525 adding to an ini file without a final newline breaks config (#526)

* Add failing test

* Fix newline issue in INI integration

* Adjust tests to reflect corrected behaviour

Author: nathanjmcdougall

src/usethis/_integrations/file/ini/io_.py

@@ -305,6 +305,7 @@ def _validated_set(
             raise NotImplementedError(msg)
 
         if section_key not in root:
+            _ensure_newline(root)
             root.add_section(section_key)
 
         root.set(section=section_key, option=option_key, value=value)
@@ -554,3 +555,13 @@ def _itermatches(values: Iterable[str], /, *, key: Key):
                 yield value
         else:
             assert_never(key)
+
+
+def _ensure_newline(root: INIDocument) -> None:
+    """Add a newline to the INI file."""
+    sections = list(root.iter_sections())
+    if not sections:
+        # No newline necessary
+        return
+    final_section = sections[-1]
+    final_section.add_after.space(newlines=1)

tests/usethis/_core/test_core_tool.py

@@ -284,6 +284,7 @@ def test_coverage_rc_file(self, uv_init_dir: Path):
 [run]
 source =
     src
+
 [report]
 exclude_also =
     if TYPE_CHECKING:
@@ -311,6 +312,7 @@ def test_tox_ini_file(self, uv_init_dir: Path):
 [coverage:run]
 source =
     src
+
 [coverage:report]
 exclude_also =
     if TYPE_CHECKING:

tests/usethis/_integrations/file/ini/test_ini_io_.py

@@ -1001,6 +1001,32 @@ def relative_path(self) -> Path:
 key =
     new_value
     other
+"""
+            )
+
+        def test_existing_file_no_newline(self, tmp_path: Path):
+            # Arrange
+            class MyINIFileManager(INIFileManager):
+                @property
+                def relative_path(self) -> Path:
+                    return Path("valid.ini")
+
+            valid_file = tmp_path / "valid.ini"
+            valid_file.write_text("[section]\nkey = value")
+
+            # Act
+            with change_cwd(tmp_path), MyINIFileManager() as manager:
+                manager.set_value(
+                    keys=["new_section", "new_key"], value="new_value", exists_ok=True
+                )
+
+            # Assert
+            assert (tmp_path / "valid.ini").read_text() == (
+                """\
+[section]
+key = value
+[new_section]
+new_key = new_value
 """
             )