How to Mock the \'open\' Handle in the \'with\' Statement Using Mock in Python?

Mocking Open Handle in a With Statement Using Mock in Python

When writing unit tests for code that utilizes the with statement and file operations, mocking the open handle becomes necessary to isolate the tested code's behavior. Here's how to achieve this using the Mock framework in Python:

Python 3

Using the unittest.mock module, you can patch built-in functions like open. Here's how:

<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>

Python 2

For Python 2, the syntax differs slightly. Instead of patching builtins.open, you need to patch __builtin__.open and import mock separately:

<code class="python">import mock

with mock.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>

Patching with a Decorator

You can also use the patch decorator for a cleaner syntax. However, using mock_open's result directly in the new= argument for patch can be tricky. Instead, leverage the new_callable= argument as described in the documentation:

<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: Keep in mind that in this case, the patched object will be passed as an argument to your test function.

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