Format one-tuples and one-subscripts as multi-line if already multi-line

Fix psf#3918.

CHANGES.md

@@ -10,6 +10,11 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Magic trailing commas are now respected in one-tuples and single-element
+  subscripts when they are already split to multiple lines. So a multiline
+  one-tuple is no longer collapsed to a single line when magic trailing
+  commas are enabled. (#4038)
+
 ### Preview style
 
 <!-- Changes that affect Black's preview style -->

src/black/lines.py

@@ -333,10 +333,18 @@ def contains_multiline_strings(self) -> bool:
     def has_magic_trailing_comma(
         self, closing: Leaf, ensure_removable: bool = False
     ) -> bool:
-        """Return True if we have a magic trailing comma, that is when:
-        - there's a trailing comma here
-        - it's not a one-tuple
-        - it's not a single-element subscript
+        """Return whether we have a magic trailing comma.
+
+        This is when:
+
+        * there's a trailing comma here
+        * it's not a one-tuple
+        * it's not a single-element subscript
+
+        Additionally, if ``ensure_removable`` is false:
+
+        * it's a one-tuple or single-element subscript that is already split to
+          multiple lines
         """
         if not (
             closing.type in CLOSING_BRACKETS
@@ -360,7 +368,10 @@ def has_magic_trailing_comma(
                     brackets=(token.LSQB, token.RSQB),
                 )
             ):
-                return False
+                return (
+                    closing.opening_bracket.lineno != closing.lineno
+                    and not ensure_removable
+                )
 
             return True
 
@@ -374,7 +385,10 @@ def has_magic_trailing_comma(
                 self.leaves,
                 brackets=(token.LPAR, token.RPAR),
             ):
-                return False
+                return (
+                    closing.opening_bracket.lineno != closing.lineno
+                    and not ensure_removable
+                )
 
             return True
 

tests/data/cases/collections.py

@@ -37,6 +37,9 @@
 ['ls', 'lsoneple/%s' % (foo,)]
 x = {"oneple": (1,)}
 y = {"oneple": (1,),}
+z = {"oneple": (
+    1,
+)}
 assert False, ("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa wraps %s" % bar)
 
 # looping over a 1-tuple should also not get wrapped
@@ -45,6 +48,18 @@
 for (x,) in (1,), (2,), (3,):
     pass
 
+(1,)
+(
+    1,
+)
+[1,]
+[
+    1,
+]
+{1,}
+{
+    1,
+}
 [1, 2, 3,]
 
 division_result_tuple = (6/2,)
@@ -127,6 +142,11 @@
 y = {
     "oneple": (1,),
 }
+z = {
+    "oneple": (
+        1,
+    )
+}
 assert False, (
     "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa wraps %s"
     % bar
@@ -138,6 +158,22 @@
 for (x,) in (1,), (2,), (3,):
     pass
 
+(1,)
+(
+    1,
+)
+[
+    1,
+]
+[
+    1,
+]
+{
+    1,
+}
+{
+    1,
+}
 [
     1,
     2,

tests/data/cases/one_element_subscript.py

@@ -1,8 +1,17 @@
 # We should not treat the trailing comma
-# in a single-element subscript.
+# in a single-line single-element subscript.
 a: tuple[int,]
 b = tuple[int,]
 
+# Trailing comma should be preserved on multi-line
+# single-element subscript.
+a: tuple[
+    int,
+]
+b = tuple[
+    int,
+]
+
 # The magic comma still applies to multi-element subscripts.
 c: tuple[int, int,]
 d = tuple[int, int,]
@@ -13,10 +22,19 @@
 
 # output
 # We should not treat the trailing comma
-# in a single-element subscript.
+# in a single-line single-element subscript.
 a: tuple[int,]
 b = tuple[int,]
 
+# Trailing comma should be preserved on multi-line
+# single-element subscript.
+a: tuple[
+    int,
+]
+b = tuple[
+    int,
+]
+
 # The magic comma still applies to multi-element subscripts.
 c: tuple[
     int,

tests/data/cases/skip_magic_trailing_comma.py

@@ -1,7 +1,13 @@
 # flags: --skip-magic-trailing-comma
 # We should not remove the trailing comma in a single-element subscript.
 a: tuple[int,]
+aa: tuple[
+    int,
+]
 b = tuple[int,]
+bb = tuple[
+    int,
+]
 
 # But commas in multiple element subscripts should be removed.
 c: tuple[int, int,]
@@ -15,6 +21,9 @@
 
 # Except single element tuples
 small_tuple = (1,)
+small_multiline_tuple = (
+    1,
+)
 
 # Trailing commas in multiple chained non-nested parens.
 zero(
@@ -50,7 +59,9 @@
 # output
 # We should not remove the trailing comma in a single-element subscript.
 a: tuple[int,]
+aa: tuple[int,]
 b = tuple[int,]
+bb = tuple[int,]
 
 # But commas in multiple element subscripts should be removed.
 c: tuple[int, int]
@@ -64,6 +75,7 @@
 
 # Except single element tuples
 small_tuple = (1,)
+small_multiline_tuple = (1,)
 
 # Trailing commas in multiple chained non-nested parens.
 zero(one).two(three).four(five)