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copied!<p>Each object has a dictionary of attributes that contains it's variables and functions. Referring to these dictionaries:</p> <pre><code>If an instance’s dictionary has an entry with the same name as a data descriptor, the data descriptor takes precedence. If an instance’s dictionary has an entry with the same name as a non-data descriptor, the dictionary entry takes precedence. </code></pre> <p>This is what they were talking about.</p> <p>To show this:</p> <pre><code>#data descriptor class A(object): def __get__(self, obj, type): print "hello from get A" def __set__(self, obj, val): print "hello from set A" #non data descriptor class B(object): def __get__(self, obj, type): print "hello from get B" class C(object): #our data descriptor a = A() #our non data descriptor b = B() >>> c = C() >>> c.a hello from get A >>> c.b hello from get B >>> c.a = 0 hello from set A >>> c.a #notice after reassignment, A.__get__ is still called hello from set A >>> c.b = 0 #instance variable with the same name as the non data-descriptor >>> c.b #notice how B.__get__ isn't called anymore 0 </code></pre> <p>Basically it is saying that when <code>__get__</code> and <code>__set__</code> are user defined for an object (data descriptor), they will be called instead of the default methods. If only <code>__get__</code> is user defined for an object(non-data descriptor), the instance can reassign am instance variable.</p> <p>So when calling <code>g.x = 0</code>: if x is a data descriptor then x's user-defined <code>__set__</code> method is called, when x is an instance variable, or non-data descriptor the default behavior is called.</p> <p>With a <strong>data descriptor</strong>, the class is controlling all access and modification to the variables. <strong>All access to variables of your data descriptor type will go through <code>__get__</code> and <code>__set__</code></strong>. So <code>c.a = 0</code> calls <code>A.__set__</code> and c.a is changed how the class defined it. There is no way to create an instance variable 'c.a' that is not of the type A. </p> <p>With a <strong>non data descriptor</strong>, the class only controls access, so when <code>c.b = 0</code> is called, since <code>__set__</code> isn't defined, a new instance variable is made(the default behavior). There is no user-defined behavior on setting the variable, so you can create an instance variable with the same name, without the <code>__get__</code> behavior.</p> <p>The precedence they are talking about is the dynamics of the two. A data descriptor will always call <code>__get__</code> and <code>__set__</code>, and so an instance variable can't be created with the same name. A non data descriptor will only call <code>__get__</code> until an instance variable with the same name is created.</p>

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