fix: preserve escaped newlines inside ordinary strings

src/docformatter/format.py

@@ -200,12 +200,14 @@ def _do_update_token_indices(
         is_same_line = tokens[i].line == tokens[i - 1].line
         is_same_position = tokens[i].start[0] == _prev_end_row
         _prev_end_row = tokens[i].end[0]
-        # A backslash continuation joins two physical lines without an NL
-        # token between them, so the current token always starts on the row
-        # after the previous one, even though the rows may look the same once
-        # the previous token has been shifted.
+        # A backslash continuation between tokens joins two physical lines without
+        # an NL token.  A multiline string can start on the previous token's row
+        # and contain that backslash itself, so compare the original row positions
+        # before treating it as a continuation between tokens.
         prev_line = tokens[i - 1].line.rstrip("\r\n")
-        is_backslash_continuation = not is_same_line and prev_line.endswith("\\")
+        is_backslash_continuation = (
+            not is_same_position and not is_same_line and prev_line.endswith("\\")
+        )
 
         if not is_backslash_continuation and (
             is_multiline

tests/formatter/test_continued_string_preservation.py

@@ -0,0 +1,58 @@
+"""Preserve escaped physical newlines inside ordinary string tokens."""
+
+import ast
+import sys
+
+import pytest
+
+from docformatter import Formatter
+
+
+@pytest.mark.integration
+@pytest.mark.parametrize("args", [[""], ["--black", ""]])
+@pytest.mark.parametrize("newline", ["\n", "\r\n"])
+@pytest.mark.parametrize(
+    "source",
+    [
+        'value = "first\\\nsecond"\n',
+        "value = 'first\\\nsecond'\n",
+        'value = r"first\\\nsecond"\n',
+        'value = b"first\\\nsecond"\n',
+        'value = u"first\\\nsecond"\n',
+        'print("first\\\nsecond")\n',
+        'def value():\n    return "first\\\nsecond"\n',
+        'value = "first\\\nsecond\\\nthird"\n',
+        'value = "first\\\nsecond" "suffix"\n',
+        'value = "first\\\nsecond" + "suffix"\n',
+        'value = "first\\\nsecond"',
+        'value = (\n    "first\\\nsecond"\n)\n',
+    ],
+)
+def test_continued_string_is_preserved(source, newline, test_args, args):
+    """A continuation within a string must not duplicate surrounding Python code."""
+    source = source.replace("\n", newline)
+    formatter = Formatter(test_args, sys.stderr, sys.stdin, sys.stdout)
+
+    formatted = formatter._do_format_code(source)
+
+    assert formatted == source
+    assert ast.dump(ast.parse(formatted)) == ast.dump(ast.parse(source))
+    assert formatter._do_format_code(formatted) == formatted
+
+
+@pytest.mark.integration
+@pytest.mark.parametrize("args", [[""], ["--black", ""]])
+@pytest.mark.parametrize("newline", ["\n", "\r\n"])
+def test_continued_string_after_docstring_rewrite(newline, test_args, args):
+    """Preserve string-token positions when docstring formatting shifts later rows."""
+    source = 'class Example:\n    """Summary."""\n    value = "first\\\nsecond"\n'
+    expected = source.replace('"""Summary."""\n', '"""Summary."""\n\n')
+    source = source.replace("\n", newline)
+    expected = expected.replace("\n", newline)
+    formatter = Formatter(test_args, sys.stderr, sys.stdin, sys.stdout)
+
+    formatted = formatter._do_format_code(source)
+
+    assert formatted == expected
+    assert ast.dump(ast.parse(formatted)) == ast.dump(ast.parse(source))
+    assert formatter._do_format_code(formatted) == formatted