Unpacking on flow constructs (return/yield) now implies 3.8+ (#2700)

CHANGES.md

@@ -11,6 +11,7 @@
   (#2686)
 - No longer color diff headers white as it's unreadable in light themed terminals
   (#2691)
+- Tuple unpacking on `return` and `yield` constructs now implies 3.8+ (#2700)
 
 ## 21.12b0
 

src/black/__init__.py

@@ -1210,6 +1210,14 @@ def get_features_used(  # noqa: C901
                         if argch.type in STARS:
                             features.add(feature)
 
+        elif (
+            n.type in {syms.return_stmt, syms.yield_expr}
+            and len(n.children) >= 2
+            and n.children[1].type == syms.testlist_star_expr
+            and any(child.type == syms.star_expr for child in n.children[1].children)
+        ):
+            features.add(Feature.UNPACKING_ON_FLOW)
+
         # Python 2 only features (for its deprecation) except for integers, see above
         elif n.type == syms.print_stmt:
             features.add(Feature.PRINT_STMT)

src/black/mode.py

@@ -49,6 +49,7 @@ class Feature(Enum):
     POS_ONLY_ARGUMENTS = 9
     RELAXED_DECORATORS = 10
     PATTERN_MATCHING = 11
+    UNPACKING_ON_FLOW = 12
     FORCE_OPTIONAL_PARENTHESES = 50
 
     # __future__ flags
@@ -116,6 +117,7 @@ class Feature(Enum):
         Feature.FUTURE_ANNOTATIONS,
         Feature.ASSIGNMENT_EXPRESSIONS,
         Feature.POS_ONLY_ARGUMENTS,
+        Feature.UNPACKING_ON_FLOW,
     },
     TargetVersion.PY39: {
         Feature.UNICODE_LITERALS,
@@ -128,6 +130,7 @@ class Feature(Enum):
         Feature.ASSIGNMENT_EXPRESSIONS,
         Feature.RELAXED_DECORATORS,
         Feature.POS_ONLY_ARGUMENTS,
+        Feature.UNPACKING_ON_FLOW,
     },
     TargetVersion.PY310: {
         Feature.UNICODE_LITERALS,
@@ -140,6 +143,7 @@ class Feature(Enum):
         Feature.ASSIGNMENT_EXPRESSIONS,
         Feature.RELAXED_DECORATORS,
         Feature.POS_ONLY_ARGUMENTS,
+        Feature.UNPACKING_ON_FLOW,
         Feature.PATTERN_MATCHING,
     },
 }

tests/test_black.py

@@ -810,6 +810,14 @@ def test_get_features_used(self) -> None:
         self.assertEqual(black.get_features_used(node), {Feature.POS_ONLY_ARGUMENTS})
         node = black.lib2to3_parse("def fn(a, /, b): ...")
         self.assertEqual(black.get_features_used(node), {Feature.POS_ONLY_ARGUMENTS})
+        node = black.lib2to3_parse("def fn(): yield a, b")
+        self.assertEqual(black.get_features_used(node), set())
+        node = black.lib2to3_parse("def fn(): return a, b")
+        self.assertEqual(black.get_features_used(node), set())
+        node = black.lib2to3_parse("def fn(): yield *b, c")
+        self.assertEqual(black.get_features_used(node), {Feature.UNPACKING_ON_FLOW})
+        node = black.lib2to3_parse("def fn(): return a, *b, c")
+        self.assertEqual(black.get_features_used(node), {Feature.UNPACKING_ON_FLOW})
 
     def test_get_features_used_for_future_flags(self) -> None:
         for src, features in [