Fix bug where # fmt: skip is not being respected with one-liner functions (#4552)

CHANGES.md

@@ -14,6 +14,9 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Fix a bug where one-liner functions/conditionals marked with `# fmt: skip`
+  would still be formatted (#4552)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

docs/the_black_code_style/future_style.md

@@ -26,6 +26,9 @@ Currently, the following features are included in the preview style:
   statements, except when the line after the import is a comment or an import statement
 - `wrap_long_dict_values_in_parens`: Add parentheses around long values in dictionaries
   ([see below](labels/wrap-long-dict-values))
+- `fix_fmt_skip_in_one_liners`: Fix `# fmt: skip` behaviour on one-liner declarations,
+  such as `def foo(): return "mock"  # fmt: skip`, where previously the declaration
+  would have been incorrectly collapsed.
 
 (labels/unstable-features)=
 

src/black/comments.py

@@ -4,7 +4,7 @@
 from functools import lru_cache
 from typing import Final, Optional, Union
 
-from black.mode import Mode
+from black.mode import Mode, Preview
 from black.nodes import (
     CLOSING_BRACKETS,
     STANDALONE_COMMENT,
@@ -270,7 +270,7 @@ def generate_ignored_nodes(
     Stops at the end of the block.
     """
     if _contains_fmt_skip_comment(comment.value, mode):
-        yield from _generate_ignored_nodes_from_fmt_skip(leaf, comment)
+        yield from _generate_ignored_nodes_from_fmt_skip(leaf, comment, mode)
         return
     container: Optional[LN] = container_of(leaf)
     while container is not None and container.type != token.ENDMARKER:
@@ -309,11 +309,12 @@ def generate_ignored_nodes(
 
 
 def _generate_ignored_nodes_from_fmt_skip(
-    leaf: Leaf, comment: ProtoComment
+    leaf: Leaf, comment: ProtoComment, mode: Mode
 ) -> Iterator[LN]:
     """Generate all leaves that should be ignored by the `# fmt: skip` from `leaf`."""
     prev_sibling = leaf.prev_sibling
     parent = leaf.parent
+    ignored_nodes: list[LN] = []
     # Need to properly format the leaf prefix to compare it to comment.value,
     # which is also formatted
     comments = list_comments(leaf.prefix, is_endmarker=False)
@@ -321,11 +322,54 @@ def _generate_ignored_nodes_from_fmt_skip(
         return
     if prev_sibling is not None:
         leaf.prefix = ""
+
+        if Preview.fix_fmt_skip_in_one_liners not in mode:
             siblings = [prev_sibling]
-        while "\n" not in prev_sibling.prefix and prev_sibling.prev_sibling is not None:
+            while (
+                "\n" not in prev_sibling.prefix
+                and prev_sibling.prev_sibling is not None
+            ):
                 prev_sibling = prev_sibling.prev_sibling
                 siblings.insert(0, prev_sibling)
             yield from siblings
+            return
+
+        # Generates the nodes to be ignored by `fmt: skip`.
+
+        # Nodes to ignore are the ones on the same line as the
+        # `# fmt: skip` comment, excluding the `# fmt: skip`
+        # node itself.
+
+        # Traversal process (starting at the `# fmt: skip` node):
+        # 1. Move to the `prev_sibling` of the current node.
+        # 2. If `prev_sibling` has children, go to its rightmost leaf.
+        # 3. If there’s no `prev_sibling`, move up to the parent
+        # node and repeat.
+        # 4. Continue until:
+        #    a. You encounter an `INDENT` or `NEWLINE` node (indicates
+        #       start of the line).
+        #    b. You reach the root node.
+
+        # Include all visited LEAVES in the ignored list, except INDENT
+        # or NEWLINE leaves.
+
+        current_node = prev_sibling
+        ignored_nodes = [current_node]
+        if current_node.prev_sibling is None and current_node.parent is not None:
+            current_node = current_node.parent
+        while "\n" not in current_node.prefix and current_node.prev_sibling is not None:
+            leaf_nodes = list(current_node.prev_sibling.leaves())
+            current_node = leaf_nodes[-1] if leaf_nodes else current_node
+
+            if current_node.type in (token.NEWLINE, token.INDENT):
+                current_node.prefix = ""
+                break
+
+            ignored_nodes.insert(0, current_node)
+
+            if current_node.prev_sibling is None and current_node.parent is not None:
+                current_node = current_node.parent
+        yield from ignored_nodes
     elif (
         parent is not None and parent.type == syms.suite and leaf.type == token.NEWLINE
     ):
@@ -333,7 +377,6 @@ def _generate_ignored_nodes_from_fmt_skip(
         # statements. The ignored nodes should be previous siblings of the
         # parent suite node.
         leaf.prefix = ""
-        ignored_nodes: list[LN] = []
         parent_sibling = parent.prev_sibling
         while parent_sibling is not None and parent_sibling.type != syms.suite:
             ignored_nodes.insert(0, parent_sibling)

src/black/mode.py

@@ -203,6 +203,7 @@ class Preview(Enum):
     wrap_long_dict_values_in_parens = auto()
     multiline_string_handling = auto()
     always_one_newline_after_import = auto()
+    fix_fmt_skip_in_one_liners = auto()
 
 
 UNSTABLE_FEATURES: set[Preview] = {

src/black/resources/black.schema.json

@@ -83,7 +83,8 @@
           "hug_parens_with_braces_and_square_brackets",
           "wrap_long_dict_values_in_parens",
           "multiline_string_handling",
-          "always_one_newline_after_import"
+          "always_one_newline_after_import",
+          "fix_fmt_skip_in_one_liners"
         ]
       },
       "description": "Enable specific features included in the `--unstable` style. Requires `--preview`. No compatibility guarantees are provided on the behavior or existence of any unstable features."

tests/data/cases/fmtskip10.py

@@ -0,0 +1,9 @@
+# flags: --preview
+def foo(): return "mock"  # fmt: skip
+if True: print("yay")  # fmt: skip
+for i in range(10): print(i)  # fmt: skip
+
+j =     1 # fmt: skip
+while j < 10: j += 1  # fmt: skip
+
+b = [c for c in "A very long string that would normally generate some kind of collapse, since it is this long"] # fmt: skip