fix: preserve leading newline of multiline string built with string() (#551)

`tomlkit.string(value, multiline=True)` rendered the value as
`"""<value>"""`. When `value` starts with a newline the output looks
like `"""\n..."""`, and per the TOML spec a newline immediately
following the opening delimiter is trimmed on parse. So a value that
begins with a newline was silently lost on the very next round-trip:

    >>> s = tomlkit.string("\nfoo", multiline=True)
    >>> str(s)
    '\nfoo'
    >>> s.as_string()
    '"""\nfoo"""'
    >>> str(tomlkit.parse(f"k = {s.as_string()}")["k"])
    'foo'                      # leading newline dropped

The value the String object reports is correct; only its serialized
form was wrong, so the corruption only surfaced after re-parsing.
Both tomllib and tomlkit's own parser agree the old output decodes to
the wrong value, so this is a genuine serialization bug (not a TOML
1.0 vs 1.1 difference). Affects basic and literal multiline strings,
including values that start with `\r\n`.

Fix: in `String.from_raw`, when the string is multiline and its
rendered body starts with a newline, emit an extra leading newline
(the one the parser trims) so the value survives round-tripping. This
is exactly how the spec suggests representing such a value. Single-line
strings and multiline strings without a leading newline are unchanged.

An existing `test_create_string` case asserted the old lossy output
(`"""\nMy\nString\n"""`); it only checked `as_string()`, never a
round-trip, so it had locked in the bug. Updated it to the correct,
round-trippable rendering and added a dedicated round-trip regression
test covering basic/literal multiline and `\r\n`-leading values.

Author: Sanjays2402

CHANGELOG.md

@@ -21,6 +21,7 @@
 
 ### Fixed
 
+- Fix `string()` dropping a leading newline of a multiline string on round-trip: a value beginning with a newline is now rendered with an extra leading newline (the one the parser trims after the opening delimiter) so it survives re-parsing.
 - Fix invalid serialization with a duplicated comma when removing a non-edge element from a parsed inline table. ([#486](https://github.com/python-poetry/tomlkit/pull/486))
 - Fix invalid serialization with a duplicated comma when appending or inserting into a comma-first formatted array. ([#499](https://github.com/python-poetry/tomlkit/pull/499))
 - Fix `ParseError` when a sub-table extends the last element of an array of tables after an unrelated table. ([#261](https://github.com/python-poetry/tomlkit/issues/261))

tests/test_api.py

@@ -506,7 +506,7 @@ def test_create_super_table_with_aot() -> None:
         ({}, "My String\x12", '"My String\\u0012"'),
         ({}, "My String\x7f", '"My String\\u007f"'),
         ({"escape": False}, "My String\u0001", '"My String\u0001"'),
-        ({"multiline": True}, "\nMy\nString\n", '"""\nMy\nString\n"""'),
+        ({"multiline": True}, "\nMy\nString\n", '"""\n\nMy\nString\n"""'),
         ({"multiline": True}, 'My"String', '"""My"String"""'),
         ({"multiline": True}, 'My""String', '"""My""String"""'),
         ({"multiline": True}, 'My"""String', '"""My""\\"String"""'),
@@ -537,6 +537,30 @@ def test_create_string(kwargs: dict[str, Any], example: str, expected: str) -> N
     assert value.as_string() == expected
 
 
+@pytest.mark.parametrize(
+    "kwargs, example",
+    [
+        ({"multiline": True}, "\n"),
+        ({"multiline": True}, "\nMy\nString\n"),
+        ({"multiline": True}, "\r\nMy\nString"),
+        ({"multiline": True, "literal": True}, "\nMy\nString"),
+        ({"multiline": True, "literal": True}, "\r\nMy\nString"),
+    ],
+)
+def test_create_multiline_string_with_leading_newline_round_trips(
+    kwargs: dict[str, Any], example: str
+) -> None:
+    """A multiline string whose value starts with a newline must survive a
+    round-trip. The parser trims a newline immediately following the opening
+    delimiter, so the rendered form has to account for it (otherwise the
+    leading newline is silently dropped when the output is parsed again).
+    """
+    value = tomlkit.string(example, **kwargs)
+    assert str(value) == example
+    reparsed = parse(f"k = {value.as_string()}\n")["k"]
+    assert str(reparsed) == example
+
+
 @pytest.mark.parametrize(
     "kwargs, example",
     [

tomlkit/items.py

@@ -2251,6 +2251,13 @@ def from_raw(
         escaped = type_.escaped_sequences
         string_value = escape_string(value, escaped) if escape and escaped else value
 
+        if type_.is_multiline() and string_value[:1] in ("\n", "\r"):
+            # A newline immediately following the opening delimiter of a
+            # multiline string is trimmed by the parser, so a value that
+            # starts with a newline would otherwise lose it on round-trip.
+            # Emit an extra leading newline (the trimmed one) to preserve it.
+            string_value = "\n" + string_value
+
         return cls(type_, decode(value), string_value, Trivia())