Cannot monkey patch module variable in Python unit tests

Question

I am trying to test the module:

package/module.py

DATA_PATH = os.path.join(os.path.dirname(__file__), "data")
class SomeClass:
 def __init__(self):
 self.filename = os.path.join(DATA_PATH, "ABC.txt")

in tests/module_test.py I am trying to do

from package import module
@patch("package.module.DATA_PATH", "data_path_here") # doesn't work
class TestSomeClass(unittest.TestCase):
 def setUp(self):
 module.DATA_PATH = "data_path_here" # doesn't work
 self.obj= SomeClass()
 @patch("package.module.DATA_PATH", "data_path_here") # doesn't work either
 def test_constructor(self):
 self.assertEqual(r"data_path_here\ABC.txt", self.obj.filename)

but the DATA_PATH is still not mocked out. I think that I tried all the possible options to mock it out but it still returns original path instead of "data_path_here"

What am I doing wrong?

EDIT: It is not a duplicate of Modifying global variables in Python unittest framework Because that solution does not work

• quamrana: Possible duplicate of Modifying global variables in Python unittest framework
• Alicja Głowacka: it is not a duplicate because that solution does not work
• F1Rumors: You might want to confirm a forward vs back slash issue on the test ... is the path really separated with a \ or a / ?? That would make the "assignment" approach in the setup work.

Answer 1

You don't need patching since you are using a global variable from another module:

#module.py
DATA_PATH = 1
def getData():
 return DATA_PATH
#tests.py
from package import module
print(module.DATA_PATH, module.getData())
module.DATA_PATH = 2
print(module.DATA_PATH, module.getData())

Output:

1 1
2 2

• Gang: this is better and cleaner without mocking.
• Nick Brady: amazing, thank you. I had to import the module and change the value on the module as you've done. e.g. import module and module.var = <some_val>. from module import var and then setting var = <some_val> did not make a persistent change.
• quamrana: When you use from module import var and then var = something you are only using your own module's var and you don't affect module.var. Mandatory link to Ned Batchelder
• TDAK: This should be the correct answer, works for my case!

Answer 2

For me, using mock/patch is a painful exercise. On the other hand, it was trivial to accomplish by setting (and restoring) the global for the tests:

import mock_module
class TestSomeClass(unittest2.TestCase):
 def setUp(self):
 self._original = mock_module.DATA_PATH
 mock_module.DATA_PATH = 'data_path_here'
 def tearDown(self):
 mock_module.DATA_PATH = self._original
 def test_constructor(self):
 obj = mock_module.SomeClass()
 self.assertEqual(r"data_path_here\ABC.txt", obj.filename)

Note that for my os path join, the separator is \ but your usage may differ.

Ran 1 test in 0.005s

OK