self install: in this context the project is not a package so we should set package-mode

src/poetry/console/commands/self/self_command.py

@@ -73,6 +73,7 @@ def generate_system_pyproject(self) -> None:
         package.python_versions = ".".join(str(v) for v in self.env.version_info[:3])
 
         content = Factory.create_pyproject_from_package(package=package)
+        content["tool"]["poetry"]["package-mode"] = False  # type: ignore[index]
 
         for key in preserved:
             content["tool"]["poetry"][key] = preserved[key]  # type: ignore[index]

tests/console/commands/self/test_install.py

@@ -0,0 +1,62 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+from poetry.console.commands.self.install import SelfInstallCommand
+
+
+if TYPE_CHECKING:
+    from cleo.testers.command_tester import CommandTester
+
+    from tests.types import CommandTesterFactory
+
+
+@pytest.fixture
+def tester(command_tester_factory: CommandTesterFactory) -> CommandTester:
+    return command_tester_factory("self install")
+
+
+@pytest.mark.parametrize(
+    "pyproject_content",
+    (
+        None,
+        """\
+[tool.poetry]
+name = "poetry-instance"
+version = "1.2"
+description = ""
+authors = []
+license = ""
+# no package-mode -> defaults to true
+
+[tool.poetry.dependencies]
+python = "3.9"
+poetry = "1.2"
+""",
+    ),
+)
+def test_self_install(
+    tester: CommandTester,
+    pyproject_content: str | None,
+) -> None:
+    command = tester.command
+    assert isinstance(command, SelfInstallCommand)
+    pyproject_path = command.system_pyproject
+    if pyproject_content:
+        pyproject_path.write_text(pyproject_content)
+    else:
+        assert not pyproject_path.exists()
+
+    tester.execute()
+
+    expected_output = """\
+Updating dependencies
+Resolving dependencies...
+
+Writing lock file
+"""
+
+    assert tester.io.fetch_output() == expected_output
+    assert tester.io.fetch_error() == ""