notes on magic mock py

zk/Pytest_Magic_Mock.md

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+---
+tags: [python]
+---
+
+An instance of the general Python `Mock` that is pre-configured with "magic"
+methods that the standard `Mock` lacks.
+
+These methods mean that it can automaticaly behave like the following objects
+without special configuration:
+
+- a [context manager](./Using_a_context_manager_in_Python.md)
+  - supports `__enter__`, `__exit__` and `with` statement
+
+- a list or other container
+  - supports `__iter__`, `__getitem`, `__len__`
+
+- calls to a methods
+
+## Example: mocking return from API call
+
+Mock:
+
+```py
+def get_user_data(api_client):
+    response = api_client.get_user(id=123)
+    return response
+
+```
+
+As follows:
+
+```py
+from unittest.mock import MagicMock
+
+def test_get_user_data():
+    mock_api = MagicMock()
+    mock_api.get_user.return_value = {"name": "Alice", "role": "Admin"}
+    result = get_user_data(mock_api)
+
+    assert result["name"] == "Alice"
+    mock_api.get_user.assert_called_once_with(id=123)
+```
+
+## Example: testing behaviour of code that relies on context managers:
+
+```py
+def log_to_file(file_system):
+    with file_system.open("log.txt") as f:
+        f.write("Hello World")
+
+def test_log_to_file():
+    mock_fs = MagicMock()
+
+    log_to_file(mock_fs)
+
+    mock_fs.open.assert_called_with("log.txt")
+    mock_fs.open().__enter__().write.assert_called_with("Hello World")
+```