Mocking two functions with patch for a unit test

You can simplify your test by using the patch decorator and nesting them like so (they are MagicMock objects by default):

from unittest.mock import patch

@patch('cPickle.dump')
@patch('__builtin__.open')
def test_write_out(mock_open, mock_pickle):
    path="~/collection"
    f = mock_open.return_value
    f.method.return_value = path
    
    write_out(path, 'data')
    
    mock_open.assert_called_once_with('~/collection', 'wb')
    mock_pickle.assert_called_once_with('data', f)
    f.close.assert_any_call()

Calls to a MagicMock instance return a new MagicMock instance, so you can check that the returned value was called just like any other mocked object. In this case f is a MagicMock named 'open()' (try printing f).