Parenthesize conditional expressions

black.py

@@ -1755,6 +1755,25 @@ def visit_default(self, node: LN) -> Iterator[Line]:
                 self.current_line.append(node)
         yield from super().visit_default(node)
 
+    def visit_test(self, node: LN) -> Iterator[Line]:
+        """Visit an `x if y else z` test"""
+
+        # parenthesize conditional expressions which span multiple lines
+        already_parenthesized = (
+            node.prev_sibling and node.prev_sibling.type == token.LPAR
+        )
+        as_str = str(node)
+        multiline = "\n" in as_str and not (
+            as_str.startswith("\n") or as_str.endswith("\n")
+        )
+        if not already_parenthesized and multiline:
+            lpar = Leaf(token.LPAR, "(")
+            rpar = Leaf(token.RPAR, ")")
+            node.insert_child(0, lpar)
+            node.append_child(rpar)
+
+        yield from self.visit_default(node)
+
     def visit_INDENT(self, node: Leaf) -> Iterator[Line]:
         """Increase indentation level, maybe yield a line."""
         # In blib2to3 INDENT never holds comments.

tests/data/conditional_expression_kwargs.py

@@ -0,0 +1,19 @@
+aaa = my_function(
+    foo="test, this is a sample value",
+    bar=some_long_value_name_foo_bar_baz
+    if some_boolean_variable
+    else some_fallback_value_foo_bar_baz,
+    baz="hello, this is a another value",
+)
+
+# output
+
+aaa = my_function(
+    foo="test, this is a sample value",
+    bar=(
+        some_long_value_name_foo_bar_baz
+        if some_boolean_variable
+        else some_fallback_value_foo_bar_baz
+    ),
+    baz="hello, this is a another value",
+)

tests/test_black.py

@@ -280,6 +280,14 @@ def test_function2(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_conditional_expression_kwargs(self) -> None:
+        source, expected = read_data("conditional_expression_kwargs")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_function_trailing_comma(self) -> None:
         source, expected = read_data("function_trailing_comma")