Mock open() function used in a class method

Question

I tried to mock the open function used in a method of my class. I found this thread How do I mock an open used in a with statement (using the Mock framework in Python)? but could not solve my issue. Also the unittest documention shows a solution which also didn't mock my open https://docs.python.org/3/library/unittest.mock-examples.html#patch-decorators

This is my class with the method where the open function is used:

#__init.py__

import json

class MyClass:

    def save_data_to_file(self, data):
        with open('/tmp/data.json', 'w') as file:
            json.dump(data, file)
...
mc = MyClass()

Now I found a little different solution. This is my test:

#save_to_file_test.py

from mymodule import MyClass
from mock import mock_open, patch
import ast

class SaveToFileTest(unittest.TestCase):

    def setUp(self):
        self.mc = MyClass()
        self.data = [
            {'id': 5414470, 'name': 'peter'},
            {'id': 5414472, 'name': 'tom'},
            {'id': 5414232, 'name': 'pit'},
        ]

    def test_save_data_to_file(self):
        m = mock_open()
        with patch('mymodule.open', m, create=True):
            self.mc.save_data_to_file(self.data)
            string = ''
            for call in m.return_value.write.mock_calls:
                string += (call[1][0])
            list = ast.literal_eval(string)
            assertEquals = (list, self.data)

I'm not sure if this is the best way to test the content which should be written to a file. When I test the mock_calls (call_args_list is the same) this are the arguments which are passed to the file handle. Any advice, improvements and suggestions are welcome.

Answer 1

TL;DR

The heart of your problem is that you should be also mocking json.dump to be able to properly test the data that is going to be written to your file. I actually had a hard time running your code until a few important adjustments were made to your test method.

• Mock with builtins.open and not mymmodule.open
• You are in a context manager, so you should be checking m.return_value.__enter__.write, however you are actually calling the write from json.dump which is where the write will be called. (Details below on a suggested solution)
• You should also mock json.dump to simply validate it is called with your data

In short, with the issues mentioned above, the method can be re-written as:

Details about all this below

def test_save_data_to_file(self):
    with patch('builtins.open', new_callable=mock_open()) as m:
        with patch('json.dump') as m_json:
            self.mc.save_data_to_file(self.data)

            # simple assertion that your open was called 
            m.assert_called_with('/tmp/data.json', 'w')

            # assert that you called m_json with your data
            m_json.assert_called_with(self.data, m.return_value)

Detailed Explanation

To focus on the problems I see in your code, the first thing I strongly suggest doing, since open is a builtin, is to mock from builtins, furthermore, you can save yourself a line of code by making use of new_callable and as, so you can simply do this:

with patch('builtins.open', new_callable=mock_open()) as m:

The next problem that I see with your code as I had trouble running this until I actually made the following adjustment when you started looping over your calls:

m.return_value.__enter__.return_value.write.mock_calls

To dissect that, what you have to keep in mind is that your method is using a context manager. In using a context manager, the work of your write will actually be done inside your __enter__ method. So, from the return_value of your m, you want to then get the return_value of __enter__.

However, this brings us to the heart of the problem with what you are trying to test. Because of how the json.dump works when writing to the file, your mock_calls for your write after inspecting the code, will actually look like this:

<MagicMock name='open().write' id='4348414496'>
call('[')
call('{')
call('"name"')
call(': ')
call('"peter"')
call(', ')
call('"id"')
call(': ')
call('5414470')
call('}')
call(', ')
call('{')
call('"name"')
call(': ')
call('"tom"')
call(', ')
call('"id"')
call(': ')
call('5414472')
call('}')
call(', ')
call('{')
call('"name"')
call(': ')
call('"pit"')
call(', ')
call('"id"')
call(': ')
call('5414232')
call('}')
call(']')
call.__str__()

That is not going to be fun to test. So, this brings us to the next solution you can try out; Mock json.dump.

You shouldn't be testing json.dump, you should be testing calling it with the right parameters. With that being said, you can follow similar fashion with your mocking and do something like this:

with patch('json.dump') as m_json:

Now, with that, you can significantly simplify your test code, to simply validate that the method gets called with your data that you are testing with. So, with that, when you put it all together, you will have something like this:

def test_save_data_to_file(self):
    with patch('builtins.open', new_callable=mock_open()) as m:
        with patch('json.dump') as m_json:
            self.mc.save_data_to_file(self.data)

            # simple assertion that your open was called 
            m.assert_called_with('/tmp/data.json', 'w')

            # assert that you called m_json with your data
            m_json.assert_called_with(self.data, m.return_value.__enter__.return_value)

If you're interested in further refactoring to make your test method a bit cleaner, you could also set up your patching as a decorator, leaving your code cleaner inside the method:

@patch('json.dump')
@patch('builtins.open', new_callable=mock_open())
def test_save_data_to_file(self, m, m_json):
    self.mc.save_data_to_file(self.data)

    # simple assertion that your open was called
    m.assert_called_with('/tmp/data.json', 'w')

    # assert that you called m_json with your data
    m_json.assert_called_with(self.data, m.return_value.__enter__.return_value)

Inspecting is your best friend here, to see what methods are being called at what steps, to further help with the testing. Good luck.