How to Mock Open Files in Python Unittest.Mock?

Mocking Open Files in Python Unittest.Mock

When testing Python code that utilizes file operations, effectively mocking the behavior of open files is essential. One specific scenario arises when open is used within a with statement, as seen in the following code:

<code class="python">def testme(filepath):
    with open(filepath) as f:
        return f.read()</code>

Python 3

To mock this open operation, utilize unittest.mock in conjunction with mock_open, which is part of the mock framework. patch serves as a context manager, returning the object used to replace the patched one:

<code class="python">from unittest.mock import patch, mock_open
with patch("builtins.open", mock_open(read_data="data")) as mock_file:
    assert open("path/to/open").read() == "data"
mock_file.assert_called_with("path/to/open")</code>

Alternatively, patch can be employed as a decorator. However, using mock_open()'s result as the new argument may be cumbersome. Instead, utilize patch's new_callable argument and remember that additional arguments not used by patch will be passed to the new_callable function:

<code class="python">@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>

Note that patch will provide the mocked object as an argument to the test function in this instance.

Python 2

For Python 2, it is necessary to patch __builtin__.open instead and import mock separately:

<code class="python">from mock import patch, mock_open
with patch("__builtin__.open", mock_open(read_data="data")) as mock_file:
    assert open("path/to/open").read() == "data"
mock_file.assert_called_with("path/to/open")</code>

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