isolated build: fallback to PyPI only pool

When isolated builder context manager is invoked outside a Poetry
project context, fall back to a PyPI only package source.

src/poetry/utils/isolated_build.py

@@ -116,9 +116,18 @@ def isolated_builder(
 
     from poetry.factory import Factory
 
-    # we recreate the project's Poetry instance in order to retrieve the correct repository pool
-    # when a pool is not provided
-    pool = pool or Factory().create_poetry().pool
+    try:
+        # we recreate the project's Poetry instance in order to retrieve the correct repository pool
+        # when a pool is not provided
+        pool = pool or Factory().create_poetry().pool
+    except RuntimeError:
+        # the context manager is not being called within a Poetry project context
+        # fallback to a default pool using only PyPI as source
+        from poetry.repositories import RepositoryPool
+        from poetry.repositories.pypi_repository import PyPiRepository
+
+        # fallback to using only PyPI
+        pool = RepositoryPool(repositories=[PyPiRepository()])
 
     python_executable = (
         python_executable or EnvManager.get_system_env(naive=True).python

tests/utils/test_isolated_build.py

@@ -15,6 +15,7 @@
 from poetry.utils.env import ephemeral_environment
 from poetry.utils.isolated_build import IsolatedBuildInstallError
 from poetry.utils.isolated_build import IsolatedEnv
+from poetry.utils.isolated_build import isolated_builder
 from tests.helpers import get_dependency
 
 
@@ -24,6 +25,7 @@
     from pytest_mock import MockerFixture
 
     from poetry.repositories.pypi_repository import PyPiRepository
+    from tests.types import FixtureDirGetter
 
 
 @pytest.fixture()
@@ -78,3 +80,16 @@ def test_isolated_env_install_failure(
         with pytest.raises(IsolatedBuildInstallError) as e:
             env.install({"a", "b>1"})
         assert e.value.requirements == {"a", "b>1"}
+
+
+def test_isolated_builder_outside_poetry_project_context(
+    tmp_working_directory: Path, fixture_dir: FixtureDirGetter
+) -> None:
+    source = fixture_dir("project_with_setup")
+    destination = tmp_working_directory / "dist"
+
+    try:
+        with isolated_builder(source, "wheel") as builder:
+            builder.metadata_path(destination)
+    except RuntimeError:
+        pytest.fail("Isolated builder did not fallback to default repository pool")