fix: Remove parenthesis around sole list items

CHANGES.md

@@ -18,6 +18,10 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Remove parenthesis around sole list items and rename
+  `hug_parens_with_braces_and_square_brackets` unstable style to
+  `concise_nested_brackets` (#4312)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

docs/the_black_code_style/future_style.md

@@ -47,8 +47,8 @@ The unstable style additionally includes the following features:
   ([see below](labels/wrap-long-dict-values))
 - `multiline_string_handling`: more compact formatting of expressions involving
   multiline strings ([see below](labels/multiline-string-handling))
-- `hug_parens_with_braces_and_square_brackets`: more compact formatting of nested
-  brackets ([see below](labels/hug-parens))
+- `concise_nested_brackets`: more compact formatting of nested brackets
+  ([see below](labels/hug-parens))
 
 (labels/hug-parens)=
 
@@ -124,6 +124,55 @@ foo(
 )
 ```
 
+Parenthesises around sole list items will also be removed for similar reasons. Trailing
+commas are respected here as well. For example:
+
+```python
+items = [(123)]
+items = [(True)]
+items = [((True,))]
+items = [(())]
+
+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
+    ),
+]
+```
+
+will be changed to:
+
+```python
+items = [123]
+items = [True]
+items = [(True,)]
+items = [()]
+
+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
+    ),
+]
+```
+
 (labels/string-processing)=
 
 ### Improved string processing

src/black/linegen.py

@@ -506,6 +506,19 @@ 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.concise_nested_brackets 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)
@@ -855,7 +868,7 @@ def _first_right_hand_split(
 
     body: Optional[Line] = None
     if (
-        Preview.hug_parens_with_braces_and_square_brackets in line.mode
+        Preview.concise_nested_brackets in line.mode
         and tail_leaves[0].value
         and tail_leaves[0].opening_bracket is head_leaves[-1]
     ):

src/black/mode.py

@@ -173,7 +173,7 @@ class Preview(Enum):
     # NOTE: string_processing requires wrap_long_dict_values_in_parens
     # for https://github.com/psf/black/issues/3117 to be fixed.
     string_processing = auto()
-    hug_parens_with_braces_and_square_brackets = auto()
+    concise_nested_brackets = auto()
     unify_docstring_detection = auto()
     no_normalize_fmt_skip_whitespace = auto()
     wrap_long_dict_values_in_parens = auto()
@@ -193,7 +193,7 @@ class Preview(Enum):
     # See issue #4159
     Preview.multiline_string_handling,
     # See issue #4036 (crash), #4098, #4099 (proposed tweaks)
-    Preview.hug_parens_with_braces_and_square_brackets,
+    Preview.concise_nested_brackets,
 }
 
 

src/black/resources/black.schema.json

@@ -80,7 +80,7 @@
         "enum": [
           "hex_codes_in_unicode_sequences",
           "string_processing",
-          "hug_parens_with_braces_and_square_brackets",
+          "concise_nested_brackets",
           "unify_docstring_detection",
           "no_normalize_fmt_skip_whitespace",
           "wrap_long_dict_values_in_parens",

tests/data/cases/preview_remove_sole_list_item_parens.py

@@ -0,0 +1,168 @@
+# flags: --unstable
+items = [(123)]
+items = [(True)]
+items = [(((((True)))))]
+items = [(((((True,)))))]
+items = [((((()))))]
+
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        if some_var == "longstring"
+        else { "key": "val" }
+    )
+]
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        if some_var == "longstring"
+        else { "key": "val" }
+    ),
+]
+items = [(
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    if some_var == "longstring"
+    else { "key": "val" }
+)]
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    if some_var == "longstring"
+    else {"key": "val"}
+]
+items = [
+    (
+        {"key1": "val1", "key2": "val2"} if some_var == "longstring" else { "key": "val" }
+    )
+]
+items = [(
+    {"key1": "val1", "key2": "val2"} if some_var == "" else { "key": "val" }
+)]
+items = [
+    {"key1": "val1", "key2": "val2"} if some_var == "longstring" else { "key": "val" }
+]
+items = [
+    ({"key1": "val1", "key2": "val2"} if some_var == "longstring" 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 == "longstrings"
+        else "123467890123467890"
+    )
+]
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    and some_var == "longstring"
+    and {"key": "val"}
+]
+items = [
+    (
+        "123456890123457890123468901234567890"
+        and some_var == "longstrings"
+        and "123467890123467890"
+    )
+]
+items = [(
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    and some_var == "longstring"
+    and {"key": "val"}
+)]
+
+
+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 == "longstring"
+    else {"key": "val"}
+]
+items = [
+    (
+        {"key1": "val1", "key2": "val2", "key3": "val3"}
+        if some_var == "longstring"
+        else {"key": "val"}
+    ),
+]
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    if some_var == "longstring"
+    else {"key": "val"}
+]
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    if some_var == "longstring"
+    else {"key": "val"}
+]
+items = [
+    {"key1": "val1", "key2": "val2"} if some_var == "longstring" else {"key": "val"}
+]
+items = [{"key1": "val1", "key2": "val2"} if some_var == "" else {"key": "val"}]
+items = [
+    {"key1": "val1", "key2": "val2"} if some_var == "longstring" else {"key": "val"}
+]
+items = [
+    {"key1": "val1", "key2": "val2"} if some_var == "longstring" 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 == "longstrings"
+    else "123467890123467890"
+]
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    and some_var == "longstring"
+    and {"key": "val"}
+]
+items = [
+    "123456890123457890123468901234567890"
+    and some_var == "longstrings"
+    and "123467890123467890"
+]
+items = [
+    {"key1": "val1", "key2": "val2", "key3": "val3"}
+    and some_var == "longstring"
+    and {"key": "val"}
+]
+
+
+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
+    ),
+]