gh-137829: Fix shelve tests for backend compatibility

[furkanonder]

• Issue: test_shelve.py crashes on WSL? #137829

[furkanonder]

@picnixz

I also added a new test case for wrong types from custom serializers. For example when serializers return None (Only bytes and strings are supported as values), dbm fails. The new test ensure proper TypeError/dbm.error exceptions are raised.

[picnixz]

If you want to test with None and with something that is not None, I would suggest using a parametrized test:

@support.subTests("serialized", [None, ["invalid type"]])
def test_custom_invalid_serializer(self, serialized):
    ...
    def serializer(obj, protocol=None):
        return serialized
    ...

and update the comment saying that the TypeError is due to the return
value of the serializer.

[furkanonder]

Thanks for the suggestion! I've implemented the parameterized test approach as you recommended. I explored two different implementations and wanted to share the trade-offs:

Option 1: Using @subTests decorator (as you suggested)

@subTests("serialized", [None, ["invalid type"]])
def test_custom_invalid_serializer(self, serialized):
    # Create unique directory for each subtest to avoid conflicts
    test_dir = f"{self.dirname}_{id(serialized)}"
    os.mkdir(test_dir)
    self.addCleanup(os_helper.rmtree, test_dir)
    test_fn = os.path.join(test_dir, "shelftemp.db")
    
    def serializer(obj, protocol=None):
        return serialized
    # ... rest of test logic

Option 2: Using self.subTest() with a simple loop

def test_custom_invalid_serializer(self):
    os.mkdir(self.dirname)
    self.addCleanup(os_helper.rmtree, self.dirname)
    
    for serialized in [None, ["invalid type"]]:
        with self.subTest(serialized=serialized):
            def serializer(obj, protocol=None):
                return serialized
            # ... rest of test logic

Trade-offs:

• Option 1: More explicit parameterization and complete isolation, but requires complex directory management
• Option 2: Simpler, more readable, and uses standard unittest patterns, but subtests share the same directory

Which approach do you prefer?

[picnixz]

Use 1, it was meant for such cases.