Parenthesize conditional expressions

CHANGES.md

@@ -18,6 +18,7 @@
 - Fix determination of f-string expression spans (#2654)
 - Fix parser error location on invalid syntax in a `match` statement (#2649)
 - Functions and classes in blocks now have more consistent surrounding spacing (#2472)
+- Multiline conditional expressions are now always parenthesized (#2278)
 
 ## 21.11b1
 

src/black/linegen.py

@@ -101,6 +101,21 @@ def visit_default(self, node: LN) -> Iterator[Line]:
                 self.current_line.append(node)
         yield from super().visit_default(node)
 
+    def visit_test(self, node: Node) -> Iterator[Line]:
+        """Visit an `x if y else z` test"""
+
+        already_parenthesized = (
+            node.prev_sibling and node.prev_sibling.type == token.LPAR
+        )
+
+        if not already_parenthesized:
+            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.py

@@ -0,0 +1,116 @@
+long_kwargs_single_line = 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",
+)
+
+multiline_kwargs_indented = 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",
+)
+
+imploding_kwargs = my_function(
+    foo="test, this is a sample value",
+    bar=a
+    if foo
+    else b,
+    baz="hello, this is a another value",
+)
+
+imploding_line = (
+    1
+    if 1 + 1 == 2
+    else 0
+)
+
+exploding_line = "hello this is a slightly long string" if some_long_value_name_foo_bar_baz else "this one is a little shorter"
+
+positional_argument_test(some_long_value_name_foo_bar_baz if some_boolean_variable else some_fallback_value_foo_bar_baz)
+
+def weird_default_argument(x=some_long_value_name_foo_bar_baz
+        if SOME_CONSTANT
+        else some_fallback_value_foo_bar_baz):
+    pass
+
+nested = "hello this is a slightly long string" if (some_long_value_name_foo_bar_baz if
+                                                    nesting_test_expressions else some_fallback_value_foo_bar_baz) \
+    else "this one is a little shorter"
+
+generator_expression = (
+    some_long_value_name_foo_bar_baz if some_boolean_variable else some_fallback_value_foo_bar_baz for some_boolean_variable in some_iterable
+)
+
+# output
+
+long_kwargs_single_line = 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",
+)
+
+multiline_kwargs_indented = 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",
+)
+
+imploding_kwargs = my_function(
+    foo="test, this is a sample value",
+    bar=a if foo else b,
+    baz="hello, this is a another value",
+)
+
+imploding_line = 1 if 1 + 1 == 2 else 0
+
+exploding_line = (
+    "hello this is a slightly long string"
+    if some_long_value_name_foo_bar_baz
+    else "this one is a little shorter"
+)
+
+positional_argument_test(
+    some_long_value_name_foo_bar_baz
+    if some_boolean_variable
+    else some_fallback_value_foo_bar_baz
+)
+
+
+def weird_default_argument(
+    x=(
+        some_long_value_name_foo_bar_baz
+        if SOME_CONSTANT
+        else some_fallback_value_foo_bar_baz
+    ),
+):
+    pass
+
+
+nested = (
+    "hello this is a slightly long string"
+    if (
+        some_long_value_name_foo_bar_baz
+        if nesting_test_expressions
+        else some_fallback_value_foo_bar_baz
+    )
+    else "this one is a little shorter"
+)
+
+generator_expression = (
+    (
+        some_long_value_name_foo_bar_baz
+        if some_boolean_variable
+        else some_fallback_value_foo_bar_baz
+    )
+    for some_boolean_variable in some_iterable
+)

tests/test_format.py

@@ -30,6 +30,7 @@
     "comment_after_escaped_newline",
     "composition",
     "composition_no_trailing_comma",
+    "conditional_expression",
     "docstring",
     "empty_lines",
     "expression",