fix: do not add a line continuation to a file without a trailing newline

src/docformatter/classify.py

@@ -469,14 +469,35 @@ def is_newline_continuation(
     """
     if (
         token.type in (tokenize.NEWLINE, tokenize.NL)
-        and token.line.strip()
+        and not _is_blank_line(token.line)
         and token.line.strip() in prev_token.line.strip()
     ):
         return True
 
     return False
 
 
+def _is_blank_line(line: str) -> bool:
+    """Determine if a physical line is blank.
+
+    A blank line is a non-empty line that holds nothing but whitespace, e.g.
+    "\\n" or "    \\n".  The empty string is *not* a blank line; tokenize uses
+    it as the line of the NEWLINE token it synthesizes for source that has no
+    trailing newline, as well as for DEDENT and ENDMARKER tokens.
+
+    Parameters
+    ----------
+    line : str
+        The physical line the token was read from; tokenize.TokenInfo.line.
+
+    Returns
+    -------
+    bool
+        True if the line holds only whitespace and is not empty.
+    """
+    return bool(line) and not line.strip()
+
+
 def is_string_variable(
     token: tokenize.TokenInfo,
     prev_token: tokenize.TokenInfo,

tests/_data/string_files/do_format_code.toml

@@ -1251,3 +1251,12 @@ pass
 [issue_355]
 source="def foo():\n    \"\"\"Summary.\"\"\"\n    x = 1\n    # next line has 4 spaces of trailing whitespace\n    \n    return x\n"
 expected="def foo():\n    \"\"\"Summary.\"\"\"\n    x = 1\n    # next line has 4 spaces of trailing whitespace\n    \n    return x\n"
+
+# Source that does not end with a newline.  tokenize synthesizes a NEWLINE
+# token for it whose line attribute is the empty string (CPython < 3.12); that
+# token must still be treated as part of the preceding line, otherwise
+# untokenize bridges the row gap with a backslash continuation and the
+# formatted file no longer parses.
+[issue_360_no_trailing_newline]
+source="def foo():\n    \"\"\"\n    Hello foo.\n    \"\"\"\n    x = 1"
+expected="def foo():\n    \"\"\"Hello foo.\"\"\"\n    x = 1"

tests/formatter/test_do_format_code.py

@@ -142,6 +142,7 @@
         ("do_not_break_f_string_single_quotes", NO_ARGS),
         ("issue_331_black_module_docstring", ["--black", ""]),
         ("issue_355", NO_ARGS),
+        ("issue_360_no_trailing_newline", NO_ARGS),
     ],
 )
 def test_do_format_code(test_key, test_args, args):