Initial commit

Author: 1Mark

.gitignore

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+__pycache__
+.vscode
+.swp

README.md

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+# Python unit testing: How to monkeypatch and mock methods
+
+Tutorial based on this blog post [https://alexmarandon.com/articles/python_mock_gotchas/](https://alexmarandon.com/articles/python_mock_gotchas/)
+
+## Problem statement
+Monkeypatching and mocking can be somewhat confusing in python. There is a lack of clear documentation on the topic.
+
+The tests provided should give a complete view of what can and cannot be done.

data_source.py

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+def get_name():
+    return "Alice"

decorators.py

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+def noise_logger(func):
+    def wrapped(self):
+        result = func(self)
+        # In a real-world scenario, the decorator would access an external
+        # resource which we don't want our tests to depend on, such as a
+        # caching service.
+        print("Pet made noise: ", result)
+        return result
+    return wrapped

person.py

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+from data_source import get_name
+from decorators import noise_logger
+
+class Person(object):
+    def __init__(self):
+        self.pet = Pet()
+
+    def name(self):
+        return get_name()
+
+class Pet():
+    @noise_logger
+    def noise(self):
+        return "Woof"

test_decorators.py

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+from unittest.mock import patch
+patch('decorators.noise_logger', lambda x: x).start()
+from person import Person
+
+def test_decorator():
+    person = Person()
+    assert person.pet.noise() == "Woof"

test_instance.py

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+from unittest.mock import patch
+from person import Person
+import pytest
+
+@pytest.mark.xfail
+@patch('person.Pet')
+def test_dog_noise_failure_to_mock(mock_pet):
+    """Trying to patch the instance
+    We cannot do this. Since the class definition of `person.Pet` was defined
+    at import time, any modifications to it will not reflect on class instances."""
+    mock_pet.noise.return_value = "Meoow"
+    person = Person()
+    assert person.pet.noise() == "Meoow"
+
+def test_dog_noise_with_patch_context_manager():
+    """We patch the module.
+    To get the instance we must call `mock_pet.return_value`.
+    ---------> This is why they came up with patch.object
+    """
+    with patch('person.Pet') as mock_pet:
+        # If `person` is defined outside the context manager this wont work.
+        person = Person()
+        mock_pet.return_value.noise.return_value = "Meoow"
+        assert person.pet.noise() == "Meoow"
+
+@patch('person.Pet')
+def test_dog_noise_with_patch_decorator(mock_pet):
+    """We patch the module.
+    To get the instance we must call `mock_pet.return_value`.
+    ---------> This is why they came up with patch.object
+    """
+    person = Person()
+    mock_pet.return_value.noise.return_value = "Meoow"
+    assert person.pet.noise() == "Meoow"
+
+def test_dog_noise_with_patch_object_context_manager():
+    """We patch the instance."""
+    person = Person()
+    with patch.object(person.pet, "noise", return_value="Meoow"):
+        assert person.pet.noise() == "Meoow"
+
+def test_dog_noise_with_monkeypatch(monkeypatch):
+    """We patch the instance."""
+    person = Person()
+    monkeypatch.setattr(person.pet, "noise", lambda: "Meoow")
+    assert person.pet.noise() == "Meoow"

test_method_call.py

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+from person import Person
+from unittest.mock import patch
+import pytest
+
+@pytest.mark.xfail(strict=True)
+def test_name_with_patch_context_manager_failure():
+    """We are patching the method within the module.
+    But this isn't exactly what's used by Person"""
+    the_person = Person()
+    with patch("data_source.get_name", return_value="Bob"):
+        name = the_person.name()
+    assert name == "Bob"
+
+def test_name_with_patch_context_manager():
+    """We are patching the method within the module"""
+    the_person = Person()
+    with patch("person.get_name", return_value="Bob"):
+        name = the_person.name()
+    assert name == "Bob"
+
+@patch("person.get_name")
+def test_name_with_patch_decorator(mock_get_name):
+    """We are patching the method within the module"""
+    mock_get_name.return_value="Bob"
+    the_person = Person()
+    assert the_person.name() == "Bob"
+
+def test_name_with_patch_object_context_manager():
+    """We are patching the instance we created."""
+    the_person = Person()
+    with patch.object(the_person, "name", return_value="Bob"):
+        name = the_person.name()
+    assert name == "Bob"
+
+def test_name_with_monkeypatch(monkeypatch):
+    """We are patching the instance we created."""
+    the_person = Person()
+    monkeypatch.setattr(the_person, "name", lambda: "Bob")
+    assert the_person.name() == "Bob"