{"id":30225,"date":"2024-01-01T20:51:50","date_gmt":"2024-01-01T15:21:50","guid":{"rendered":"http:\/\/www.pythonpool.com\/?p=30225"},"modified":"2026-07-14T13:36:04","modified_gmt":"2026-07-14T08:06:04","slug":"pytest-assert-function-called","status":"publish","type":"post","link":"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/","title":{"rendered":"Pytest Assert Function Called With Mock"},"content":{"rendered":"<p>In pytest, the cleanest way to assert that a function was called is usually to replace the dependency with a mock, run the behavior under test, and then inspect that mock. Pytest does not need a special assertion API for this: the call assertions come from Python&#8217;s <code>unittest.mock<\/code> module, and pytest reports a failed mock assertion as a normal test failure.<\/p>\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_85 counter-hierarchy ez-toc-counter ez-toc-transparent ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #990303;color:#990303\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #990303;color:#990303\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 eztoc-toggle-hide-by-default' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Quick_Answer\" >Quick Answer<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Mock_The_Dependency_Boundary\" >Mock The Dependency Boundary<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Choose_The_Right_Call_Assertion\" >Choose The Right Call Assertion<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Assert_A_Call_And_Its_Arguments\" >Assert A Call And Its Arguments<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Patch_Where_The_Code_Looks_Up_The_Name\" >Patch Where The Code Looks Up The Name<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Use_pytest_monkeypatch_For_Small_Replacements\" >Use pytest monkeypatch For Small Replacements<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Mock_Return_Values_And_Side_Effects\" >Mock Return Values And Side Effects<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Call_Assertions_For_Async_Functions\" >Call Assertions For Async Functions<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-9\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Common_Mock_Assertion_Mistakes\" >Common Mock Assertion Mistakes<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-10\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Separate_Return_Value_Assertions_From_Call_Assertions\" >Separate Return Value Assertions From Call Assertions<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-11\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Use_Autospec_When_The_Signature_Matters\" >Use Autospec When The Signature Matters<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-12\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Keep_Mock_Assertions_Readable\" >Keep Mock Assertions Readable<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-13\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Frequently_Asked_Questions\" >Frequently Asked Questions<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-14\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#How_do_I_assert_a_function_was_called_in_pytest\" >How do I assert a function was called in pytest?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-15\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#What_is_the_difference_between_assert_called_and_assert_called_once\" >What is the difference between assert_called and assert_called_once?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-16\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#Where_should_I_patch_a_function_in_pytest\" >Where should I patch a function in pytest?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-17\" href=\"https:\/\/www.pythonpool.com\/pytest-assert-function-called\/#How_do_I_check_arguments_for_multiple_calls\" >How do I check arguments for multiple calls?<\/a><\/li><\/ul><\/li><\/ul><\/nav><\/div>\n<h2><span class=\"ez-toc-section\" id=\"Quick_Answer\"><\/span>Quick Answer<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Patch the dependency where the code under test looks it up, execute the behavior, then use <code>assert_called()<\/code>, <code>assert_called_once()<\/code>, <code>assert_called_with()<\/code>, or <code>assert_called_once_with()<\/code> according to the contract you actually need. Inspect <code>call_args<\/code> or <code>call_args_list<\/code> when the arguments matter.<\/p>\n<figure class=\"pythonpool-article-visual\"><img src=\"https:\/\/www.pythonpool.com\/wp-content\/uploads\/2026\/07\/pytest-mock-call-assertions.png\" alt=\"Pytest Mock call assertion flow from code under test to assert_called_once_with\" width=\"1536\" height=\"1024\" loading=\"lazy\" decoding=\"async\"><figcaption>Mock the dependency boundary, run the real behavior, then assert the call and arguments that matter.<\/figcaption><\/figure>\n<p>The Python <a href=\"https:\/\/docs.python.org\/3\/library\/unittest.mock.html\" target=\"_blank\" rel=\"noopener noreferrer\">unittest.mock reference<\/a> documents the call assertion methods, while the <a href=\"https:\/\/docs.pytest.org\/en\/stable\/\" target=\"_blank\" rel=\"noopener noreferrer\">pytest documentation<\/a> covers fixtures, monkeypatching, and test discovery. Use the mock for an external interaction and an ordinary <code>assert<\/code> for a return value.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Mock_The_Dependency_Boundary\"><\/span>Mock The Dependency Boundary<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Suppose an application sends a welcome email. The test should not send a real email; it should replace the email service and verify the interaction.<\/p>\n<pre><code class=\"language-python\">from unittest.mock import Mock\n\ndef send_welcome(email_service, address):\n    email_service.send(address, subject='Welcome')\n\ndef test_sends_welcome_email():\n    email_service = Mock()\n\n    send_welcome(email_service, 'user@example.com')\n\n    email_service.send.assert_called_once_with(\n        'user@example.com',\n        subject='Welcome',\n    )\n<\/code><\/pre>\n<p>The assertion checks both that the method was called and that its arguments match. It does not prove that an SMTP server accepted a message; that belongs in an integration test.<\/p>\n<p><!-- Python Pool visual layout repair 2026-07-13 --><\/p>\n<figure class=\"pythonpool-article-visual pythonpool-supporting-visual\"><img src=\"https:\/\/www.pythonpool.com\/wp-content\/uploads\/2026\/07\/pytest-mock-patch-b179.png\" alt=\"Python Pool infographic showing a pytest test, patched dependency, mock object, and function under test\" width=\"1536\" height=\"1054\" loading=\"lazy\" decoding=\"async\"><figcaption>Patch dependency: A pytest test, patched dependency, mock object, and function under test.<\/figcaption><\/figure>\n<h2><span class=\"ez-toc-section\" id=\"Choose_The_Right_Call_Assertion\"><\/span>Choose The Right Call Assertion<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Use the least restrictive assertion that expresses the behavior:<\/p>\n<ul>\n<li><code>assert_called()<\/code> means the mock was called at least once.<\/li>\n<li><code>assert_called_once()<\/code> means it was called exactly once, regardless of arguments.<\/li>\n<li><code>assert_called_with()<\/code> checks the most recent call and its arguments.<\/li>\n<li><code>assert_called_once_with()<\/code> checks that there was exactly one call and that its arguments match.<\/li>\n<li><code>assert_not_called()<\/code> verifies that the dependency was not used.<\/li>\n<\/ul>\n<p>Do not use <code>assert_called_once_with()<\/code> merely because it sounds stronger. If retries are a valid part of the behavior, an exact-once assertion can make a correct test fail.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Assert_A_Call_And_Its_Arguments\"><\/span>Assert A Call And Its Arguments<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>When a function may be called several times, inspect the recorded calls rather than asserting only the last one.<\/p>\n<pre><code class=\"language-python\">from unittest.mock import Mock, call\n\ndef publish_all(client, messages):\n    for message in messages:\n        client.publish(message)\n\ndef test_publishes_each_message():\n    client = Mock()\n\n    publish_all(client, ['one', 'two'])\n\n    assert client.publish.call_args_list == [\n        call('one'),\n        call('two'),\n    ]\n<\/code><\/pre>\n<p><code>call_args<\/code> contains the arguments from the most recent call. <code>call_args_list<\/code> preserves the sequence, which is useful when order is part of the contract. <code>mock_calls<\/code> can include nested calls when the dependency has a chain of methods.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Patch_Where_The_Code_Looks_Up_The_Name\"><\/span>Patch Where The Code Looks Up The Name<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>If a module imports a function directly, patch the name inside that module rather than patching the original library globally. This is one of the most common reasons a mock appears not to work.<\/p>\n<pre><code class=\"language-python\">from unittest.mock import patch\n\ndef load_user(user_id):\n    response = requests.get('\/users\/' + str(user_id))\n    return response.json()\n\ndef test_load_user():\n    fake_response = type('Response', (), {\n        'json': lambda self: {'id': 7}\n    })()\n\n    with patch(__name__ + '.requests.get',\n               return_value=fake_response) as get:\n        assert load_user(7) == {'id': 7}\n        get.assert_called_once_with('\/users\/7')\n<\/code><\/pre>\n<p>In a real module, use the import path of the module under test. If the code says <code>from requests import get<\/code>, patch <code>module_under_test.get<\/code>. If it says <code>import requests<\/code>, patch <code>module_under_test.requests.get<\/code>.<\/p>\n<figure class=\"pythonpool-article-visual pythonpool-supporting-visual\"><img src=\"https:\/\/www.pythonpool.com\/wp-content\/uploads\/2026\/07\/pytest-mock-call-b179.png\" alt=\"Python Pool infographic mapping a function call through a Mock to call arguments and count\" width=\"1536\" height=\"1054\" loading=\"lazy\" decoding=\"async\"><figcaption>Record call: A function call through a Mock to call arguments and count.<\/figcaption><\/figure>\n<h2><span class=\"ez-toc-section\" id=\"Use_pytest_monkeypatch_For_Small_Replacements\"><\/span>Use pytest monkeypatch For Small Replacements<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>pytest&#8217;s <code>monkeypatch<\/code> fixture is useful when a test needs to replace an attribute, environment variable, or current directory for the duration of the test.<\/p>\n<pre><code class=\"language-python\">def read_mode(config):\n    return config.mode\n\ndef test_read_mode(monkeypatch):\n    config = type('Config', (), {'mode': 'test'})()\n    monkeypatch.setattr('module_under_test.config', config)\n\n    assert read_mode(config) == 'test'\n<\/code><\/pre>\n<p>Use a Mock when you need call history or configurable return values. Use monkeypatch when the primary job is to replace a value or attribute and no call assertion is required.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Mock_Return_Values_And_Side_Effects\"><\/span>Mock Return Values And Side Effects<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Configure a mock before running the code under test. A return value models a successful dependency; a side effect can model an exception or a sequence of results.<\/p>\n<pre><code class=\"language-python\">from unittest.mock import Mock\n\nclient = Mock()\nclient.fetch.return_value = {'status': 'ok'}\nclient.fetch.side_effect = [ValueError('temporary'), {'status': 'ok'}]\n<\/code><\/pre>\n<p>Do not put so much implementation detail into a mock that the test only proves the code called itself in the expected order. Focus on the observable collaboration that matters to the feature.<\/p>\n<figure class=\"pythonpool-article-visual pythonpool-supporting-visual\"><img src=\"https:\/\/www.pythonpool.com\/wp-content\/uploads\/2026\/07\/pytest-mock-assert-b179.png\" alt=\"Python Pool infographic comparing assert_called_once, assert_called_with, call count, and failure\" width=\"1536\" height=\"1054\" loading=\"lazy\" decoding=\"async\"><figcaption>Assert called: Assert_called_once, assert_called_with, call count, and failure.<\/figcaption><\/figure>\n<h2><span class=\"ez-toc-section\" id=\"Call_Assertions_For_Async_Functions\"><\/span>Call Assertions For Async Functions<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>For an async dependency, use <code>AsyncMock<\/code> and await the code under test. A normal Mock can record a call but does not automatically behave like an awaitable dependency.<\/p>\n<pre><code class=\"language-python\">from unittest.mock import AsyncMock\n\nasync def test_fetch_async():\n    client = AsyncMock()\n    client.fetch.return_value = {'status': 'ok'}\n\n    result = await client.fetch('\/health')\n\n    assert result == {'status': 'ok'}\n    client.fetch.assert_awaited_once_with('\/health')\n<\/code><\/pre>\n<p>Use the async assertion methods when the contract is about awaiting. This catches a subtle bug where code calls an async dependency but forgets to await it.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Common_Mock_Assertion_Mistakes\"><\/span>Common Mock Assertion Mistakes<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<ul>\n<li>Creating a Mock but never injecting it into the code under test.<\/li>\n<li>Patching the library&#8217;s original name instead of the imported name used by the module.<\/li>\n<li>Using an exact-once assertion when retries are valid.<\/li>\n<li>Asserting a private call sequence instead of a meaningful behavior.<\/li>\n<li>Using a normal Mock for an awaited dependency.<\/li>\n<\/ul>\n<p>The practical rule is simple: isolate the side effect, run the real behavior, and assert the smallest interaction contract that matters. That produces tests that are strict about important behavior without becoming coupled to harmless refactors.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Separate_Return_Value_Assertions_From_Call_Assertions\"><\/span>Separate Return Value Assertions From Call Assertions<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>A mock call assertion answers whether the code collaborated with a dependency. It does not automatically prove that the function returned the right value. Keep those questions separate so a failure tells you which behavior changed.<\/p>\n<p>For pure logic, prefer a normal value assertion and avoid a mock entirely. For side effects, assert the dependency call and use a small fake response when the code needs to continue through a success path.<\/p>\n<figure class=\"pythonpool-article-visual pythonpool-supporting-visual\"><img src=\"https:\/\/www.pythonpool.com\/wp-content\/uploads\/2026\/07\/pytest-mock-check-b179.png\" alt=\"Python Pool infographic testing patch target, autospec, side effects, return values, and validation\" width=\"1536\" height=\"1054\" loading=\"lazy\" decoding=\"async\"><figcaption>Mock checks: Patch target, autospec, side effects, return values, and validation.<\/figcaption><\/figure>\n<h2><span class=\"ez-toc-section\" id=\"Use_Autospec_When_The_Signature_Matters\"><\/span>Use Autospec When The Signature Matters<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><code>patch()<\/code> can create an autospecced mock from the object being replaced. This catches misspelled methods and incompatible arguments earlier, especially when the dependency has a stable public interface.<\/p>\n<p>Autospec is not a reason to mock every internal helper. Use it at a meaningful boundary, and keep tests focused on the behavior that callers rely on.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Keep_Mock_Assertions_Readable\"><\/span>Keep Mock Assertions Readable<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Long expected call expressions can hide the behavior under test. Assign a meaningful expected value, use <code>call<\/code> objects for repeated calls, and name fixtures after the dependency they represent.<\/p>\n<p>When a test fails, the mock&#8217;s assertion message should show the expected and actual arguments without requiring a reader to reconstruct a large fixture graph. Readable failure output is part of test maintenance.<\/p>\n<p data-pythonpool-link-set=\"2026-07-13-final\">For adjacent test assertions, compare pytest with unittest and assertEqual patterns. Read <a href=\"https:\/\/www.pythonpool.com\/python-unittest-vs-pytest\/\">python unittest vs pytest<\/a> and <a href=\"https:\/\/www.pythonpool.com\/assertequal-python\/\">assertequal python<\/a> for the related workflow.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Frequently_Asked_Questions\"><\/span>Frequently Asked Questions<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<h3><span class=\"ez-toc-section\" id=\"How_do_I_assert_a_function_was_called_in_pytest\"><\/span>How do I assert a function was called in pytest?<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>Replace the dependency with a Mock, run the behavior under test, and call assert_called(), assert_called_once(), or an argument-aware assertion such as assert_called_once_with().<\/p>\n<h3><span class=\"ez-toc-section\" id=\"What_is_the_difference_between_assert_called_and_assert_called_once\"><\/span>What is the difference between assert_called and assert_called_once?<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>assert_called() requires at least one call, while assert_called_once() requires exactly one call. Choose the one that matches whether retries or repeated calls are valid.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"Where_should_I_patch_a_function_in_pytest\"><\/span>Where should I patch a function in pytest?<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>Patch the name where the code under test looks it up. If a module imported a function directly, patch that module&#8217;s imported name rather than the original library name.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"How_do_I_check_arguments_for_multiple_calls\"><\/span>How do I check arguments for multiple calls?<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>Use call_args_list or mock_calls and compare them with call objects. This preserves call order and lets the test check every argument.<\/p>\n<p><script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"How do I assert a function was called in pytest?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Replace the dependency with a Mock, run the behavior under test, and call assert_called(), assert_called_once(), or an argument-aware assertion such as assert_called_once_with().\"}},{\"@type\":\"Question\",\"name\":\"What is the difference between assert_called and assert_called_once?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"assert_called() requires at least one call, while assert_called_once() requires exactly one call. 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This preserves call order and lets the test check every argument.\"}}]}<\/script><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Learn how to assert function calls in pytest with unittest.mock, choose the right call assertion, patch the correct import, and test side effects.<\/p>\n","protected":false},"author":38,"featured_media":33340,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_mi_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[15],"tags":[],"class_list":["post-30225","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-tutorials","infinite-scroll-item"],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v20.1 (Yoast SEO v28.0) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Pytest Assert Function Called With 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