rewrite new toml-data will has a new whitespace line

rewrite toml data will has a new whitespace line.

eg toml:

toml

# xxx test


[env.pro1.rst]
    name = "x7"

[env2]
    name = 2

[env3]
    name = 3

modified with rewrite new tool data, like this

file_ = "test.toml"

def update_toml():
    with open(file_, "r") as f:
        t = tomlkit.load(f)

    t = merge_dicts(
        t,
        {'env': {'pro1': {'rst': {'name': "x7"}}}}
    )

    with open(file_, "w") as f:
        tomlkit.dump(t, f)

def merge_dicts(a: dict, b: dict):
    # ret = a.copy()
    ret = a

    for k, v in b.items():
        if k in a and isinstance(a[k], (dict, OrderedDict, Container)):
            ret[k] = merge_dicts(a[k], v)

    return ret

the result (has a new line after name = "x7")

# xxx test


[env.pro1.rst]
    name = "x7"


[env2]
    name = 2

[env3]
    name = 3

and if I edit in local with code

def merge_dicts(a: dict, b: dict):
    # ret = a.copy()
    return merge_dict_to_toml_local(a, b)


def merge_dict_to_toml_local(a: tomlkit.TOMLDocument, b: dict):

    for k, v in b.items():
        if k in a and isinstance(a[k], (dict, OrderedDict, Container)):
            merge_dict_to_toml_local(a[k], v)
        else:
            a[k] = v

There will be no more line.

I'm not sure if this is a bug, so I checked the source code.
in file tomlkit/container.py, line 703 with function _replace_at , it will add a whitespace-line where there is not has whitespace-line.
how check ? the end of tomlkit/container.py

def ends_with_whitespace(it: Any) -> bool:
    """Returns ``True`` if the given item ``it`` is a ``Table`` or ``AoT`` object
    ending with a ``Whitespace``.
    """
    return (
        isinstance(it, Table) and isinstance(it.value._previous_item(), Whitespace)
    ) or (isinstance(it, AoT) and len(it) > 0 and isinstance(it[-1], Whitespace))

ends_with_whitespace only check last and current without sub-tree.
Should subdata be checked ?

`tomlkit` may omit the newline: workaround limitations with regexes

Does not reproduce today

Does not reproduce today

It does, even with tomlkit 0.15.0.

Proof:

#!/usr/bin/env -S uv run --script
# /// script
# requires-python = ">=3.10"
# dependencies = ["tomlkit==0.15.0"]
# ///
"""Reproduce tomlkit#352 against tomlkit 0.15.0."""
from __future__ import annotations

import tomlkit
from tomlkit.container import Container

SOURCE = """\
[env.pro1.rst]
name = "x7"

[env2]
name = 2

[env3]
name = 3
"""


def merge_dicts(a, b):
    """Walk ``b`` into ``a``, reassigning nested tables back onto their parent."""
    for k, v in b.items():
        if k in a and isinstance(a[k], (dict, Container)):
            a[k] = merge_dicts(a[k], v)
    return a


def main() -> None:
    print(f"tomlkit {tomlkit.__version__}")

    doc = tomlkit.parse(SOURCE)
    merge_dicts(doc, {"env": {"pro1": {"rst": {"name": "x7"}}}})
    output = tomlkit.dumps(doc)

    print("--- input  ---")
    print(SOURCE, end="")
    print("--- output ---")
    print(output, end="")
    print("--------------")

    if output == SOURCE:
        print("OK: round-trip preserved")
        return

    extra = output.count("\n\n\n") - SOURCE.count("\n\n\n")
    print(f"BUG: round-trip diverged ({extra} extra blank line(s) inserted)")
    raise SystemExit(1)


if __name__ == "__main__":
    main()

Which produces:

$ uv run ./issue_352-test.py
tomlkit 0.15.0
--- input  ---
[env.pro1.rst]
name = "x7"

[env2]
name = 2

[env3]
name = 3
--- output ---
[env.pro1.rst]
name = "x7"


[env2]
name = 2

[env3]
name = 3
--------------
BUG: round-trip diverged (1 extra blank line(s) inserted)

oh I see - the original report provided a repro that did not actually do anything! So indeed nothing changed when I "executed" it...

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rewrite new toml-data will has a new whitespace line #352

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rewrite new toml-data will has a new whitespace line #352

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dimbleby commented on Jun 6, 2026

kdeldycke commented on Jun 10, 2026

dimbleby commented on Jun 10, 2026

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