Remove unnecessary parentheses from except clauses (#2939)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

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 `except` statements (#2939)
 - Remove unnecessary parentheses from tuple unpacking in `for` loops (#2945)
 - Avoid magic-trailing-comma in single-element subscripts (#2942)
 

src/black/linegen.py

@@ -318,7 +318,12 @@ def __post_init__(self) -> None:
         self.visit_try_stmt = partial(
             v, keywords={"try", "except", "else", "finally"}, parens=Ø
         )
-        self.visit_except_clause = partial(v, keywords={"except"}, parens=Ø)
+        if self.mode.preview:
+            self.visit_except_clause = partial(
+                v, keywords={"except"}, parens={"except"}
+            )
+        else:
+            self.visit_except_clause = partial(v, keywords={"except"}, parens=Ø)
         self.visit_with_stmt = partial(v, keywords={"with"}, parens=Ø)
         self.visit_funcdef = partial(v, keywords={"def"}, parens=Ø)
         self.visit_classdef = partial(v, keywords={"class"}, parens=Ø)

tests/data/remove_except_parens.py

@@ -0,0 +1,79 @@
+# These brackets are redundant, therefore remove.
+try:
+    a.something
+except (AttributeError) as err:
+    raise err
+
+# This is tuple of exceptions.
+# Although this could be replaced with just the exception,
+# we do not remove brackets to preserve AST.
+try:
+    a.something
+except (AttributeError,) as err:
+    raise err
+
+# This is a tuple of exceptions. Do not remove brackets.
+try:
+    a.something
+except (AttributeError, ValueError) as err:
+    raise err
+
+# Test long variants.
+try:
+    a.something
+except (some.really.really.really.looooooooooooooooooooooooooooooooong.module.over89.chars.Error) as err:
+    raise err
+
+try:
+    a.something
+except (some.really.really.really.looooooooooooooooooooooooooooooooong.module.over89.chars.Error,) as err:
+    raise err
+
+try:
+    a.something
+except (some.really.really.really.looooooooooooooooooooooooooooooooong.module.over89.chars.Error, some.really.really.really.looooooooooooooooooooooooooooooooong.module.over89.chars.Error) as err:
+    raise err
+
+# output
+# These brackets are redundant, therefore remove.
+try:
+    a.something
+except AttributeError as err:
+    raise err
+
+# This is tuple of exceptions.
+# Although this could be replaced with just the exception,
+# we do not remove brackets to preserve AST.
+try:
+    a.something
+except (AttributeError,) as err:
+    raise err
+
+# This is a tuple of exceptions. Do not remove brackets.
+try:
+    a.something
+except (AttributeError, ValueError) as err:
+    raise err
+
+# Test long variants.
+try:
+    a.something
+except (
+    some.really.really.really.looooooooooooooooooooooooooooooooong.module.over89.chars.Error
+) as err:
+    raise err
+
+try:
+    a.something
+except (
+    some.really.really.really.looooooooooooooooooooooooooooooooong.module.over89.chars.Error,
+) as err:
+    raise err
+
+try:
+    a.something
+except (
+    some.really.really.really.looooooooooooooooooooooooooooooooong.module.over89.chars.Error,
+    some.really.really.really.looooooooooooooooooooooooooooooooong.module.over89.chars.Error,
+) as err:
+    raise err

tests/test_format.py

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