Python unittest to create a mock .json file
Question:
I have function that looks like this:
def file1_exists(directory):
file1_path = os.path.join(directory, 'file1.json')
return os.path.exists(file1_path)
def file2_exists(directory):
log_path = os.path.join(directory, 'file2.log')
return os.path.exists(file2_path)
def create_file1(directory):
if file1_exists(directory):
return
if not file2_exists(directory):
return
mod_time = os.stat(os.path.join(directory, 'file2.log')).st_mtime
timestamp = {
"creation_timestamp": datetime.datetime.fromtimestamp(mod_time).isoformat()
}
with open(os.path.join(directory, "file1.json"), "w") as f:
json.dump(timestamp, f)
And I need to create a unittest that uses mock files.
The 3 Unittests that I need are:
- A mock myfile.json file where I will assert that the function will return None (based on the 1st if statement, since the file exists)
- A way to mock-hide the data.txt item in order to assert that the function will return None (based on the second if statement)
- A mock myfile.json file where I write the required data and then assert that the return matches the expected outcome.
So far I’ve tried tests 1. and 2. with variations of this but I’ve been unsuccessful:
class TestAdminJsonCreation(unittest.TestCase):
@patch('os.path.exists', return_value=True)
def test_existing_admin_json(self):
self.assertNone(postprocess_results.create_json_file())
I’ve also read about other solutions such as:
Python testing: using a fake file with mock & io.StringIO
But I haven’t found a way to successfully do what I need…
Answers:
You want to be able to provide different return values for each call to os.path.exists
. Since you know the order of the calls, you can use side_effects
to supply a list of values to be used in order.
class TestAdminJsonCreation(unittest.TestCase):
# No JSON file
@patch('os.path.exists', return_value=True)
def test_existing_admin_json(self):
self.assertNone(postprocess_results.create_json_file())
# JSON file, log file
@patch('os.path.exists', side_effects=[True, False])
def test_existing_admin_json(self):
self.assertNone(postprocess_results.create_json_file())
# JSON file, no log file
@patch('os.path.exists', side_effects=[True, True])
def test_existing_admin_json(self):
...
The third test requires an actual file system, or for you to mock open
.
So, I ended up breaking my original function into 3 different functions for easier testing.
The tests are performed by checking what the result of the ‘def create_file1’ would be when we feed it different return_values from the other 2 functions and when we add valid data.
class TestFile1JsonCreation(unittest.TestCase):
@patch('builtins.open', new_callable=mock_open())
@patch('os.stat')
@patch('file1_exists', return_value=True)
@patch('file2_exists', return_value=False)
def test_existing_file1_json(self, file2_exists, file1_existsmock, stat, mopen):
create_file1('.')
# file1.json should not have been written
mopen.assert_not_called()
@patch('builtins.open', new_callable=mock_open())
@patch('os.stat')
@patch('file1_exists', return_value=False)
@patch('file2_exists', return_value=False)
def test_missing_file2(self, file2_exists, file1_existsmock, stat, mopen):
create_file1('.')
# file1.json should not have been written
mopen.assert_not_called()
@patch('builtins.open', new_callable=mock_open())
@patch('os.stat')
@patch('file1_exists', return_value=False)
@patch('file2_exists', return_value=True)
def test_write_data(self, file2_exists, file1_existsmock, stat, mopen):
class FakeStat:
st_mtime = 1641992788
stat.return_value = FakeStat()
create_file1('.')
# file1.json should have been written
mopen.assert_called_once_with('./file1.json', 'w')
written_data = ''.join(
c[1][0]
for c in mopen().__enter__().write.mock_calls
)
expected_data = {"creation_timestamp": "2022-01-12T13:06:28"}
written_dict_data = json.loads(written_data)
self.assertEqual(written_dict_data, expected_data)
I have function that looks like this:
def file1_exists(directory):
file1_path = os.path.join(directory, 'file1.json')
return os.path.exists(file1_path)
def file2_exists(directory):
log_path = os.path.join(directory, 'file2.log')
return os.path.exists(file2_path)
def create_file1(directory):
if file1_exists(directory):
return
if not file2_exists(directory):
return
mod_time = os.stat(os.path.join(directory, 'file2.log')).st_mtime
timestamp = {
"creation_timestamp": datetime.datetime.fromtimestamp(mod_time).isoformat()
}
with open(os.path.join(directory, "file1.json"), "w") as f:
json.dump(timestamp, f)
And I need to create a unittest that uses mock files.
The 3 Unittests that I need are:
- A mock myfile.json file where I will assert that the function will return None (based on the 1st if statement, since the file exists)
- A way to mock-hide the data.txt item in order to assert that the function will return None (based on the second if statement)
- A mock myfile.json file where I write the required data and then assert that the return matches the expected outcome.
So far I’ve tried tests 1. and 2. with variations of this but I’ve been unsuccessful:
class TestAdminJsonCreation(unittest.TestCase):
@patch('os.path.exists', return_value=True)
def test_existing_admin_json(self):
self.assertNone(postprocess_results.create_json_file())
I’ve also read about other solutions such as:
Python testing: using a fake file with mock & io.StringIO
But I haven’t found a way to successfully do what I need…
You want to be able to provide different return values for each call to os.path.exists
. Since you know the order of the calls, you can use side_effects
to supply a list of values to be used in order.
class TestAdminJsonCreation(unittest.TestCase):
# No JSON file
@patch('os.path.exists', return_value=True)
def test_existing_admin_json(self):
self.assertNone(postprocess_results.create_json_file())
# JSON file, log file
@patch('os.path.exists', side_effects=[True, False])
def test_existing_admin_json(self):
self.assertNone(postprocess_results.create_json_file())
# JSON file, no log file
@patch('os.path.exists', side_effects=[True, True])
def test_existing_admin_json(self):
...
The third test requires an actual file system, or for you to mock open
.
So, I ended up breaking my original function into 3 different functions for easier testing.
The tests are performed by checking what the result of the ‘def create_file1’ would be when we feed it different return_values from the other 2 functions and when we add valid data.
class TestFile1JsonCreation(unittest.TestCase):
@patch('builtins.open', new_callable=mock_open())
@patch('os.stat')
@patch('file1_exists', return_value=True)
@patch('file2_exists', return_value=False)
def test_existing_file1_json(self, file2_exists, file1_existsmock, stat, mopen):
create_file1('.')
# file1.json should not have been written
mopen.assert_not_called()
@patch('builtins.open', new_callable=mock_open())
@patch('os.stat')
@patch('file1_exists', return_value=False)
@patch('file2_exists', return_value=False)
def test_missing_file2(self, file2_exists, file1_existsmock, stat, mopen):
create_file1('.')
# file1.json should not have been written
mopen.assert_not_called()
@patch('builtins.open', new_callable=mock_open())
@patch('os.stat')
@patch('file1_exists', return_value=False)
@patch('file2_exists', return_value=True)
def test_write_data(self, file2_exists, file1_existsmock, stat, mopen):
class FakeStat:
st_mtime = 1641992788
stat.return_value = FakeStat()
create_file1('.')
# file1.json should have been written
mopen.assert_called_once_with('./file1.json', 'w')
written_data = ''.join(
c[1][0]
for c in mopen().__enter__().write.mock_calls
)
expected_data = {"creation_timestamp": "2022-01-12T13:06:28"}
written_dict_data = json.loads(written_data)
self.assertEqual(written_dict_data, expected_data)