Avoid removing whitespace for walrus operators within subscripts (#3823)

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

CHANGES.md

@@ -80,6 +80,7 @@
   (#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/mode.py

@@ -183,6 +183,7 @@ class Preview(Enum):
     wrap_long_dict_values_in_parens = auto()
     wrap_multiple_context_managers_in_parens = auto()
     dummy_implementations = auto()
+    walrus_subscript = auto()
 
 
 class Deprecated(UserWarning):

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, Preview
 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 +346,11 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa: C901
 
             return NO
 
+        elif Preview.walrus_subscript in mode 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 +1,6 @@
+x[(a:=0):]
+x[:(a:=0)]
+
+# output
+x[(a := 0):]
+x[:(a := 0)]

tests/data/preview_py_310/pep_572.py

@@ -0,0 +1,12 @@
+x[a:=0]
+x[a := 0]
+x[a := 0, b := 1]
+x[5, b := 0]
+x[a:=0,b:=1]
+
+# output
+x[a := 0]
+x[a := 0]
+x[a := 0, b := 1]
+x[5, b := 0]
+x[a := 0, b := 1]

tests/test_format.py

@@ -56,6 +56,13 @@ def test_preview_context_managers_targeting_py39() -> None:
     assert_format(source, expected, mode, minimum_version=(3, 9))
 
 
+@pytest.mark.parametrize("filename", all_data_cases("preview_py_310"))
+def test_preview_python_310(filename: str) -> None:
+    source, expected = read_data("preview_py_310", filename)
+    mode = black.Mode(target_versions={black.TargetVersion.PY310}, preview=True)
+    assert_format(source, expected, mode, minimum_version=(3, 10))
+
+
 @pytest.mark.parametrize(
     "filename", all_data_cases("preview_context_managers/auto_detect")
 )