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/nodes.py

@@ -345,6 +345,9 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa: C901
 
             return NO
 
+        elif t == token.COLONEQUAL or prev.type == token.COLONEQUAL:
+            return SPACE
+
         elif not complex_subscript:
             return NO
 

tests/data/py_310/pep_572_py310.py

@@ -1,7 +1,7 @@
 # Unparenthesized walruses are now allowed in indices since Python 3.10.
-x[a:=0]
-x[a:=0, b:=1]
-x[5, b:=0]
+x[a := 0]
+x[a := 0, b := 1]
+x[5, b := 0]
 
 # Walruses are allowed inside generator expressions on function calls since 3.10.
 if any(match := pattern_error.match(s) for s in buffer):