Fix unnecessary parentheses in indexed assignment RHS

CHANGES.md

@@ -20,6 +20,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Fix unnecessary parentheses around short RHS expressions in indexed assignments like
+  `x[key] = expr` (#5095)
 - Prevent string merger from creating unsplittable long lines when a pragma comment
   (e.g. `# type: ignore`) follows the closing bracket (#5096)
 - Improve heuristics around whether blank lines should appear before, within and after

docs/the_black_code_style/future_style.md

@@ -28,6 +28,9 @@ Currently, the following features are included in the preview style:
 - `pyi_overload_group_blank_lines`: In `.pyi` stub files, improve heuristics around when
   blank lines should appear before, after and within decorated function groups.
   ([see below](labels/pyi-overload-group))
+- `fix_unnecessary_parens_in_indexed_assignment`: Remove unnecessary parentheses around
+  the right-hand side of indexed assignments (e.g. `x[key] = expr`) when the expression
+  is short enough. ([see below](labels/fix-unnecessary-parens-indexed-assignment))
 
 (labels/wrap-comprehension-in)=
 
@@ -189,6 +192,33 @@ def foo(x: str) -> str: ...
 def bar(x): ...
 ```
 
+(labels/fix-unnecessary-parens-indexed-assignment)=
+
+### Unnecessary parentheses in indexed assignments
+
+When an assignment target contains brackets (e.g. indexed access like `x[key] = expr`),
+Black would previously wrap the right-hand side expression in unnecessary parentheses
+when the line was too long. With this feature enabled, Black removes the unnecessary
+parentheses when the RHS expression is short enough.
+
+For example:
+
+```python
+# Before
+dictionary_of_arrays["long_key_name_for_the_example"][
+    very_long_index_name, index_zero
+] = (10 - 5)
+```
+
+will be formatted to:
+
+```python
+# After (with --preview)
+dictionary_of_arrays["long_key_name_for_the_example"][
+    very_long_index_name, index_zero
+] = 10 - 5
+```
+
 ## Unstable style
 
 (labels/unstable-style)=

src/black/linegen.py

@@ -1066,7 +1066,7 @@ def _maybe_split_omitting_optional_parens(
         # in this case; attempting a split without them is a waste of time)
         and not line.is_import
         # and we can actually remove the parens
-        and can_omit_invisible_parens(rhs, mode.line_length)
+        and can_omit_invisible_parens(rhs, mode.line_length, mode)
     ):
         omit = {id(rhs.closing_bracket), *omit}
         try:

src/black/lines.py

@@ -1350,6 +1350,7 @@ def can_be_split(line: Line) -> bool:
 def can_omit_invisible_parens(
     rhs: RHSResult,
     line_length: int,
+    mode: Mode,
 ) -> bool:
     """Does `rhs.body` have a shape safe to reformat without optional parens around it?
 
@@ -1479,6 +1480,25 @@ def can_omit_invisible_parens(
         if _can_omit_closing_paren(line, last=last, line_length=line_length):
             return True
 
+    # For assignment RHS where the LHS contains brackets (e.g. indexed
+    # assignments like `x[key] = expr`), allow omitting optional parens
+    # if the body is short enough.  The split will happen at the LHS
+    # brackets instead, and _prefer_split_rhs_oop_over_rhs will decide
+    # whether it actually produces better output.
+    if (
+        Preview.fix_unnecessary_parens_in_indexed_assignment in mode
+        and len(rhs.head.leaves) >= 2
+        and rhs.head.leaves[-2].type == token.EQUAL
+        and any(leaf.type in BRACKETS for leaf in rhs.head.leaves[:-2])
+    ):
+        # Only when the body is short enough to fit on the tail line
+        # after the LHS bracket split (e.g. `] = 10 - 5`).
+        body_length = 4 * line.depth
+        for _index, _leaf, leaf_length in line.enumerate_with_length():
+            body_length += leaf_length
+        if body_length <= line_length // 2:
+            return True
+
     return False
 
 

src/black/mode.py

@@ -258,6 +258,7 @@ class Preview(Enum):
     wrap_long_dict_values_in_parens = auto()
     fix_if_guard_explosion_in_case_statement = auto()
     pyi_overload_group_blank_lines = auto()
+    fix_unnecessary_parens_in_indexed_assignment = auto()
 
 
 UNSTABLE_FEATURES: set[Preview] = {

src/black/resources/black.schema.json

@@ -87,7 +87,8 @@
           "simplify_power_operator_hugging",
           "wrap_long_dict_values_in_parens",
           "fix_if_guard_explosion_in_case_statement",
-          "pyi_overload_group_blank_lines"
+          "pyi_overload_group_blank_lines",
+          "fix_unnecessary_parens_in_indexed_assignment"
         ]
       },
       "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_prefer_rhs_split_indexed_assignment.py

@@ -0,0 +1,21 @@
+# flags: --preview
+
+# Indexed assignment with a short RHS expression should not get unnecessary parens.
+dictionary_of_arrays["long_key_name_for_the_example"][
+    very_long_index_name, index_zero
+] = 10 - 5
+
+# Unformatted input: the unnecessary parens should be removed.
+dictionary_of_arrays["long_key_name_for_the_example"][very_long_index_name, index_zero] = (10 - 5)
+
+# output
+
+# Indexed assignment with a short RHS expression should not get unnecessary parens.
+dictionary_of_arrays["long_key_name_for_the_example"][
+    very_long_index_name, index_zero
+] = 10 - 5
+
+# Unformatted input: the unnecessary parens should be removed.
+dictionary_of_arrays["long_key_name_for_the_example"][
+    very_long_index_name, index_zero
+] = 10 - 5