Python 3

Patch builtins.open and use mock_open, which is part of the mock framework. patch used as a context manager returns the object used to replace the patched one:

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")

If you want to use patch as a decorator, using mock_open()‘s result as the new= argument to patch can be a little bit weird. Instead, use patch‘s new_callable= argument and remember that every extra argument that patch doesn’t use will be passed to the new_callable function, as described in the patch documentation:

patch() takes arbitrary keyword arguments. These will be passed to the Mock (or new_callable) on construction.

@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")

Remember that in this case patch will pass the mocked object as an argument to your test function.

Python 2

You need to patch __builtin__.open instead of builtins.open and mock is not part of unittest, you need to pip install and import it separately:

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")