{"id":2739,"date":"2021-11-05T10:01:22","date_gmt":"2021-11-05T10:01:22","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2739"},"modified":"2021-11-14T07:03:37","modified_gmt":"2021-11-14T07:03:37","slug":"python-data-descriptors","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-data-descriptors\/","title":{"rendered":"Python Data vs. Non-data Descriptors"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you&#8217;ll learn the differences between data and non-data descriptors.<\/p>\n\n\n\n<p><a href=\"https:\/\/www.pythontutorial.net\/python-oop\/python-descriptors\/\">Descriptors<\/a> have two types:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Data descriptors are objects of a class that implements <code>__set__<\/code> method (and\/or <code>__delete__<\/code> method)<\/li><li>Non-data descriptors are objects of a class that have the <code>__get__<\/code> method only.<\/li><\/ul>\n\n\n\n<p>Both descriptor types can optionally implement the <code>__set_name__<\/code> method. The <code>__set_name__<\/code> method doesn&#8217;t affect the classification of the descriptors.<\/p>\n\n\n\n<p>The descriptor types determine how Python resolves object&#8217;s attributes lookup.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='non-data-descriptor'>Non-data descriptor <a href=\"#non-data-descriptor\" class=\"anchor\" id=\"non-data-descriptor\" title=\"Anchor for Non-data descriptor\">#<\/a><\/h2>\n\n\n\n<p>If a class uses a non-data descriptor, Python will search the attribute in instance attributes first (<code>instance.__dict__<\/code>). If Python doesn&#8217;t find the attribute in the instance attributes, it&#8217;ll use the data descriptor.<\/p>\n\n\n\n<p>Let&#8217;s take a look at the following example.<\/p>\n\n\n\n<p>First, define a non-data descriptor class <code>FileCount<\/code> that has the <code>__get__<\/code> method which returns the number of files in a folder:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-1\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\"><span class=\"hljs-class\"><span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">FileCount<\/span>:<\/span>\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">__get__<\/span><span class=\"hljs-params\">(self, instance, owner)<\/span>:<\/span>\n        print(<span class=\"hljs-string\">'The __get__ was called'<\/span>)\n        <span class=\"hljs-keyword\">return<\/span> len(os.listdir(instance.path))<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-1\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Second, define a <code>Folder<\/code> class that uses the <code>FileCount<\/code> descriptor:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-2\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\"><span class=\"hljs-class\"><span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">Folder<\/span>:<\/span>\n    count = FileCount()\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">__init__<\/span><span class=\"hljs-params\">(self, path)<\/span>:<\/span>\n        self.path = path<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-2\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Third, create an instance of the <code>Folder<\/code> class and access the <code>count<\/code> attribute:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-3\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">folder = Folder(<span class=\"hljs-string\">'\/'<\/span>)\nprint(<span class=\"hljs-string\">'file count: '<\/span>, folder.count)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-3\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Python called the <code>__get__<\/code> descriptor:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-4\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">The __get__ was called\nfile count:  <span class=\"hljs-number\">32<\/span><\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-4\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>After that, set the <code>count<\/code> attribute of the <code>folder<\/code> instance to <code>100<\/code> and access the count attribute:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-5\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">folder.__dict__&#91;<span class=\"hljs-string\">'count'<\/span>] = <span class=\"hljs-number\">100<\/span>\nprint(<span class=\"hljs-string\">'file count: '<\/span>, folder.count)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-5\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Output:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-6\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">file count:  <span class=\"hljs-number\">100<\/span><\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-6\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>In this example, Python can find the <code>count<\/code> attribute in the instance dictionary <code>__dict__<\/code>. Therefore, it does not use data descriptors.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='data-descriptor'>Data descriptor <a href=\"#data-descriptor\" class=\"anchor\" id=\"data-descriptor\" title=\"Anchor for Data descriptor\">#<\/a><\/h2>\n\n\n\n<p>When a class has a data descriptor, Python will look for an instance&#8217;s attribute in the data descriptor first. If Python doesn&#8217;t find the attribute, it&#8217;ll look for the attribute in the instance dictionary (<code>__dict__<\/code>). For example:<\/p>\n\n\n\n<p>First, define a <code>Coordinate<\/code> descriptor class:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-7\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\"><span class=\"hljs-class\"><span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">Coordinate<\/span>:<\/span>\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">__get__<\/span><span class=\"hljs-params\">(self, instance, owner)<\/span>:<\/span>\n        print(<span class=\"hljs-string\">'The __get__ was called'<\/span>)\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">__set__<\/span><span class=\"hljs-params\">(self, instance, value)<\/span>:<\/span>\n        print(<span class=\"hljs-string\">'The __set__ was called'<\/span>)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-7\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Second, define a <code>Point<\/code> class that uses the <code>Coordinate<\/code> descriptor:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-8\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\"><span class=\"hljs-class\"><span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">Point<\/span>:<\/span>\n    x = Coordinate()\n    y = Coordinate()<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-8\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Third, create a new instance of the <code>Point<\/code> class and assign a value to the <code>x<\/code> attribute of the <code>p<\/code> instance:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-9\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">p = Point()\np.x = <span class=\"hljs-number\">10<\/span><\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-9\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Output:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-10\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">The __set__ was called<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-10\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Python called the <code>__set__<\/code> method of the <code>x<\/code> descriptor.<\/p>\n\n\n\n<p>Finally, access the <code>x<\/code> attribute of the <code>p<\/code> instance:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-11\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">p.x<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-11\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Output:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-12\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">The __get__ was called<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-12\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">Python<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">python<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Python called the <code>__get__<\/code> method of the <code>x<\/code> descriptor.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='summary'>Summary <a href=\"#summary\" class=\"anchor\" id=\"summary\" title=\"Anchor for Summary\">#<\/a><\/h2>\n\n\n\n<ul class=\"wp-block-list\"><li>Data descriptors are objects of a class that implements <code>__set__<\/code> method (and\/or <code>__delete__<\/code> method)<\/li><li>Non-data descriptors are objects of a class that have the <code>__get__<\/code> method only.<\/li><li>When accessing object&#8217;s attributes, data descriptors override the instance&#8217;s attributes and instance&#8217;s attributes override non-data descriptors.<\/li><\/ul>\n<div class=\"helpful-block-content\" data-title=\"\">\n\t<header>\n\t\t<div class=\"wth-question\">Was this tutorial helpful ?<\/div>\n\t\t<div class=\"wth-thumbs\">\n\t\t\t<button\n\t\t\t\tdata-post=\"2739\"\n\t\t\t\tdata-post-url=\"https:\/\/www.pythontutorial.net\/python-oop\/python-data-descriptors\/\"\n\t\t\t\tdata-post-title=\"Python Data vs. Non-data Descriptors\"\n\t\t\t\tdata-response=\"1\"\n\t\t\t\tclass=\"wth-btn-rounded wth-yes-btn\"\n\t\t\t>\n\t\t\t\t<svg\n\t\t\t\t\txmlns=\"http:\/\/www.w3.org\/2000\/svg\"\n\t\t\t\t\tviewBox=\"0 0 24 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non-data 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