fix: Don't normalize whitespace before fmt:skip comments

CHANGES.md

@@ -24,6 +24,7 @@
 - For stubs, fix logic to enforce empty line after nested classes with bodies (#4141)
 - Fix crash when using a walrus in a dictionary (#4155)
 - Fix unnecessary parentheses when wrapping long dicts (#4135)
+- Stop normalizing spaces before `# fmt: skip` comments (#4146)
 
 ### Configuration
 

src/black/comments.py

@@ -46,6 +46,7 @@ class ProtoComment:
     newlines: int  # how many newlines before the comment
     consumed: int  # how many characters of the original leaf's prefix did we consume
     form_feed: bool  # is there a form feed before the comment
+    leading_whitespace: str  # leading whitespace before the comment, if any
 
 
 def generate_comments(leaf: LN) -> Iterator[Leaf]:
@@ -88,7 +89,9 @@ def list_comments(prefix: str, *, is_endmarker: bool) -> List[ProtoComment]:
     form_feed = False
     for index, full_line in enumerate(re.split("\r?\n", prefix)):
         consumed += len(full_line) + 1  # adding the length of the split '\n'
-        line = full_line.lstrip()
+        match = re.match(r"^(\s*)(\S.*|)$", full_line)
+        assert match
+        whitespace, line = match.groups()
         if not line:
             nlines += 1
             if "\f" in full_line:
@@ -113,6 +116,7 @@ def list_comments(prefix: str, *, is_endmarker: bool) -> List[ProtoComment]:
                 newlines=nlines,
                 consumed=consumed,
                 form_feed=form_feed,
+                leading_whitespace=whitespace,
             )
         )
         form_feed = False
@@ -230,7 +234,11 @@ def convert_one_fmt_off_pair(
                 standalone_comment_prefix += fmt_off_prefix
                 hidden_value = comment.value + "\n" + hidden_value
             if _contains_fmt_skip_comment(comment.value, mode):
-                hidden_value += "  " + comment.value
+                hidden_value += (
+                    comment.leading_whitespace
+                    if Preview.no_normalize_fmt_skip_whitespace in mode
+                    else "  "
+                ) + comment.value
             if hidden_value.endswith("\n"):
                 # That happens when one of the `ignored_nodes` ended with a NEWLINE
                 # leaf (possibly followed by a DEDENT).

src/black/mode.py

@@ -197,6 +197,7 @@ class Preview(Enum):
     allow_form_feeds = auto()
     unify_docstring_detection = auto()
     respect_east_asian_width = auto()
+    no_normalize_fmt_skip_whitespace = auto()
 
 
 class Deprecated(UserWarning):

tests/data/cases/fmtskip9.py

@@ -0,0 +1,9 @@
+# flags: --preview
+print () # fmt: skip
+print () # fmt:skip
+
+
+# output
+
+print () # fmt: skip
+print () # fmt:skip