0.15.0 serializes invalid TOML when appending to a parsed inline table #476

Summary

In tomlkit 0.15.0, appending a new key to a parsed inline table that already contains multiple key/value pairs can produce invalid TOML: the newly appended key is serialized without a comma separator before it.

This appears to be a regression from 0.14.0.

Reproducer

import tomlkit

print(tomlkit.__version__)

doc = tomlkit.parse('x = { a = 1, b = 2 }\n')
doc['x']['c'] = 3

out = doc.as_string()
print(out, end='')

tomlkit.parse(out)

Actual behavior on 0.15.0

0.15.0
x = { a = 1, b = 2 c = 3}
Traceback ...
tomlkit.exceptions.UnexpectedCharError: Unexpected character: 'c' at line 1 col 19

The serialized TOML is invalid because the inline table is missing a comma between b = 2 and c = 3.

Expected behavior

as_string() should produce valid TOML, for example:

x = { a = 1, b = 2, c = 3}

The same reproducer works on 0.14.0:

0.14.0
x = { a = 1, b = 2, c = 3}

Environment

• Python: 3.13.2
• tomlkit: 0.15.0
• Last known good version tested: 0.14.0

Notes

This only seems to happen when mutating a parsed inline table with multiple existing entries. Creating/replacing an inline table from scratch still serializes commas correctly.