Avoid removing whitespace for walrus operators within subscripts

CHANGES.md

@@ -66,6 +66,7 @@
   (#3740)
 - Fix error in AST validation when _Black_ removes trailing whitespace in a type comment
   (#3773)
+- Fix a bug whereby spaces were removed from walrus operators within subscript (#3823)
 
 ### Preview style
 

src/black/lines.py

@@ -81,7 +81,9 @@ def append(
             # Note: at this point leaf.prefix should be empty except for
             # imports, for which we only preserve newlines.
             leaf.prefix += whitespace(
-                leaf, complex_subscript=self.is_complex_subscript(leaf)
+                leaf,
+                complex_subscript=self.is_complex_subscript(leaf),
+                mode=self.mode,
             )
         if self.inside_brackets or not preformatted or track_bracket:
             self.bracket_tracker.mark(leaf)

src/black/nodes.py

@@ -13,6 +13,7 @@
 from mypy_extensions import mypyc_attr
 
 from black.cache import CACHE_DIR
+from black.mode import Mode
 from black.strings import has_triple_quotes
 from blib2to3 import pygram
 from blib2to3.pgen2 import token
@@ -171,7 +172,7 @@ def visit_default(self, node: LN) -> Iterator[T]:
                 yield from self.visit(child)
 
 
-def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa: C901
+def whitespace(leaf: Leaf, *, complex_subscript: bool, mode: Mode) -> str:  # noqa: C901
     """Return whitespace prefix if needed for the given `leaf`.
 
     `complex_subscript` signals whether the given leaf is part of a subscription
@@ -345,6 +346,9 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa: C901
 
             return NO
 
+        elif mode.preview and (t == token.COLONEQUAL or prev.type == token.COLONEQUAL):
+            return SPACE
+
         elif not complex_subscript:
             return NO
 

tests/data/preview/pep_572.py

@@ -0,0 +1,3 @@
+x[a := 0]
+x[a := 0, b := 1]
+x[5, b := 0]