code>.

<P>
</DD>
</DL>

<P>
For instance bindings, the precedence of descriptor invocation depends
on the which descriptor methods are defined.  Data descriptors define
both <tt class="method">__get__()</tt> and <tt class="method">__set__()</tt>.  Non-data descriptors have
just the <tt class="method">__get__()</tt> method.  Data descriptors always override
a redefinition in an instance dictionary.  In contrast, non-data
descriptors can be overridden by instances.

<P>
Python methods (including <tt class="function">staticmethod()</tt> and <tt class="function">classmethod()</tt>)
are implemented as non-data descriptors.  Accordingly, instances can
redefine and override methods.  This allows individual instances to acquire
behaviors that differ from other instances of the same class.

<P>
The <tt class="function">property()</tt> function is implemented as a data descriptor.
Accordingly, instances cannot override the behavior of a property.

<P>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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