I was surprised that in the following __init__ is actually called once per test function, although nothing in documentation implied so.

a.py
------

import unittest

class A(unittest.TestCase):
   def __init__(self, *args, **kwargs):
       print('called only once!')
       super().__init__(*args, **kwargs)

   def test(self):
       self.assertEqual(1, 1)
 
   def test2(self):
       self.assertEqual(2, 2)

if __name__ == '__main__':
   unittest.main()

$ python a.py
called only once!
called only once!
..
----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK



The attached patch adds a note regarding this behaviour for the TestCase class.

I'm guessing it is that a new TestCase instance is instantiated each time an individual test is run.  I don't know that this is part of the API, though, so I'm not sure how it should be documented.

This sentence seems to cover it: "Each instance of the TestCase will only be used to run a single test method, so a new fixture is created for each test."  from http://docs.python.org/2/library/unittest.html

The word "fixture" here is being used oddly, but that's the sentence.

"Each instance of the TestCase will only be used to run a single test method, so a new fixture is created for each test."

Yes, this seems to be, but it is not clear what fixture means in this context (as in "the preparation needed to perform one or *more* tests"?). Perhaps enhancing the message a little will suffice.

I'd be happy with a doc change from "fixture" to instance (or maybe add instance in parentheses). I'd rather a simple change than making the documentation much longer.

The reason for a separate instance per test is for test isolation. Each test can create and modify instance attributes without them affecting other tests from the same class.

How about the following:

"Each instance of the TestCase will only be used to run a single test method from it, so a new instance is created for each test method."


It replaces `fixture` with `instance` and it emphasizes that the called method belongs to that particular TestCase instance.

In Python 3 docs there is a hint in the documentation for `loadTestsFromModule`:

"This method searches module for classes derived from TestCase and creates an instance of the class for each test method defined for the class."

The phrase with a fixture per test from Python 2 docs is gone though. It would be nice if the same explanation from loadTestsFromModule could be applied to TestCase documentation.

New changeset dff46758f267ad6c13096c69c4e1dee17f9969aa by Gregory P. Smith in branch 'master':
bpo-19950: Clarify unittest TestCase instance use. (GH-6875)
https://github.com/python/cpython/commit/dff46758f267ad6c13096c69c4e1dee17f9969aa

New changeset a5f33a899f6450de96f5e4cd154de4486d50bdd7 by Gregory P. Smith (Miss Islington (bot)) in branch '3.6':
bpo-19950: Clarify unittest TestCase instance use. (GH-6875) (GH-6939)
https://github.com/python/cpython/commit/a5f33a899f6450de96f5e4cd154de4486d50bdd7

New changeset 436972e295f5057fe7cdd7312f543c2fa884705d by Gregory P. Smith (Miss Islington (bot)) in branch '3.7':
bpo-19950: Clarify unittest TestCase instance use. (GH-6875) (GH-6938)
https://github.com/python/cpython/commit/436972e295f5057fe7cdd7312f543c2fa884705d