Expose a new tox_extend_envs hook in plugins API

docs/changelog/3510.feature.rst

@@ -0,0 +1 @@
+3591.feature.rst

docs/changelog/3591.feature.rst

@@ -0,0 +1,10 @@
+A new tox life cycle event is now exposed for use via :doc:`Plugins
+API </plugins>` -- by :user:`webknjaz`.
+
+The corresponding hook point is :func:`tox_extend_envs
+<tox.plugin.spec.tox_extend_envs>`. It allows plugin authors to
+declare ephemeral environments that they can then populate through
+the in-memory configuration loader interface.
+
+This patch was made possible thanks to pair programming with
+:user:`gaborbernat` at PyCon US 2025.

src/tox/config/main.py

@@ -2,8 +2,9 @@
 
 import os
 from collections import OrderedDict, defaultdict
+from itertools import chain
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Iterator, Sequence, TypeVar
+from typing import TYPE_CHECKING, Any, Iterable, Iterator, Sequence, TypeVar
 
 from .sets import ConfigSet, CoreConfigSet, EnvConfigSet
 
@@ -22,18 +23,20 @@
 class Config:
     """Main configuration object for tox."""
 
-    def __init__(
+    def __init__(  # noqa: PLR0913  # <- no way around many args
         self,
         config_source: Source,
         options: Parsed,
         root: Path,
         pos_args: Sequence[str] | None,
         work_dir: Path,
+        extra_envs: Iterable[str],
     ) -> None:
         self._pos_args = None if pos_args is None else tuple(pos_args)
         self._work_dir = work_dir
         self._root = root
         self._options = options
+        self._extra_envs = extra_envs
 
         self._overrides: OverrideMap = defaultdict(list)
         for override in options.override:
@@ -78,7 +81,7 @@ def src_path(self) -> Path:
 
     def __iter__(self) -> Iterator[str]:
         """:return: an iterator that goes through existing environments"""
-        return self._src.envs(self.core)
+        return chain(self._src.envs(self.core), self._extra_envs)
 
     def sections(self) -> Iterator[Section]:
         yield from self._src.sections()
@@ -91,7 +94,7 @@ def __contains__(self, item: str) -> bool:
         return any(name for name in self if name == item)
 
     @classmethod
-    def make(cls, parsed: Parsed, pos_args: Sequence[str] | None, source: Source) -> Config:
+    def make(cls, parsed: Parsed, pos_args: Sequence[str] | None, source: Source, extra_envs: Iterable[str]) -> Config:
         """Make a tox configuration object."""
         # root is the project root, where the configuration file is at
         # work dir is where we put our own files
@@ -106,6 +109,7 @@ def make(cls, parsed: Parsed, pos_args: Sequence[str] | None, source: Source) ->
             pos_args=pos_args,
             root=root,
             work_dir=work_dir,
+            extra_envs=extra_envs,
         )
 
     @property

src/tox/plugin/manager.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 import os
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Iterable
 
 import pluggy
 
@@ -80,6 +80,12 @@ def _load_external_plugins(self) -> None:
             self.manager.set_blocked(name)
         self.manager.load_setuptools_entrypoints(NAME)
 
+    def tox_extend_envs(self) -> list[Iterable[str]]:
+        additional_env_names_hook_value = self.manager.hook.tox_extend_envs()
+        # NOTE: S101 is suppressed below to allow for type narrowing in MyPy
+        assert isinstance(additional_env_names_hook_value, list)  # noqa: S101
+        return additional_env_names_hook_value
+
     def tox_add_option(self, parser: ToxParser) -> None:
         self.manager.hook.tox_add_option(parser=parser)
 

src/tox/plugin/spec.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Iterable
 
 import pluggy
 
@@ -29,6 +29,24 @@ def tox_register_tox_env(register: ToxEnvRegister) -> None:
     """
 
 
+@_spec
+def tox_extend_envs() -> Iterable[str]:
+    """Declare additional environment names.
+
+    This hook is called without any arguments early in the lifecycle. It
+    is expected to return an iterable of strings with environment names
+    for tox to consider. It can be used to facilitate dynamic creation of
+    additional environments from within tox plugins.
+
+    This is ideal to pair with :func:`tox_add_core_config
+    <tox.plugin.spec.tox_add_core_config>` that has access to
+    ``state.conf.memory_seed_loaders`` allowing to extend it with instances of
+    :class:`tox.config.loader.memory.MemoryLoader` early enough before tox
+    starts caching configuration values sourced elsewhere.
+    """
+    return ()  # <- Please MyPy
+
+
 @_spec
 def tox_add_option(parser: ToxParser) -> None:
     """
@@ -108,6 +126,7 @@ def tox_env_teardown(tox_env: ToxEnv) -> None:
     "tox_after_run_commands",
     "tox_before_run_commands",
     "tox_env_teardown",
+    "tox_extend_envs",
     "tox_on_install",
     "tox_register_tox_env",
 ]

src/tox/session/state.py

@@ -1,11 +1,13 @@
 from __future__ import annotations
 
 import sys
+from itertools import chain
 from typing import TYPE_CHECKING, Sequence
 
 from tox.config.main import Config
 from tox.journal import Journal
 from tox.plugin import impl
+from tox.plugin.manager import MANAGER
 
 from .env_select import EnvSelector
 
@@ -18,7 +20,8 @@ class State:
     """Runtime state holder."""
 
     def __init__(self, options: Options, args: Sequence[str]) -> None:
-        self.conf = Config.make(options.parsed, options.pos_args, options.source)
+        extended_envs = chain.from_iterable(MANAGER.tox_extend_envs())
+        self.conf = Config.make(options.parsed, options.pos_args, options.source, extended_envs)
         self.conf.core.add_constant(
             keys=["on_platform"],
             desc="platform we are running on",

tests/config/cli/test_cli_ini.py

@@ -166,6 +166,7 @@ def test_conf_arg(tmp_path: Path, conf_arg: str, filename: str, content: str) ->
         Parsed(work_dir=dest, override=[], config_file=config_file, root_dir=None),
         pos_args=[],
         source=source,
+        extra_envs=(),
     )
 
 

tests/config/loader/conftest.py

@@ -32,6 +32,7 @@ def example(conf: str, pos_args: list[str] | None = None) -> str:
             root=tmp_path,
             pos_args=pos_args,
             work_dir=tmp_path,
+            extra_envs=(),
         )
         loader = config.get_env("py").loaders[0]
         args = ConfigLoadArgs(chain=[], name="a", env_name="a")

tests/conftest.py

@@ -62,6 +62,7 @@ def func(conf: str, override: Sequence[Override] | None = None) -> Config:
             Parsed(work_dir=dest, override=override or [], config_file=config_file, root_dir=None),
             pos_args=[],
             source=source,
+            extra_envs=(),
         )
 
     return func

tests/plugin/test_inline.py

@@ -4,7 +4,9 @@
 
 if TYPE_CHECKING:
     from tox.config.cli.parser import ToxParser
+    from tox.config.sets import ConfigSet
     from tox.pytest import ToxProjectCreator
+    from tox.session.state import State
 
 
 def test_inline_tox_py(tox_project: ToxProjectCreator) -> None:
@@ -22,3 +24,38 @@ def tox_add_option(parser: ToxParser) -> None:
     result = project.run("-h")
     result.assert_success()
     assert "--magic" in result.out
+
+
+def test_toxfile_py_w_ephemeral_envs(tox_project: ToxProjectCreator) -> None:
+    """Ensure additional ephemeral tox envs can be plugin-injected."""
+
+    def plugin() -> None:  # pragma: no cover # the code is copied to a python file
+        from tox.config.loader.memory import MemoryLoader  # noqa: PLC0415
+        from tox.plugin import impl  # noqa: PLC0415
+
+        env_name = "sentinel-env-name"
+
+        @impl
+        def tox_extend_envs() -> tuple[str]:
+            return (env_name,)
+
+        @impl
+        def tox_add_core_config(core_conf: ConfigSet, state: State) -> None:  # noqa: ARG001
+            in_memory_config_loader = MemoryLoader(
+                base=["sentinel-base"],
+                description="sentinel-description",
+            )
+            state.conf.memory_seed_loaders[env_name].append(
+                in_memory_config_loader,  # src/tox/provision.py:provision()
+            )
+
+    project = tox_project({"toxfile.py": plugin})
+
+    tox_list_result = project.run("list", "-qq")
+    tox_list_result.assert_success()
+    expected_additional_env_txt = "\n\nadditional environments:\nsentinel-env-name -> sentinel-description"
+    assert expected_additional_env_txt in tox_list_result.out
+
+    tox_config_result = project.run("config", "-e", "sentinel-env-name", "-qq")
+    tox_config_result.assert_success()
+    assert "base = sentinel-base" in tox_config_result.out