Disable the stability check with --line-ranges for now. (#4034)

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

CHANGES.md

@@ -21,6 +21,9 @@
 
 <!-- Changes to how Black can be configured -->
 
+- `--line-ranges` now skips _Black_'s internal stability check in `--safe` mode. This
+  avoids a crash on rare inputs that have many unformatted same-content lines. (#4034)
+
 ### Packaging
 
 - Upgrade to mypy 1.6.1 (#4049)

docs/usage_and_configuration/the_basics.md

@@ -196,6 +196,12 @@ Example: `black --line-ranges=1-10 --line-ranges=21-30 test.py` will format line
 
 This option is mainly for editor integrations, such as "Format Selection".
 
+```{note}
+Due to [#4052](https://github.com/psf/black/issues/4052), `--line-ranges` might format
+extra lines outside of the ranges when ther are unformatted lines with the exact
+content. It also disables _Black_'s formatting stability check in `--safe` mode.
+```
+
 #### `--color` / `--no-color`
 
 Show (or do not show) colored diff. Only applies when `--diff` is given.

src/black/__init__.py

@@ -1465,11 +1465,16 @@ def assert_stable(
     src: str, dst: str, mode: Mode, *, lines: Collection[Tuple[int, int]] = ()
 ) -> None:
     """Raise AssertionError if `dst` reformats differently the second time."""
+    if lines:
+        # Formatting specified lines requires `adjusted_lines` to map original lines
+        # to the formatted lines before re-formatting the previously formatted result.
+        # Due to less-ideal diff algorithm, some edge cases produce incorrect new line
+        # ranges. Hence for now, we skip the stable check.
+        # See https://github.com/psf/black/issues/4033 for context.
+        return
     # We shouldn't call format_str() here, because that formats the string
     # twice and may hide a bug where we bounce back and forth between two
     # versions.
-    if lines:
-        lines = adjusted_lines(lines, src, dst)
     newdst = _format_str_once(dst, mode=mode, lines=lines)
     if dst != newdst:
         log = dump_to_file(

tests/data/cases/line_ranges_diff_edge_case.py

@@ -0,0 +1,28 @@
+# flags: --line-ranges=10-11
+# NOTE: If you need to modify this file, pay special attention to the --line-ranges=
+# flag above as it's formatting specifically these lines.
+
+# Reproducible example for https://github.com/psf/black/issues/4033.
+# This can be fixed in the future if we use a better diffing algorithm, or make Black
+# perform formatting in a single pass.
+
+print ( "format me" )
+print ( "format me" )
+print ( "format me" )
+print ( "format me" )
+print ( "format me" )
+
+# output
+# flags: --line-ranges=10-11
+# NOTE: If you need to modify this file, pay special attention to the --line-ranges=
+# flag above as it's formatting specifically these lines.
+
+# Reproducible example for https://github.com/psf/black/issues/4033.
+# This can be fixed in the future if we use a better diffing algorithm, or make Black
+# perform formatting in a single pass.
+
+print ( "format me" )
+print("format me")
+print("format me")
+print("format me")
+print("format me")