fix: Remove parenthesis around sole list items

CHANGES.md

@@ -16,6 +16,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Remove parenthesis around sole list items (#4312)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

docs/the_black_code_style/future_style.md

@@ -49,6 +49,8 @@ The unstable style additionally includes the following features:
   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))
+- `remove_lone_list_item_parens`: remove redundant parenthesis around lone list items
+  ([see below](labels/lone-list-item-parens))
 
 (labels/hug-parens)=
 
@@ -124,6 +126,62 @@ foo(
 )
 ```
 
+(labels/lone-list-item-parens)=
+
+### Remove keys around sole list item
+
+Parentheses 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 formatted as:
+
+```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
+    ),
+]
+```
+
+This style depends on `hug_parens_with_braces_and_square_brackets` to remove parenthesis
+in all cases.
+
 (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.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

@@ -206,6 +206,9 @@ 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()
 
 
 UNSTABLE_FEATURES: Set[Preview] = {
@@ -217,6 +220,8 @@ class Preview(Enum):
     Preview.multiline_string_handling,
     # See issue #4036 (crash), #4098, #4099 (proposed tweaks)
     Preview.hug_parens_with_braces_and_square_brackets,
+    # Depends on hug_parens_with_braces_and_square_brackets
+    Preview.remove_lone_list_item_parens,
 }
 
 

src/black/resources/black.schema.json

@@ -90,7 +90,8 @@
           "is_simple_lookup_for_doublestar_expression",
           "docstring_check_for_newline",
           "remove_redundant_guard_parens",
-          "parens_for_long_if_clauses_in_case_block"
+          "parens_for_long_if_clauses_in_case_block",
+          "remove_lone_list_item_parens"
         ]
       },
       "description": "Enable specific features included in the `--unstable` style. Requires `--preview`. No compatibility guarantees are provided on the behavior or existence of any unstable features."

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
+    ),
+]