Deprecate Python 2 formatting support (#2523)

* Prepare for Python 2 depreciation

- Use BlackRunner and .stdout in command line test

So the next commit won't break this test. This is in its own commit so
we can just revert the depreciation commit when dropping Python 2
support completely.

* Deprecate Python 2 formatting support

CHANGES.md

@@ -10,6 +10,7 @@
 - Bumped typed-ast version minimum to 1.4.3 for 3.10 compatiblity (#2519)
 - Fixed a Python 3.10 compatibility issue where the loop argument was still being passed
   even though it has been removed (#2580)
+- Deprecate Python 2 formatting support (#2523)
 
 ### _Blackd_
 

docs/faq.md

@@ -74,13 +74,17 @@ disabled-by-default counterpart W504. E203 should be disabled while changes are
 
 ## Does Black support Python 2?
 
+```{warning}
+Python 2 support has been deprecated since 21.10b0.
+
+This support will be dropped in the first stable release, expected for January 2022.
+See [The Black Code Style](the_black_code_style/index.rst) for details.
+```
+
 For formatting, yes! [Install](getting_started.md#installation) with the `python2` extra
 to format Python 2 files too! In terms of running _Black_ though, Python 3.6 or newer is
 required.
 
-Note that this support will be dropped in the first stable release, expected for
-January 2022. See [The Black Code Style](the_black_code_style/index.rst) for details.
-
 ## Why does my linter or typechecker complain after I format my code?
 
 Some linters and other tools use magical comments (e.g., `# noqa`, `# type: ignore`) to

src/black/__init__.py

@@ -1061,6 +1061,15 @@ def f(
         versions = mode.target_versions
     else:
         versions = detect_target_versions(src_node)
+
+    # TODO: fully drop support and this code hopefully in January 2022 :D
+    if TargetVersion.PY27 in mode.target_versions or versions == {TargetVersion.PY27}:
+        msg = (
+            "DEPRECATION: Python 2 support will be removed in the first stable release"
+            "expected in January 2022."
+        )
+        err(msg, fg="yellow", bold=True)
+
     normalize_fmt_off(src_node)
     lines = LineGenerator(
         mode=mode,
@@ -1103,7 +1112,7 @@ def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
         return tiow.read(), encoding, newline
 
 
-def get_features_used(node: Node) -> Set[Feature]:
+def get_features_used(node: Node) -> Set[Feature]:  # noqa: C901
     """Return a set of (relatively) new Python features used in this file.
 
     Currently looking for:
@@ -1113,6 +1122,7 @@ def get_features_used(node: Node) -> Set[Feature]:
     - positional only arguments in function signatures and lambdas;
     - assignment expression;
     - relaxed decorator syntax;
+    - print / exec statements;
     """
     features: Set[Feature] = set()
     for n in node.pre_order():
@@ -1161,6 +1171,11 @@ def get_features_used(node: Node) -> Set[Feature]:
                         if argch.type in STARS:
                             features.add(feature)
 
+        elif n.type == token.PRINT_STMT:
+            features.add(Feature.PRINT_STMT)
+        elif n.type == token.EXEC_STMT:
+            features.add(Feature.EXEC_STMT)
+
     return features
 
 

src/black/mode.py

@@ -41,9 +41,17 @@ class Feature(Enum):
     RELAXED_DECORATORS = 10
     FORCE_OPTIONAL_PARENTHESES = 50
 
+    # temporary for Python 2 deprecation
+    PRINT_STMT = 200
+    EXEC_STMT = 201
+
 
 VERSION_TO_FEATURES: Dict[TargetVersion, Set[Feature]] = {
-    TargetVersion.PY27: {Feature.ASYNC_IDENTIFIERS},
+    TargetVersion.PY27: {
+        Feature.ASYNC_IDENTIFIERS,
+        Feature.PRINT_STMT,
+        Feature.EXEC_STMT,
+    },
     TargetVersion.PY33: {Feature.UNICODE_LITERALS, Feature.ASYNC_IDENTIFIERS},
     TargetVersion.PY34: {Feature.UNICODE_LITERALS, Feature.ASYNC_IDENTIFIERS},
     TargetVersion.PY35: {

src/blib2to3/pgen2/token.py

@@ -74,6 +74,9 @@
 COLONEQUAL: Final = 59
 N_TOKENS: Final = 60
 NT_OFFSET: Final = 256
+# temporary for Python 2 deprecation
+PRINT_STMT: Final = 316
+EXEC_STMT: Final = 288
 # --end constants--
 
 tok_name: Final[Dict[int, str]] = {}

tests/test_black.py

@@ -1401,14 +1401,14 @@ def test_docstring_reformat_for_py27(self) -> None:
         )
         expected = 'def foo():\n    """Testing\n    Testing"""\n    print "Foo"\n'
 
-        result = CliRunner().invoke(
+        result = BlackRunner().invoke(
             black.main,
             ["-", "-q", "--target-version=py27"],
             input=BytesIO(source),
         )
 
         self.assertEqual(result.exit_code, 0)
-        actual = result.output
+        actual = result.stdout
         self.assertFormatEqual(actual, expected)
 
     @staticmethod
@@ -2017,6 +2017,21 @@ def test_get_sources_with_stdin_filename_and_force_exclude(self) -> None:
         )
 
 
+@pytest.mark.parametrize("explicit", [True, False], ids=["explicit", "autodetection"])
+def test_python_2_deprecation_with_target_version(explicit: bool) -> None:
+    args = [
+        "--config",
+        str(THIS_DIR / "empty.toml"),
+        str(DATA_DIR / "python2.py"),
+        "--check",
+    ]
+    if explicit:
+        args.append("--target-version=py27")
+    with cache_dir():
+        result = BlackRunner().invoke(black.main, args)
+    assert "DEPRECATION: Python 2 support will be removed" in result.stderr
+
+
 with open(black.__file__, "r", encoding="utf-8") as _bf:
     black_source_lines = _bf.readlines()