Add support for processing .pth files in project plugins

docs/plugins.md

@@ -263,13 +263,16 @@ in the `tool.poetry.requires-plugins` section of the pyproject.toml file:
 ```toml
 [tool.poetry.requires-plugins]
 my-application-plugin = ">1.0"
+custom-plugin = {path = "custom_plugin", develop = true}
 ```
 
 If the plugin is not installed in Poetry's own environment when running `poetry install`,
 it will be installed only for the current project under `.poetry/plugins`
 in the project's directory.
 
 The syntax to specify `plugins` is the same as for [dependencies]({{< relref "managing-dependencies" >}}).
+Plugins can be installed in editable mode using path dependencies with `develop = true`,
+which is useful for plugin development.
 
 {{% warning %}}
 You can even overwrite a plugin in Poetry's own environment with another version.

src/poetry/plugins/plugin_manager.py

@@ -9,6 +9,7 @@
 from functools import cached_property
 from importlib import metadata
 from pathlib import Path
+from site import addsitedir
 from typing import TYPE_CHECKING
 
 import tomlkit
@@ -62,7 +63,10 @@ def add_project_plugin_path(directory: Path) -> None:
 
         plugin_path = pyproject_toml.parent / ProjectPluginCache.PATH
         if plugin_path.exists():
+            # insert at the beginning to allow overriding dependencies
             EnvManager.get_system_env(naive=True).sys_path.insert(0, str(plugin_path))
+            # process .pth files (among other things)
+            addsitedir(str(plugin_path))
 
     @classmethod
     def ensure_project_plugins(cls, poetry: Poetry, io: IO) -> None:

tests/plugins/test_plugin_manager.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import shutil
+import sys
 
 from pathlib import Path
 from typing import TYPE_CHECKING
@@ -116,6 +117,12 @@ def io() -> BufferedIO:
     return BufferedIO()
 
 
+@pytest.fixture(autouse=True)
+def mock_sys_path(mocker: MockerFixture) -> None:
+    sys_path_copy = sys.path.copy()
+    mocker.patch("poetry.plugins.plugin_manager.sys.path", new=sys_path_copy)
+
+
 @pytest.fixture()
 def manager_factory(poetry: Poetry, io: BufferedIO) -> ManagerFactory:
     def _manager(group: str = Plugin.group) -> PluginManager:
@@ -187,6 +194,59 @@ def test_add_project_plugin_path(
     } == {"my-application-plugin 1.0"}
 
 
+def test_add_project_plugin_path_addsitedir_called(
+    poetry_with_plugins: Poetry,
+    io: BufferedIO,
+    mocker: MockerFixture,
+) -> None:
+    """Test that addsitedir is called when plugin path exists."""
+    cache = ProjectPluginCache(poetry_with_plugins, io)
+    cache._path.mkdir(parents=True, exist_ok=True)
+
+    mock_addsitedir = mocker.patch("poetry.plugins.plugin_manager.addsitedir")
+
+    PluginManager.add_project_plugin_path(poetry_with_plugins.pyproject_path.parent)
+
+    # sys.path is mocked, so we can check it was modified
+    assert str(cache._path) in sys.path
+    assert sys.path[0] == str(cache._path)
+    mock_addsitedir.assert_called_once_with(str(cache._path))
+
+
+def test_add_project_plugin_path_no_addsitedir_when_path_missing(
+    poetry_with_plugins: Poetry,
+    mocker: MockerFixture,
+) -> None:
+    """Test that addsitedir is not called when plugin path doesn't exist."""
+    cache = ProjectPluginCache(poetry_with_plugins, BufferedIO())
+    # Ensure the plugin path does not exist
+    if cache._path.exists():
+        shutil.rmtree(cache._path)
+
+    mock_addsitedir = mocker.patch("poetry.plugins.plugin_manager.addsitedir")
+    initial_sys_path = sys.path.copy()
+
+    PluginManager.add_project_plugin_path(poetry_with_plugins.pyproject_path.parent)
+
+    assert sys.path == initial_sys_path
+    mock_addsitedir.assert_not_called()
+
+
+def test_add_project_plugin_path_no_pyproject(
+    tmp_path: Path,
+    mocker: MockerFixture,
+) -> None:
+    """Test that no action is taken when pyproject.toml is missing."""
+    mock_addsitedir = mocker.patch("poetry.plugins.plugin_manager.addsitedir")
+    initial_sys_path = sys.path.copy()
+
+    # Call with a directory that has no pyproject.toml
+    PluginManager.add_project_plugin_path(tmp_path)
+
+    assert sys.path == initial_sys_path
+    mock_addsitedir.assert_not_called()
+
+
 def test_ensure_plugins_no_plugins_no_output(poetry: Poetry, io: BufferedIO) -> None:
     PluginManager.ensure_project_plugins(poetry, io)