Custom prefix in .ini source

docs/changelog/2926.bugfix.rst

@@ -0,0 +1,2 @@
+Plugins are now able to access tox.ini config sections using a custom prefix with the same suffix / name as a tox
+``testenv`` - by :user:`masenf`

src/tox/config/source/ini.py

@@ -40,8 +40,13 @@ def sections(self) -> Iterator[IniSection]:
             yield IniSection.from_key(section)
 
     def get_loader(self, section: Section, override_map: OverrideMap) -> IniLoader | None:
-        sections = self._section_mapping.get(section.name)
-        key = sections[0] if sections else section.key
+        # look up requested section name in the generative testenv mapping to find the real config source
+        for key in self._section_mapping.get(section.name) or []:
+            if section.prefix is None or Section.from_key(key).prefix == section.prefix:
+                break
+        else:
+            # if no matching section/prefix is found, use the requested section key as-is (for custom prefixes)
+            key = section.key
         if self._parser.has_section(key):
             return IniLoader(
                 section=section,

tests/config/source/test_source_ini.py

@@ -2,7 +2,9 @@
 
 from pathlib import Path
 
+from tests.conftest import ToxIniCreator
 from tox.config.loader.section import Section
+from tox.config.sets import ConfigSet
 from tox.config.source.ini import IniSource
 
 
@@ -22,3 +24,27 @@ def test_source_ini_ignore_invalid_factor_filters(tmp_path: Path) -> None:
     loader = IniSource(tmp_path, content="[a]\nb= if c: d")
     res = list(loader.envs({"env_list": []}))  # type: ignore
     assert not res
+
+
+def test_source_ini_custom_non_testenv_sections(tox_ini_conf: ToxIniCreator) -> None:
+    """Validate that a plugin can load section with custom prefix overlapping testenv name."""
+
+    class CustomConfigSet(ConfigSet):
+        def register_config(self) -> None:
+            self.add_config(
+                keys=["a"],
+                of_type=str,
+                default="",
+                desc="d",
+            )
+
+    config = tox_ini_conf("[testenv:foo]\n[custom:foo]\na = b")
+    known_envs = list(config._src.envs(config.core))
+    assert known_envs
+    custom_section = config.get_section_config(
+        section=Section("custom", "foo"),
+        base=[],
+        of_type=CustomConfigSet,
+        for_env=None,
+    )
+    assert custom_section["a"] == "b"