add test configurations that don't contain python2 optional install

add test for negative scenario: formatting python2 code
tag python2 only tests

pyproject.toml

@@ -25,3 +25,6 @@ extend-exclude = '''
 [build-system]
 requires = ["setuptools>=41.0", "setuptools-scm", "wheel"]
 build-backend = "setuptools.build_meta"
+
+[tool.pytest.ini_options]
+markers = ['python2', "without_python2"]

tests/test_black.py

@@ -24,6 +24,7 @@
     Iterator,
     TypeVar,
 )
+import pytest
 import unittest
 from unittest.mock import patch, MagicMock
 
@@ -459,6 +460,31 @@ def test_skip_magic_trailing_comma(self) -> None:
             )
             self.assertEqual(expected, actual, msg)
 
+    @pytest.mark.without_python2
+    def test_python2_should_fail_without_optional_install(self) -> None:
+        source = "x = 1234l"
+        tmp_file = Path(black.dump_to_file(source))
+        try:
+            runner = BlackRunner()
+            result = runner.invoke(black.main, [str(tmp_file)])
+            self.assertEqual(result.exit_code, 123)
+        finally:
+            os.unlink(tmp_file)
+        actual = (
+            runner.stderr_bytes.decode()
+            .replace("\n", "")
+            .replace("\\n", "")
+            .replace("\\r", "")
+            .replace("\r", "")
+        )
+        msg = (
+            "The requested source code has invalid Python 3 syntax."
+            "If you are trying to format Python 2 files please reinstall Black"
+            " with the 'python2' extra: `python3 -m pip install black[python2]`."
+        )
+        self.assertIn(msg, actual)
+
+    @pytest.mark.python2
     @patch("black.dump_to_file", dump_to_stderr)
     def test_python2_print_function(self) -> None:
         source, expected = read_data("python2_print_function")
@@ -1971,6 +1997,7 @@ def test_bpo_2142_workaround(self) -> None:
         actual = diff_header.sub(DETERMINISTIC_HEADER, actual)
         self.assertEqual(actual, expected)
 
+    @pytest.mark.python2
     def test_docstring_reformat_for_py27(self) -> None:
         """
         Check that stripping trailing whitespace from Python 2 docstrings

tests/test_format.py

@@ -1,6 +1,7 @@
 from unittest.mock import patch
 
 import black
+import pytest
 from parameterized import parameterized
 
 from tests.util import (
@@ -46,16 +47,19 @@
     "function2",
     "function_trailing_comma",
     "import_spacing",
-    "numeric_literals_py2",
-    "python2",
-    "python2_unicode_literals",
     "remove_parens",
     "slices",
     "string_prefixes",
     "tricky_unicode_symbols",
     "tupleassign",
 ]
 
+SIMPLE_CASES_PY2 = [
+    "numeric_literals_py2",
+    "python2",
+    "python2_unicode_literals",
+]
+
 EXPERIMENTAL_STRING_PROCESSING_CASES = [
     "cantfit",
     "comments7",
@@ -86,6 +90,12 @@
 
 
 class TestSimpleFormat(BlackBaseTestCase):
+    @parameterized.expand(SIMPLE_CASES_PY2)
+    @pytest.mark.python2
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_simple_format_py2(self, filename: str) -> None:
+        self.check_file(filename, DEFAULT_MODE)
+
     @parameterized.expand(SIMPLE_CASES)
     @patch("black.dump_to_file", dump_to_stderr)
     def test_simple_format(self, filename: str) -> None:

tox.ini

@@ -7,9 +7,11 @@ skip_install = True
 deps =
     -r{toxinidir}/test_requirements.txt
 commands =
-    pip install -e .[d,python2]
+    pip install -e .[d]
     coverage erase
-    coverage run -m pytest tests {posargs}
+    coverage run -m pytest tests -m "not python2" {posargs}
+    pip install -e .[d,python2]
+    coverage run -m pytest tests -m "not without_python2" {posargs}
     coverage report
 
 [testenv:fuzz]