Remove unnecessary parentheses from except clauses

CHANGES.md

@@ -14,6 +14,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Remove unnecessary parentheses from `except` statements (#2939)
+
 ### _Blackd_
 
 <!-- Changes to blackd -->

src/black/linegen.py

@@ -313,7 +313,12 @@ 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 Preview.remove_except_parens in self.mode:
+            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)

src/black/mode.py

@@ -127,6 +127,7 @@ class Preview(Enum):
     """Individual preview style features."""
 
     string_processing = auto()
+    remove_except_parens = auto()
 
 
 class Deprecated(UserWarning):
@@ -162,9 +163,7 @@ def __contains__(self, feature: Preview) -> bool:
         """
         if feature is Preview.string_processing:
             return self.preview or self.experimental_string_processing
-        # TODO: Remove type ignore comment once preview contains more features
-        #  than just ESP
-        return self.preview  # type: ignore
+        return self.preview
 
     def get_cache_key(self) -> str:
         if self.target_versions:

tests/data/remove_except_parens.py

@@ -0,0 +1,40 @@
+# 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
+
+# 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

tests/test_format.py

@@ -79,6 +79,7 @@
     "long_strings__edge_case",
     "long_strings__regression",
     "percent_precedence",
+    "remove_except_parens",
 ]
 
 SOURCES: List[str] = [