fix: Remove parenthesis around sole list items

CHANGES.md

@@ -74,6 +74,7 @@
 
 - Fix type annotation spacing between * and more complex type variable tuple (i.e. `def
   fn(*args: *tuple[*Ts, T]) -> None: pass`) (#4440)
+- Remove parenthesis around sole list items (#4312)
 
 ### Caching
 

docs/the_black_code_style/future_style.md

@@ -38,6 +38,9 @@ Currently, the following features are included in the preview style:
   blocks when the line is too long
 - `pep646_typed_star_arg_type_var_tuple`: fix type annotation spacing between * and more
   complex type variable tuple (i.e. `def fn(*args: *tuple[*Ts, T]) -> None: pass`)
+- `remove_lone_list_item_parens`: remove redundant parenthesis around lone list items
+  (depends on unstable `hug_parens_with_braces_and_square_brackets` feature in some
+  cases)
 
 (labels/unstable-features)=
 

src/black/linegen.py

@@ -507,6 +507,19 @@ def visit_NUMBER(self, leaf: Leaf) -> Iterator[Line]:
         normalize_numeric_literal(leaf)
         yield from self.visit_default(leaf)
 
+    def visit_atom(self, node: Node) -> Iterator[Line]:
+        """Visit any atom"""
+        if (
+            Preview.remove_lone_list_item_parens in self.mode
+            and len(node.children) == 3
+            and node.children[0].type == token.LSQB
+            and node.children[-1].type == token.RSQB
+        ):
+            # Lists of one item
+            maybe_make_parens_invisible_in_atom(node.children[1], parent=node)
+
+        yield from self.visit_default(node)
+
     def visit_fstring(self, node: Node) -> Iterator[Line]:
         # currently we don't want to format and split f-strings at all.
         string_leaf = fstring_to_string(node)

src/black/mode.py

@@ -210,6 +210,9 @@ class Preview(Enum):
     docstring_check_for_newline = auto()
     remove_redundant_guard_parens = auto()
     parens_for_long_if_clauses_in_case_block = auto()
+    # NOTE: remove_lone_list_item_parens requires
+    # hug_parens_with_braces_and_square_brackets to remove parens in some cases
+    remove_lone_list_item_parens = auto()
     pep646_typed_star_arg_type_var_tuple = auto()
 
 

src/black/resources/black.schema.json

@@ -91,6 +91,7 @@
           "docstring_check_for_newline",
           "remove_redundant_guard_parens",
           "parens_for_long_if_clauses_in_case_block",
+          "remove_lone_list_item_parens",
           "pep646_typed_star_arg_type_var_tuple"
         ]
       },

tests/data/cases/preview_remove_lone_list_item_parens.py

@@ -0,0 +1,156 @@
+# flags: --unstable
+items = [(123)]
+items = [(True)]
+items = [(((((True)))))]
+items = [(((((True,)))))]
+items = [((((()))))]
+
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        if some_var == "long strings"
+        else {"key": "val"}
+    )
+]
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        if some_var == "long strings"
+        else {"key": "val"}
+    ),
+]
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    if some_var == "long strings"
+    else {"key": "val"}
+]
+items = [
+    (
+        {"key1": "val1", "key2": "val2"}
+        if some_var == ""
+        else {"key": "val"}
+    )
+]
+items = [{"key1": "val1", "key2": "val2"} if some_var == "" else {"key": "val"}]
+items = [
+    (
+        "123456890123457890123468901234567890"
+        if some_var == "long strings"
+        else "123467890123467890"
+    )
+]
+
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        and some_var == "long strings"
+        and {"key": "val"}
+    )
+]
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        and some_var == "long strings"
+        and {"key": "val"}
+    ),
+]
+items = [
+    (
+        "123456890123457890123468901234567890"
+        and some_var == "long strings"
+        and "123467890123467890"
+    )
+]
+items = [
+    (
+        "123456890123457890123468901234567890"
+        and some_var == "long strings"
+        and "123467890123467890"
+    ),
+]
+
+items = [
+    (
+        long_variable_name
+        and even_longer_variable_name
+        and yet_another_very_long_variable_name
+    )
+]
+items = [
+    (
+        long_variable_name
+        and even_longer_variable_name
+        and yet_another_very_long_variable_name
+    ),
+]
+
+
+# output
+items = [123]
+items = [True]
+items = [True]
+items = [(True,)]
+items = [()]
+
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    if some_var == "long strings"
+    else {"key": "val"}
+]
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        if some_var == "long strings"
+        else {"key": "val"}
+    ),
+]
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    if some_var == "long strings"
+    else {"key": "val"}
+]
+items = [{"key1": "val1", "key2": "val2"} if some_var == "" else {"key": "val"}]
+items = [{"key1": "val1", "key2": "val2"} if some_var == "" else {"key": "val"}]
+items = [
+    "123456890123457890123468901234567890"
+    if some_var == "long strings"
+    else "123467890123467890"
+]
+
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    and some_var == "long strings"
+    and {"key": "val"}
+]
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        and some_var == "long strings"
+        and {"key": "val"}
+    ),
+]
+items = [
+    "123456890123457890123468901234567890"
+    and some_var == "long strings"
+    and "123467890123467890"
+]
+items = [
+    (
+        "123456890123457890123468901234567890"
+        and some_var == "long strings"
+        and "123467890123467890"
+    ),
+]
+
+items = [
+    long_variable_name
+    and even_longer_variable_name
+    and yet_another_very_long_variable_name
+]
+items = [
+    (
+        long_variable_name
+        and even_longer_variable_name
+        and yet_another_very_long_variable_name
+    ),
+]