Keep valid TOML when deleting the last dotted key in an inline table

tests/test_toml_document.py

@@ -1804,3 +1804,48 @@ def test_emptied_array_of_tables_is_relative_to_the_nearest_header() -> None:
     doc["t"]["u"]["a"].pop()
 
     assert doc.as_string() == "[t]\nu.a = []\n"
+
+
+def test_delete_last_dotted_key_in_inline_table_keeps_valid_toml() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/662
+    # Removing the only child of a dotted key in an inline table left a
+    # dangling separator, producing invalid TOML like ``t = {a="", , d=1}``.
+    doc = parse('t = {a="", b.c=1, d=1}\n')
+    del doc["t"]["b"]["c"]
+
+    assert doc.as_string() == 't = {a="", b={}, d=1}\n'
+    tomllib = pytest.importorskip("tomllib")
+    assert tomllib.loads(doc.as_string()) == {"t": {"a": "", "b": {}, "d": 1}}
+
+
+def test_delete_last_dotted_key_in_inline_table_key_positions() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/662
+    # The empty slot used to surface as a leading or trailing comma depending
+    # on where the dotted key sat, and dropped ``b`` entirely when it was the
+    # only key.
+    tomllib = pytest.importorskip("tomllib")
+
+    doc = parse("t = {a=1, b.c=1}\n")
+    del doc["t"]["b"]["c"]
+    assert doc.as_string() == "t = {a=1, b={}}\n"
+    tomllib.loads(doc.as_string())
+
+    doc = parse("t = {b.c=1, a=1}\n")
+    del doc["t"]["b"]["c"]
+    assert doc.as_string() == "t = {b={}, a=1}\n"
+    tomllib.loads(doc.as_string())
+
+    doc = parse("t = {b.c=1}\n")
+    del doc["t"]["b"]["c"]
+    assert doc.as_string() == "t = {b={}}\n"
+    tomllib.loads(doc.as_string())
+
+
+def test_delete_last_nested_dotted_key_in_inline_table_keeps_valid_toml() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/662
+    doc = parse('t = {a="", b.c.d=1, e=1}\n')
+    del doc["t"]["b"]["c"]["d"]
+
+    assert doc.as_string() == 't = {a="", b.c={}, e=1}\n'
+    tomllib = pytest.importorskip("tomllib")
+    assert tomllib.loads(doc.as_string()) == {"t": {"a": "", "b": {"c": {}}, "e": 1}}

tomlkit/items.py

@@ -2182,7 +2182,14 @@ def as_string(self) -> str:
                 # A table materialized from a dotted key renders one
                 # `prefix.child = value` pair per child, so that keys added
                 # after parsing stay attached to the dotted prefix.
-                rendered = ", ".join(self._render_dotted(k, v))
+                dotted = self._render_dotted(k, v)
+                if dotted:
+                    rendered = ", ".join(dotted)
+                else:
+                    # Every child of the dotted key was removed: keep the
+                    # now-empty table rather than emitting nothing, which
+                    # would leave a dangling separator and invalid TOML.
+                    rendered = f"{k.as_string()}{k.sep or '='}{{}}"
             else:
                 rendered = (
                     f"{k.as_string() + ('.' if k.is_dotted() else '')}"
@@ -2220,7 +2227,13 @@ def _render_dotted(self, key: Key, table: Table) -> list[str]:
             if k is None:
                 continue
             if isinstance(v, Table):
-                parts.extend(f"{prefix}{sub}" for sub in self._render_dotted(k, v))
+                sub = self._render_dotted(k, v)
+                if sub:
+                    parts.extend(f"{prefix}{s}" for s in sub)
+                else:
+                    # A dotted child whose keys were all removed renders as
+                    # an empty inline table so the output stays valid TOML.
+                    parts.append(f"{prefix}{k.as_string()}{k.sep or '='}{{}}")
             else:
                 trail = v.trivia.trail.replace("\n", "")
                 parts.append(