Render an emptied array of tables as an empty array

tests/test_toml_document.py

@@ -1666,3 +1666,44 @@ def test_scalar_is_not_captured_by_table_rendered_from_dotted_key() -> None:
     doc["z"] = 2
 
     assert doc.as_string() == "a.b = 1\nz = 2\n"
+
+
+def test_emptied_array_of_tables_renders_as_empty_array() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/553
+    # An array of tables with no elements left has no `[[key]]` header to
+    # render, so it must fall back to the inline `key = []` form. Rendering
+    # nothing dropped the key entirely.
+    doc = parse("[[a]]\nx = 1\n")
+    doc["a"].pop()
+
+    assert doc.as_string() == "a = []\n"
+    assert parse(doc.as_string()) == {"a": []}
+
+
+def test_emptied_array_of_tables_is_hoisted_above_table_headers() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/553
+    # TOML only reads bare key/value pairs before the first table header, so
+    # the inline fallback has to be emitted there rather than in body order.
+    # Left in place it would be parsed back as a key of the preceding table.
+    doc = parse("[t]\nq = 2\n\n[[a]]\nx = 1\n")
+    doc["a"].pop()
+
+    assert parse(doc.as_string()) == {"t": {"q": 2}, "a": []}
+    assert doc.as_string().index("a = []") < doc.as_string().index("[t]")
+
+
+def test_emptied_array_of_tables_keeps_preceding_scalars() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/553
+    doc = parse("v = 9\n\n[[a]]\nx = 1\n")
+    doc["a"].pop()
+
+    assert parse(doc.as_string()) == {"v": 9, "a": []}
+
+
+def test_non_empty_array_of_tables_is_not_hoisted() -> None:
+    # https://github.com/python-poetry/tomlkit/issues/553
+    # Only emptied arrays of tables change form; ordinary ones must round-trip
+    # byte for byte.
+    content = "[t]\nq = 2\n\n[[a]]\nx = 1\n"
+
+    assert parse(content).as_string() == content

tomlkit/container.py

@@ -630,10 +630,36 @@ def last_item(self) -> Item | None:
             return self._body[-1][1]
         return None
 
+    def _is_empty_aot(self, item: Item) -> bool:
+        return isinstance(item, AoT) and not item.body
+
     def as_string(self) -> str:
         """Render as TOML string."""
         s = ""
-        for k, v in self._body:
+        # An emptied array of tables has no ``[[key]]`` header left to render,
+        # so it falls back to the inline ``key = []`` form. TOML only reads
+        # bare key/value pairs before the first table header, so such keys are
+        # hoisted there instead of being rendered in body order.
+        hoisted = [
+            (k, v) for k, v in self._body if k is not None and self._is_empty_aot(v)
+        ]
+        first_header = next(
+            (
+                i
+                for i, (k, v) in enumerate(self._body)
+                if isinstance(v, (Table, AoT)) and not self._is_empty_aot(v)
+            ),
+            None,
+        )
+        if first_header is None or not hoisted:
+            hoisted = []
+
+        for i, (k, v) in enumerate(self._body):
+            if hoisted and i == first_header:
+                for hk, hv in hoisted:
+                    s += self._render_aot(hk, hv)
+            if hoisted and k is not None and self._is_empty_aot(v):
+                continue
             if k is not None:
                 if isinstance(v, Table):
                     if (
@@ -745,6 +771,20 @@ def _render_aot(self, key: Key, aot: AoT, prefix: str | None = None) -> str:
         if prefix is not None:
             _key = prefix + "." + _key
 
+        if not aot.body:
+            # An array of tables with no elements has no ``[[key]]`` header to
+            # render, so fall back to the inline empty-array form. Rendering
+            # nothing would drop the key entirely.
+            trail = aot.trivia.trail or "\n"
+            return (
+                f"{aot.trivia.indent}"
+                f"{decode(_key)}"
+                f" = []"
+                f"{aot.trivia.comment_ws}"
+                f"{decode(aot.trivia.comment)}"
+                f"{trail}"
+            )
+
         cur = ""
         _key = decode(_key)
         for table in aot.body: