Python testing: using a fake file with mock & io.StringIO
Just mock out both os.path.isfile
and the open()
call, and pass in a fake filename (you are not expected to pass in an open file, after all).
The mock library includes a utility for the latter: mock_open()
:
@mock.patch('os.path.isfile')
def test_CheckConfig_with_file(mock_isfile):
mock_isfile.return_value = True
config_data = mock.mock_open(read_data='data')
with mock.patch('mymodule.open', config_data) as mock_open:
expected = parsed_file_data
actual = CheckConfig('mocked/filename').config
assert expected == actual
This causes the if isinstance(data, list):
test to be false (because data
is a string instead), followed by the elif os.path.isfile(data):
returning True
, and the open(data)
call to use your mocked data from the mock_open()
result.
You can use the mock_open
variable to assert that open()
was called with the right data (mock_open. assert_called_once_with('mocked/filename')
for example).
Demo:
>>> import os.path
>>> from unittest import mock
>>> class CheckConfig(object):
... def __init__(self, config):
... self.config = self._check_input_data(config)
... def _check_input_data(self, data):
... if isinstance(data, list):
... return self._parse(data)
... elif os.path.isfile(data):
... with open(data) as f:
... return self._parse(f.readlines())
... def _parse(self, data):
... return data
...
>>> with mock.patch('os.path.isfile') as mock_isfile:
... mock_isfile.return_value = True
... config_data = mock.mock_open(read_data='line1\nline2\n')
... with mock.patch('__main__.open', config_data) as mock_open:
... actual = CheckConfig('mocked/filename').config
...
>>> actual
['line1\n', 'line2\n']
>>> mock_open.mock_calls
[call('mocked/filename'),
call().__enter__(),
call().readlines(),
call().__exit__(None, None, None)]
In case you end up here wondering how to solve this using the pytest-mock library, here is how you do it:
def test_open(mocker):
m = mocker.patch('builtins.open', mocker.mock_open(read_data='bibble'))
with open('foo') as h:
result = h.read()
m.assert_called_once_with('foo')
assert result == 'bibble'
This code example was found (but had to be adjusted) here.