Remove unnecessary parentheses from tuple unpacking in for loops

CHANGES.md

@@ -15,6 +15,7 @@
 <!-- Changes that affect Black's preview style -->
 
 - Code cell separators `#%%` are now standardised to `# %%` (#2919)
+- Remove unnecessary parentheses from tuple unpacking in `for` loops (#2945)
 - Avoid magic-trailing-comma in single-element subscripts (#2942)
 
 ### _Blackd_

src/black/linegen.py

@@ -841,7 +841,11 @@ def normalize_invisible_parens(
 
         if check_lpar:
             if child.type == syms.atom:
-                if maybe_make_parens_invisible_in_atom(child, parent=node):
+                if maybe_make_parens_invisible_in_atom(
+                    child,
+                    parent=node,
+                    preview=preview,
+                ):
                     wrap_in_parentheses(node, child, visible=False)
             elif is_one_tuple(child):
                 wrap_in_parentheses(node, child, visible=True)
@@ -865,21 +869,35 @@ def normalize_invisible_parens(
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 
 
-def maybe_make_parens_invisible_in_atom(node: LN, parent: LN) -> bool:
+def maybe_make_parens_invisible_in_atom(
+    node: LN,
+    parent: LN,
+    preview: bool = False,
+) -> bool:
     """If it's safe, make the parens in the atom `node` invisible, recursively.
     Additionally, remove repeated, adjacent invisible parens from the atom `node`
     as they are redundant.
 
     Returns whether the node should itself be wrapped in invisible parentheses.
 
     """
+    if (
+        preview
+        and parent.type == syms.for_stmt
+        and isinstance(node.prev_sibling, Leaf)
+        and node.prev_sibling.type == token.NAME
+        and node.prev_sibling.value == "for"
+    ):
+        for_stmt_check = False
+    else:
+        for_stmt_check = True
 
     if (
         node.type != syms.atom
         or is_empty_tuple(node)
         or is_one_tuple(node)
         or (is_yield(node) and parent.type != syms.expr_stmt)
-        or max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY
+        or (max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY and for_stmt_check)
     ):
         return False
 
@@ -902,7 +920,7 @@ def maybe_make_parens_invisible_in_atom(node: LN, parent: LN) -> bool:
         # make parentheses invisible
         first.value = ""
         last.value = ""
-        maybe_make_parens_invisible_in_atom(middle, parent=parent)
+        maybe_make_parens_invisible_in_atom(middle, parent=parent, preview=preview)
 
         if is_atom_with_invisible_parens(middle):
             # Strip the invisible parens from `middle` by replacing

src/black/mode.py

@@ -127,6 +127,7 @@ class Preview(Enum):
     """Individual preview style features."""
 
     string_processing = auto()
+    remove_redundant_parens = auto()
     one_element_subscript = auto()
 
 

src/black/trans.py

@@ -365,7 +365,7 @@ def do_match(self, line: Line) -> TMatchResult:
 
         is_valid_index = is_valid_index_factory(LL)
 
-        for (i, leaf) in enumerate(LL):
+        for i, leaf in enumerate(LL):
             if (
                 leaf.type == token.STRING
                 and is_valid_index(i + 1)
@@ -570,7 +570,7 @@ def make_naked(string: str, string_prefix: str) -> str:
 
         # Build the final line ('new_line') that this method will later return.
         new_line = line.clone()
-        for (i, leaf) in enumerate(LL):
+        for i, leaf in enumerate(LL):
             if i == string_idx:
                 new_line.append(string_leaf)
 
@@ -691,7 +691,7 @@ def do_match(self, line: Line) -> TMatchResult:
 
         is_valid_index = is_valid_index_factory(LL)
 
-        for (idx, leaf) in enumerate(LL):
+        for idx, leaf in enumerate(LL):
             # Should be a string...
             if leaf.type != token.STRING:
                 continue
@@ -1713,7 +1713,7 @@ def _assert_match(LL: List[Leaf]) -> Optional[int]:
         if parent_type(LL[0]) == syms.assert_stmt and LL[0].value == "assert":
             is_valid_index = is_valid_index_factory(LL)
 
-            for (i, leaf) in enumerate(LL):
+            for i, leaf in enumerate(LL):
                 # We MUST find a comma...
                 if leaf.type == token.COMMA:
                     idx = i + 2 if is_empty_par(LL[i + 1]) else i + 1
@@ -1751,7 +1751,7 @@ def _assign_match(LL: List[Leaf]) -> Optional[int]:
         ):
             is_valid_index = is_valid_index_factory(LL)
 
-            for (i, leaf) in enumerate(LL):
+            for i, leaf in enumerate(LL):
                 # We MUST find either an '=' or '+=' symbol...
                 if leaf.type in [token.EQUAL, token.PLUSEQUAL]:
                     idx = i + 2 if is_empty_par(LL[i + 1]) else i + 1
@@ -1794,7 +1794,7 @@ def _dict_match(LL: List[Leaf]) -> Optional[int]:
         if syms.dictsetmaker in [parent_type(LL[0]), parent_type(LL[0].parent)]:
             is_valid_index = is_valid_index_factory(LL)
 
-            for (i, leaf) in enumerate(LL):
+            for i, leaf in enumerate(LL):
                 # We MUST find a colon...
                 if leaf.type == token.COLON:
                     idx = i + 2 if is_empty_par(LL[i + 1]) else i + 1

tests/data/long_strings__regression.py

@@ -599,7 +599,7 @@ def foo():
 
 
 def foo(xxxx):
-    for (xxx_xxxx, _xxx_xxx, _xxx_xxxxx, xxx_xxxx) in xxxx:
+    for xxx_xxxx, _xxx_xxx, _xxx_xxxxx, xxx_xxxx in xxxx:
         for xxx in xxx_xxxx:
             assert ("x" in xxx) or (xxx in xxx_xxx_xxxxx), (
                 "{0} xxxxxxx xx {1}, xxx {1} xx xxx xx xxxx xx xxx xxxx: xxx xxxx {2}"

tests/data/remove_for_brackets.py

@@ -0,0 +1,18 @@
+# Only remove tuple brackets after `for`
+for (k, v) in d.items():
+    print(k, v)
+
+# Don't touch tuple brackets after `in`
+for module in (core, _unicodefun):
+    if hasattr(module, "_verify_python3_env"):
+        module._verify_python3_env = lambda: None
+
+# output
+# Only remove tuple brackets after `for`
+for k, v in d.items():
+    print(k, v)
+
+# Don't touch tuple brackets after `in`
+for module in (core, _unicodefun):
+    if hasattr(module, "_verify_python3_env"):
+        module._verify_python3_env = lambda: None

tests/test_format.py

@@ -80,6 +80,7 @@
     "long_strings__edge_case",
     "long_strings__regression",
     "percent_precedence",
+    "remove_for_brackets",
     "one_element_subscript",
 ]