{"id":3680,"date":"2022-07-05T04:10:24","date_gmt":"2022-07-05T04:10:24","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=3680"},"modified":"2022-07-09T07:43:28","modified_gmt":"2022-07-09T07:43:28","slug":"python-patch","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-unit-testing\/python-patch\/","title":{"rendered":"Python patch()"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you&#8217;ll learn how to use the Python <code>patch()<\/code> to replace a target with a mock object temporarily.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='introduction-to-the-python-patch'>Introduction to the Python patch <a href=\"#introduction-to-the-python-patch\" class=\"anchor\" id=\"introduction-to-the-python-patch\" title=\"Anchor for Introduction to the Python patch\">#<\/a><\/h2>\n\n\n\n<p>The <code>unittest.mock<\/code> module has a <code>patch()<\/code> that allows you to temporarily replace a target with a mock object.<\/p>\n\n\n\n<p>A target can be a <a href=\"https:\/\/www.pythontutorial.net\/python-basics\/python-functions\/\">function<\/a>, a <a href=\"https:\/\/www.pythontutorial.net\/python-oop\/python-methods\/\">method<\/a>, or a <a href=\"https:\/\/www.pythontutorial.net\/python-oop\/python-class\/\">class<\/a>. It&#8217;s a string with the following format:<\/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-string\">'package.module.className'<\/span><\/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>To use the <code>patch()<\/code> correctly, you need to understand two important steps:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Identify the target<\/li><li>How to call <code>patch()<\/code><\/li><\/ul>\n\n\n\n<h3 class=\"wp-block-heading\" id='identifying-the-target'>Identifying the target <a href=\"#identifying-the-target\" class=\"anchor\" id=\"identifying-the-target\" title=\"Anchor for Identifying the target\">#<\/a><\/h3>\n\n\n\n<p>To identify a target:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>The target must be importable.<\/li><li>And patch the target where it is used, not where it comes from.<\/li><\/ul>\n\n\n\n<h3 class=\"wp-block-heading\" id='calling-patch'>Calling patch <a href=\"#calling-patch\" class=\"anchor\" id=\"calling-patch\" title=\"Anchor for Calling patch\">#<\/a><\/h3>\n\n\n\n<p>Python provides you with three ways to call <code>patch()<\/code>: <\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Decorators for a function or a class.<\/li><li>Context manager<\/li><li>Manual start\/stop<\/li><\/ul>\n\n\n\n<p>When you use the <code>patch()<\/code> as a decorator of a function or class, inside the function or class the target is replaced with a new object.<\/p>\n\n\n\n<p>If you use the patch in a context manager, inside the <code>with<\/code> statement, the target is replaced with a new object.<\/p>\n\n\n\n<p>In both cases, when the function or the <code>with<\/code> statement exits, the patch is undone.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='python-patch-examples'>Python patch examples <a href=\"#python-patch-examples\" class=\"anchor\" id=\"python-patch-examples\" title=\"Anchor for Python patch examples\">#<\/a><\/h2>\n\n\n\n<p>Let&#8217;s create a new module called <code>total.py<\/code> for demonstration purposes:<\/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-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">read<\/span><span class=\"hljs-params\">(filename)<\/span>:<\/span>\n    <span class=\"hljs-string\">\"\"\" read a text file and return a list of numbers \"\"\"<\/span>\n    <span class=\"hljs-keyword\">with<\/span> open(filename) <span class=\"hljs-keyword\">as<\/span> f:\n        lines = f.readlines()\n        <span class=\"hljs-keyword\">return<\/span> &#91;float(line.strip()) <span class=\"hljs-keyword\">for<\/span> line <span class=\"hljs-keyword\">in<\/span> lines]\n\n\n<span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">calculate_total<\/span><span class=\"hljs-params\">(filename)<\/span>:<\/span>\n    <span class=\"hljs-string\">\"\"\" return the sum of numbers in a text file \"\"\"<\/span>\n    numbers = read(filename)\n    <span class=\"hljs-keyword\">return<\/span> sum(numbers)<\/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>How it works.<\/p>\n\n\n\n<p>The <code>read()<\/code> function reads a text file, converts each line into a number, and returns a list of numbers. For example, a text file has the following lines:<\/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\"><span class=\"hljs-number\">1<\/span>\n<span class=\"hljs-number\">2<\/span>\n<span class=\"hljs-number\">3<\/span><\/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>the <code>read()<\/code> function will return the following list:<\/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\">&#91;<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">2<\/span>, <span class=\"hljs-number\">3<\/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>The <code>calculate_total()<\/code> function uses the <code>read()<\/code> function to get a list of numbers from a file and returns the sum of the numbers.<\/p>\n\n\n\n<p>To test <code>calculate_total()<\/code>, you can create a <code>test_total_mock.py<\/code> module and mock the <code>read()<\/code> function as follows:<\/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\"><span class=\"hljs-keyword\">import<\/span> unittest\n\n<span class=\"hljs-keyword\">from<\/span> unittest.mock <span class=\"hljs-keyword\">import<\/span> MagicMock\n\n<span class=\"hljs-keyword\">import<\/span> total\n\n\n<span class=\"hljs-class\"><span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">TestTotal<\/span><span class=\"hljs-params\">(unittest.TestCase)<\/span>:<\/span>\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">test_calculate_total<\/span><span class=\"hljs-params\">(self)<\/span>:<\/span>\n        total.read = MagicMock()\n        total.read.return_value = &#91;<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">2<\/span>, <span class=\"hljs-number\">3<\/span>]\n        result = total.calculate_total(<span class=\"hljs-string\">''<\/span>)\n        self.assertEqual(result, <span class=\"hljs-number\">6<\/span>)<\/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>Run the test:<\/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\">python -m unittest test_total_mock.py -v<\/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>Output:<\/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\">test_calculate_total (test_total_mock.TestTotal) ... ok\n\n----------------------------------------------------------------------\nRan <span class=\"hljs-number\">1<\/span> test <span class=\"hljs-keyword\">in<\/span> <span class=\"hljs-number\">0.001<\/span>s\n\nOK<\/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>Instead of using the <code>MagicMock()<\/code> object directly, you can use the <code>patch()<\/code>.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id='1-using-patch-as-a-decorator'>1) Using patch() as a decorator <a href=\"#1-using-patch-as-a-decorator\" class=\"anchor\" id=\"1-using-patch-as-a-decorator\" title=\"Anchor for 1) Using patch() as a decorator\">#<\/a><\/h3>\n\n\n\n<p>The following test module <code>test_total_with_patch_decorator.py<\/code> tests the <code>total.py<\/code> module using the <code>patch()<\/code> as a function decorator:<\/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-keyword\">import<\/span> unittest\n<span class=\"hljs-keyword\">from<\/span> unittest.mock <span class=\"hljs-keyword\">import<\/span> patch\n<span class=\"hljs-keyword\">import<\/span> total\n\n\n<span class=\"hljs-class\"><span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">TestTotal<\/span><span class=\"hljs-params\">(unittest.TestCase)<\/span>:<\/span>\n<span class=\"hljs-meta\">    @patch('total.read')<\/span>\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">test_calculate_total<\/span><span class=\"hljs-params\">(self, mock_read)<\/span>:<\/span>\n        mock_read.return_value = &#91;<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">2<\/span>, <span class=\"hljs-number\">3<\/span>]\n        result = total.calculate_total(<span class=\"hljs-string\">''<\/span>)\n        self.assertEqual(result, <span class=\"hljs-number\">6<\/span>)<\/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>How it works.<\/p>\n\n\n\n<p>First, import the patch from the <code>unittest.mock<\/code> module:<\/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\"><span class=\"hljs-keyword\">from<\/span> unittest.mock <span class=\"hljs-keyword\">import<\/span> patch<\/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>Second, decorate the <code>test_calculate_total()<\/code> test method with the <code>@patch<\/code> decorator. The target is the read function of the total module.<\/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\"><span class=\"hljs-meta\">@patch('total.read')<\/span>\n<span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">test_calculate_total<\/span><span class=\"hljs-params\">(self, mock_read)<\/span>:<\/span>\n   <span class=\"hljs-comment\"># ...<\/span><\/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>Because of the <code>@patch<\/code> decorator, the <code>test_calculate_total()<\/code> method has an additional argument mock_read which is an instance of the MagicMock. <\/p>\n\n\n\n<p>Note that you can name the parameter whatever you want.<\/p>\n\n\n\n<p>Inside the <code>test_calculate_total()<\/code> method, the <code>patch()<\/code> will replace the total.<code>read()<\/code> function with the mock_read object.<\/p>\n\n\n\n<p>Third, assign a list to the return_value of the mock object:<\/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\"> mock_read.return_value = &#91;<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">2<\/span>, <span class=\"hljs-number\">3<\/span>]<\/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>Finally, call the <code>calculate_total()<\/code> function and use the <code>assertEqual()<\/code> method to test if the total is 6. <\/p>\n\n\n\n<p>Because the mock_read object will be called instead of the total.<code>read()<\/code> function, you can pass any filename to the <code>calculate_total()<\/code> function:<\/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\">result = total.calculate_total(<span class=\"hljs-string\">''<\/span>)\nself.assertEqual(result, <span class=\"hljs-number\">6<\/span>)<\/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>Run the test:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-13\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">python -m unittest test_total_patch_decorator -v<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-13\"><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-14\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">test_calculate_total (test_total_patch_decorator.TestTotal) ... ok\n\n----------------------------------------------------------------------\nRan <span class=\"hljs-number\">1<\/span> test <span class=\"hljs-keyword\">in<\/span> <span class=\"hljs-number\">0.001<\/span>s\n\nOK<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-14\"><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<h3 class=\"wp-block-heading\" id='2-using-patch-as-a-context-manager'>2) Using patch() as a context manager <a href=\"#2-using-patch-as-a-context-manager\" class=\"anchor\" id=\"2-using-patch-as-a-context-manager\" title=\"Anchor for 2) Using patch() as a context manager\">#<\/a><\/h3>\n\n\n\n<p>The following example illustrates how to use the <code>patch()<\/code> as a context manager:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-15\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\"><span class=\"hljs-keyword\">import<\/span> unittest\n<span class=\"hljs-keyword\">from<\/span> unittest.mock <span class=\"hljs-keyword\">import<\/span> patch\n<span class=\"hljs-keyword\">import<\/span> total\n\n\n<span class=\"hljs-class\"><span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">TestTotal<\/span><span class=\"hljs-params\">(unittest.TestCase)<\/span>:<\/span>\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">test_calculate_total<\/span><span class=\"hljs-params\">(self)<\/span>:<\/span>\n        <span class=\"hljs-keyword\">with<\/span> patch(<span class=\"hljs-string\">'total.read'<\/span>) <span class=\"hljs-keyword\">as<\/span> mock_read:\n            mock_read.return_value = &#91;<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">2<\/span>, <span class=\"hljs-number\">3<\/span>]\n            result = total.calculate_total(<span class=\"hljs-string\">''<\/span>)\n            self.assertEqual(result, <span class=\"hljs-number\">6<\/span>)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-15\"><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>How it works.<\/p>\n\n\n\n<p>First, patch <code>total.read()<\/code> function using as the <code>mock_read<\/code> object in a context manager:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-16\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\"><span class=\"hljs-keyword\">with<\/span> patch(<span class=\"hljs-string\">'total.read'<\/span>) <span class=\"hljs-keyword\">as<\/span> mock_read:<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-16\"><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>It means that within the <code>with<\/code> block, the <code>patch()<\/code> replaces the <code>total.read()<\/code> function with the mock_read object.<\/p>\n\n\n\n<p>Second, assign a list of numbers to the <code>return_value<\/code> property of the <code>mock_read<\/code> object:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-17\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">mock_read.return_value = &#91;<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">2<\/span>, <span class=\"hljs-number\">3<\/span>]<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-17\"><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, call the <code>calculate_total()<\/code> function and test if the result of the <code>calculate_total()<\/code> function is equal 6 using the <code>assertEqual()<\/code> method:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-18\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">result = total.calculate_total(<span class=\"hljs-string\">''<\/span>)\nself.assertEqual(result, <span class=\"hljs-number\">6<\/span>)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-18\"><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>Run the test:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-19\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">python -m unittest test_total_patch_ctx_mgr -v<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-19\"><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-20\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">test_calculate_total (test_total_patch_ctx_mgr.TestTotal) ... ok\n\n----------------------------------------------------------------------\nRan <span class=\"hljs-number\">1<\/span> test <span class=\"hljs-keyword\">in<\/span> <span class=\"hljs-number\">0.001<\/span>s\n\nOK<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-20\"><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<h3 class=\"wp-block-heading\" id='3-using-patch-manually'>3) Using patch() manually <a href=\"#3-using-patch-manually\" class=\"anchor\" id=\"3-using-patch-manually\" title=\"Anchor for 3) Using patch() manually\">#<\/a><\/h3>\n\n\n\n<p>The following test module (<code>test_total_patch_manual.py<\/code>) shows how to use <code>patch()<\/code> manually:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-21\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\"><span class=\"hljs-keyword\">import<\/span> unittest\n<span class=\"hljs-keyword\">from<\/span> unittest.mock <span class=\"hljs-keyword\">import<\/span> patch\n<span class=\"hljs-keyword\">import<\/span> total\n\n\n<span class=\"hljs-class\"><span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">TestTotal<\/span><span class=\"hljs-params\">(unittest.TestCase)<\/span>:<\/span>\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">test_calculate_total<\/span><span class=\"hljs-params\">(self)<\/span>:<\/span>\n        <span class=\"hljs-comment\"># start patching<\/span>\n        patcher = patch(<span class=\"hljs-string\">'total.read'<\/span>)\n\n        <span class=\"hljs-comment\"># create a mock object<\/span>\n        mock_read = patcher.start()\n\n        <span class=\"hljs-comment\"># assign the return value<\/span>\n        mock_read.return_value = &#91;<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">2<\/span>, <span class=\"hljs-number\">3<\/span>]\n\n        <span class=\"hljs-comment\"># test the calculate_total<\/span>\n        result = total.calculate_total(<span class=\"hljs-string\">''<\/span>)\n        self.assertEqual(result, <span class=\"hljs-number\">6<\/span>)\n\n        <span class=\"hljs-comment\"># stop patching<\/span>\n        patcher.stop()<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-21\"><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>How it works.<\/p>\n\n\n\n<p>First, start a patch by calling <code>patch()<\/code> with a target is the <code>read()<\/code> function of the <code>total<\/code> module:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-22\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">patcher = patch(<span class=\"hljs-string\">'total.read'<\/span>)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-22\"><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>Next, create a mock object for the <code>read()<\/code> function:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-23\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">mock_read = patcher.start()<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-23\"><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>Then, assign a list of numbers to the <code>return_value<\/code> of the <code>mock_read<\/code> object:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-24\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">result = total.calculate_total(<span class=\"hljs-string\">''<\/span>)\nself.assertEqual(result, <span class=\"hljs-number\">6<\/span>)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-24\"><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, call the <code>calculate_total()<\/code> and test its result.<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-25\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\"><span class=\"hljs-function\"><span class=\"hljs-keyword\">def<\/span> <span class=\"hljs-title\">test_calculate_total<\/span><span class=\"hljs-params\">(self)<\/span>:<\/span>\n    self.mock_read.return_value = &#91;<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">2<\/span>, <span class=\"hljs-number\">3<\/span>]\n    result = total.calculate_total(<span class=\"hljs-string\">''<\/span>)\n    self.assertEqual(result, <span class=\"hljs-number\">6<\/span>)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-25\"><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>Finally, stop patching by calling the <code>stop()<\/code> method of the patcher object:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-26\" data-shcb-language-name=\"Python\" data-shcb-language-slug=\"python\"><span><code class=\"hljs language-python\">patcher.stop()<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-26\"><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<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>Use the <code>patch()<\/code> from <code>unittest.mock<\/code> module to temporarily replace a target with a mock object.<\/li><li>Use the <code>patch()<\/code> as a decorator, a context manager, or manually call <code>start()<\/code> and <code>stop()<\/code> patching.<\/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=\"3680\"\n\t\t\t\tdata-post-url=\"https:\/\/www.pythontutorial.net\/python-unit-testing\/python-patch\/\"\n\t\t\t\tdata-post-title=\"Python patch()\"\n\t\t\t\tdata-response=\"1\"\n\t\t\t\tclass=\"wth-btn-rounded 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target with a mock object 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