How do I set up the test data and the test cases in a single python script

Question

I am writing a unit test and trying to set up the test data to use for the test cases in the same python script.

However, When I run the script, it does create the test data, but prints an error message that the data do not exist, leading to the test failure. It's only when I run the script again that the test succeeds.

Below is a simplified script that I wrote to figure out what's going on.

import unittest
from ddt import ddt, file_data
import pandas

@ddt
class TestWhatever(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        cls.setup_test_data()
        print("setUpClass is running")

    @classmethod
    def tearDownClass(cls):
        print("tearDownClass is running")

    @classmethod
    def setup_test_data(cls):
        data = pandas.DataFrame({'msg':["testing"]})
        data = data.transpose()
        with open("practice_test.json", "w") as file:
           file.write(data.to_json())
        print("setup_test_data is running")

    @file_data("practice_test.json")
    def test_whatever_possible(self, msg):
        print("test_whatever_possible is running :", msg)
        self.assertEqual('q', 'q')

    def test_whatever_impossible(self):
        print("test_whatever_impossible is running")
        self.assertEqual('n', 'n')

When I run the script above, it prints :

setup_test_data is running
setUpClass is running
test_whatever_impossible is running
.EtearDownClass is running

======================================================================
ERROR: test_whatever_possible_00001_error (main.TestWhatever)
Error!
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\ddt.py", line 145, in wrapper
    return func(self, *args, **kwargs)
  File "C:\ddt.py", line 187, in func
    raise ValueError(message % file_attr)
ValueError: practice_test.json does not exist

----------------------------------------------------------------------
Ran 2 tests in 0.006s

FAILED (errors=1)

Then on the second run :

setup_test_data is running
setUpClass is running
test_whatever_impossible is running
.test_whatever_possible is running : testing
.tearDownClass is running

----------------------------------------------------------------------
Ran 2 tests in 0.005s

OK

I'm pretty much lost at this point... Does anyone have a clue on this?

Answer 1

As a workaround you could write your own decorator to create the data file e.g.

def create_data(name):
    def decorator(fn):
        def decorated(*args, **kwargs):
            data = pandas.DataFrame({"msg": ["testing"]})
            data = data.transpose()
            with open(name, "w") as file:
                file.write(data.to_json())
            res = fn(*args, **kwargs)
            os.unlink(name)
            return res
        return decorated
    return decorator

and then stack this for your test (and remove the setup_test_data step):

@ddt
class TestWhatever(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        print("setUpClass is running")

    @classmethod
    def tearDownClass(cls):
        print("tearDownClass is running")

    @create_data("practice_test.json")
    @file_data("practice_test.json")
    def test_whatever_possible(self):
        print("test_whatever_possible is running :")
        self.assertEqual("q", "q")

Update: Jan 31st

The following example preserves the ddt behaviour of passing the parameter value into the test function:

import json
import unittest
from ddt import ddt, FILE_ATTR


def create_file_data(value, data):
    def wrapper(func):
        with open(value, "w") as file:
            file.write(json.dumps(data))
        # this is what the file_data decorator does
        setattr(func, FILE_ATTR, value)
        return func

    return wrapper


@ddt
class TestWhatever(unittest.TestCase):
    @create_file_data("practice_test.json", {"msg": ["testing"]})
    def test_file_data(self, msg):
        print("test_file_data msg:", msg)


unittest.main()

When this is run the output is:

test_file_data msg: ['testing']
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

Using the two examples above you should be able to derive an appropriate solution for your problem.