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)
 - Improve heuristics around whether blank lines should appear before, within and after
   groups of same-name decorated functions (such as `@overload` groups) in `.pyi` stub
   files (#5021)

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 = Mode(),
 ) -> bool:
     """Does `rhs.body` have a shape safe to reformat without optional parens around it?
 
@@ -1479,6 +1480,23 @@ 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 = str_width(str(line).strip("\n"))
+        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] = {

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