How to Mock Objects Used in Context Managers with unittest.mock?

How to Mock an Object Used in a Context Manager with unittest.mock

When testing code that uses a with statement, it can be challenging to mock the underlying object. Let's consider the following example:

def testme(filepath):
    with open(filepath) as f:
        return f.read()

To test this function with unittest.mock, we need to mock the open function. Here's how:

Python 3:

1. Patch builtins.open and use mock_open from unittest.mock.
2. Use patch as a context manager or decorator. In both cases, pass mock_open with the desired read data and verify that the mocked file is called with the expected arguments.

Python 2:

1. Patch __builtin__.open instead of builtins.open.
2. Use mock as a context manager and pass mock_open as before.

Example:

<code class="python">from unittest.mock import patch, mock_open

@patch("builtins.open", new_callable=mock_open, read_data="data")
def test_patch(mock_file):
    assert open("path/to/open").read() == "data"
    mock_file.assert_called_with("path/to/open")</code>

Remember, in Python 3, patch will pass the mocked object as an argument to your test function, while in Python 2, you must assert on the mocked file explicitly.

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