How do I mock an open used in a with statement (using the Mock framework in Python)?
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 theMock
(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")
The way to do this has changed in mock 0.7.0 which finally supports mocking the python protocol methods (magic methods), particularly using the MagicMock:
http://www.voidspace.org.uk/python/mock/magicmock.html
An example of mocking open as a context manager (from the examples page in the mock documentation):
>>> open_name = '%s.open' % __name__
>>> with patch(open_name, create=True) as mock_open:
... mock_open.return_value = MagicMock(spec=file)
...
... with open('/some/path', 'w') as f:
... f.write('something')
...
<mock.Mock object at 0x...>
>>> file_handle = mock_open.return_value.__enter__.return_value
>>> file_handle.write.assert_called_with('something')
With the latest versions of mock, you can use the really useful mock_open helper:
mock_open(mock=None, read_data=None)
A helper function to create a mock to replace the use of open. It works for open called directly or used as a context manager.
The mock argument is the mock object to configure. If None (the default) then a MagicMock will be created for you, with the API limited to methods or attributes available on standard file handles.
read_data is a string for the read method of the file handle to return. This is an empty string by default.
>>> from mock import mock_open, patch
>>> m = mock_open()
>>> with patch('{}.open'.format(__name__), m, create=True):
... with open('foo', 'w') as h:
... h.write('some stuff')
>>> m.assert_called_once_with('foo', 'w')
>>> handle = m()
>>> handle.write.assert_called_once_with('some stuff')