Discover whether a file is Python 3.6+ also by stars in calls

Fixes a pathological situation where if a function signature used a trailing
comma but was later reformatted to a single line (with the trailing comma
removed), Black would change its mind whether a file is Python
3.6-compatible between runs.

README.md

@@ -549,6 +549,11 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 * fixed not splitting long from-imports with only a single name
 
+* fixed Python 3.6+ file discovery by also looking at function calls with
+  unpacking. This fixed non-deterministic formatting if trailing commas
+  where used both in function signatures with stars and function calls
+  with stars but the former would be reformatted to a single line.
+
 
 ### 18.4a4
 

black.py

@@ -2337,7 +2337,7 @@ def is_python36(node: Node) -> bool:
 
     Currently looking for:
     - f-strings; and
-    - trailing commas after * or ** in function signatures.
+    - trailing commas after * or ** in function signatures and calls.
     """
     for n in node.pre_order():
         if n.type == token.STRING:
@@ -2346,7 +2346,7 @@ def is_python36(node: Node) -> bool:
                 return True
 
         elif (
-            n.type == syms.typedargslist
+            n.type in {syms.typedargslist, syms.arglist}
             and n.children
             and n.children[-1].type == token.COMMA
         ):
@@ -2354,6 +2354,11 @@ def is_python36(node: Node) -> bool:
                 if ch.type in STARS:
                     return True
 
+                if ch.type == syms.argument:
+                    for argch in ch.children:
+                        if argch.type in STARS:
+                            return True
+
     return False
 
 

tests/function.py

@@ -81,6 +81,15 @@ def trailing_comma():
     C: 0.1 * (10.0 / 12),
     D: 0.1 * (10.0 / 12),
 }
+def f(
+  a,
+  **kwargs,
+) -> A:
+    return A(
+        very_long_argument_name1=very_long_value_for_the_argument,
+        very_long_argument_name2=very_long_value_for_the_argument,
+        **kwargs,
+    )
 
 # output
 
@@ -212,3 +221,11 @@ def trailing_comma():
         C: 0.1 * (10.0 / 12),
         D: 0.1 * (10.0 / 12),
     }
+
+
+def f(a, **kwargs) -> A:
+    return A(
+        very_long_argument_name1=very_long_value_for_the_argument,
+        very_long_argument_name2=very_long_value_for_the_argument,
+        **kwargs,
+    )

tests/function2.py

@@ -0,0 +1,18 @@
+def f(
+  a,
+  **kwargs,
+) -> A:
+    return A(
+        very_long_argument_name1=very_long_value_for_the_argument,
+        very_long_argument_name2=very_long_value_for_the_argument,
+        **kwargs,
+    )
+
+# output
+
+def f(a, **kwargs) -> A:
+    return A(
+        very_long_argument_name1=very_long_value_for_the_argument,
+        very_long_argument_name2=very_long_value_for_the_argument,
+        **kwargs,
+    )

tests/test_black.py

@@ -167,6 +167,14 @@ def test_function(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, line_length=ll)
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_function2(self) -> None:
+        source, expected = read_data("function2")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, line_length=ll)
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_expression(self) -> None:
         source, expected = read_data("expression")