24.7.2: Testing fails without the `TOX_PACKAGE` environment variable

Start with tests/test_examples.py, especially build_and_install at line 35, and run the example tests without TOX_PACKAGE. Trace how the tests obtain the incremental package, using the reported failures as a guide. Done means the eight example tests pass when that environment variable is unset.

When I run tests without the TOX_PACKAGE environment variable set then several tests fails:

py39: commands[5]> coverage run /usr/bin/trial tests/test_examples.py
test_examples
  ExampleTests
    test_hatchling_get_version ...                                      [ERROR]
    test_noop ...                                                       [ERROR]
    test_setuppy_version ...                                            [ERROR]
    test_setuptools_bad_versionpy ...                                    [FAIL]
    test_setuptools_missing_versionpy ...                               [ERROR]
    test_setuptools_no_optin ...                                        [ERROR]
    test_setuptools_no_package ...                                      [ERROR]
    test_setuptools_version ...                                         [ERROR]

===============================================================================
[FAIL]
Traceback (most recent call last):
  File "tests/test_examples.py", line 184, in test_setuptools_bad_versionpy
    build_and_install(src)
  File "/usr/lib/python3.9/vendor-packages/twisted/trial/_synctest.py", line 355, in __exit__
    self._testCase.fail(
twisted.trial.unittest.FailTest: builtins.KeyError raised instead of BuildBackendException:
 Traceback (most recent call last):
  File "/usr/lib/python3.9/vendor-packages/twisted/trial/_asynctest.py", line 124, in _run
    d = defer.maybeDeferred(
  File "/usr/lib/python3.9/vendor-packages/twisted/internet/defer.py", line 212, in maybeDeferred
    result = f(*args, **kwargs)
  File "/usr/lib/python3.9/vendor-packages/twisted/internet/utils.py", line 223, in runWithWarningsSuppressed
    result = f(*a, **kw)
  File "tests/test_examples.py", line 184, in test_setuptools_bad_versionpy

--- <exception caught here> ---
  File "tests/test_examples.py", line 184, in test_setuptools_bad_versionpy

  File "tests/test_examples.py", line 35, in build_and_install

  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
builtins.KeyError: 'TOX_PACKAGE'


test_examples.ExampleTests.test_setuptools_bad_versionpy
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "tests/test_examples.py", line 190, in test_hatchling_get_version
    build_and_install(TEST_DIR.child("example_hatchling"))
  File "tests/test_examples.py", line 35, in build_and_install
    "incremental @ file://" + os.environ["TOX_PACKAGE"],
  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
builtins.KeyError: 'TOX_PACKAGE'

test_examples.ExampleTests.test_hatchling_get_version
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "tests/test_examples.py", line 205, in test_noop
    build_and_install(TEST_DIR.child("example_noop"))
  File "tests/test_examples.py", line 35, in build_and_install
    "incremental @ file://" + os.environ["TOX_PACKAGE"],
  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
builtins.KeyError: 'TOX_PACKAGE'

test_examples.ExampleTests.test_noop
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "tests/test_examples.py", line 54, in test_setuppy_version
    build_and_install(TEST_DIR.child("example_setuppy"))
  File "tests/test_examples.py", line 35, in build_and_install
    "incremental @ file://" + os.environ["TOX_PACKAGE"],
  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
builtins.KeyError: 'TOX_PACKAGE'

test_examples.ExampleTests.test_setuppy_version
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "tests/test_examples.py", line 149, in test_setuptools_missing_versionpy
    build_and_install(src)
  File "tests/test_examples.py", line 35, in build_and_install
    "incremental @ file://" + os.environ["TOX_PACKAGE"],
  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
builtins.KeyError: 'TOX_PACKAGE'

test_examples.ExampleTests.test_setuptools_missing_versionpy
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "tests/test_examples.py", line 94, in test_setuptools_no_optin
    build_and_install(src)
  File "tests/test_examples.py", line 35, in build_and_install
    "incremental @ file://" + os.environ["TOX_PACKAGE"],
  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
builtins.KeyError: 'TOX_PACKAGE'

test_examples.ExampleTests.test_setuptools_no_optin
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "tests/test_examples.py", line 115, in test_setuptools_no_package
    build_and_install(src)
  File "tests/test_examples.py", line 35, in build_and_install
    "incremental @ file://" + os.environ["TOX_PACKAGE"],
  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
builtins.KeyError: 'TOX_PACKAGE'

test_examples.ExampleTests.test_setuptools_no_package
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "tests/test_examples.py", line 65, in test_setuptools_version
    build_and_install(TEST_DIR.child("example_setuptools"))
  File "tests/test_examples.py", line 35, in build_and_install
    "incremental @ file://" + os.environ["TOX_PACKAGE"],
  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
builtins.KeyError: 'TOX_PACKAGE'

test_examples.ExampleTests.test_setuptools_version
-------------------------------------------------------------------------------
Ran 8 tests in 0.070s

FAILED (failures=1, errors=7)