Prefix every line of a multiline comment with #

CHANGELOG.md

@@ -20,6 +20,7 @@
 - Fix a table replaced by a plain value being serialized inside the preceding table's body when other tables follow; the value now moves before the first table like other root-level values. ([#504](https://github.com/python-poetry/tomlkit/issues/504))
 - Restore `dumps()` rendering mapping-like wrappers around a parsed document (e.g. `dotty_dict`'s `Dotty`) through their delegated `as_string`, preserving the original table order and layout instead of re-encoding through a plain dict — a 0.15.0 regression. ([#482](https://github.com/python-poetry/tomlkit/issues/482))
 - Fix uncontrolled recursion when parsing deeply nested documents: crafted input could crash the process with a `RecursionError`. Values nested more than 100 levels deep and keys with more than 100 dotted fragments now raise `ParseError`. ([#459](https://github.com/python-poetry/tomlkit/issues/459))
+- Fix `comment()` producing invalid TOML for a multiline string by prefixing every line with `#`, not just the first. ([#449](https://github.com/python-poetry/tomlkit/issues/449))
 
 ## [0.15.0] - 2026-05-10
 

tests/test_api.py

@@ -263,6 +263,22 @@ def test_datetime() -> None:
         tomlkit.time("1979-05-13")
 
 
+def test_comment_single_line() -> None:
+    doc = tomlkit.document()
+    doc.add(tomlkit.comment("a comment"))
+    assert doc.as_string() == "# a comment\n"
+
+
+def test_comment_multiline_is_valid_toml() -> None:
+    """A multiline comment prefixes every line so the output re-parses (#449)."""
+    doc = tomlkit.document()
+    doc.add(tomlkit.comment("line one\nline two\n\nline four"))
+    rendered = doc.as_string()
+    assert rendered == "# line one\n# line two\n#\n# line four\n"
+    # The rendered document must round-trip.
+    assert parse(rendered).as_string() == rendered
+
+
 def test_array() -> None:
     a = tomlkit.array()
 

tomlkit/api.py

@@ -306,8 +306,14 @@ def nl() -> Whitespace:
 
 
 def comment(string: str) -> Comment:
-    """Create a comment item."""
-    return Comment(Trivia(comment_ws="  ", comment="# " + string))
+    """Create a comment item.
+
+    A multiline string produces one ``#``-prefixed line per line so that the
+    result is still valid TOML.
+    """
+    lines = string.split("\n")
+    rendered = "\n".join(f"# {line}" if line else "#" for line in lines)
+    return Comment(Trivia(comment_ws="  ", comment=rendered))
 
 
 def register_encoder(encoder: E) -> E: