Handle missing virtualenv in shell command

src/poetry_plugin_shell/command.py

@@ -36,11 +36,19 @@ def handle(self) -> int:
 
             return 0
 
-        self.line(f"Spawning shell within <info>{self.env.path}</>")
-
         # Be sure that we have the right type of environment.
         env = self.env
-        assert env.is_venv()
+        if not env.is_venv():
+            self.line_error(
+                "<error>Poetry could not find a virtual environment to activate.</>\n"
+                "If <info>virtualenvs.create</> is set to <comment>false</>, "
+                "enable virtual environment creation or create/select a virtual "
+                "environment before running <info>poetry shell</>."
+            )
+            return 1
+
+        self.line(f"Spawning shell within <info>{self.env.path}</>")
+
         env = cast("VirtualEnv", env)
 
         # Setting this to avoid spawning unnecessary nested shells

tests/test_shell_command.py

@@ -7,6 +7,8 @@
 
 import pytest
 
+from poetry.utils.env import MockEnv
+
 from poetry_plugin_shell.command import ShellCommand
 
 
@@ -50,6 +52,25 @@ def test_shell_already_active(tester: CommandTester, mocker: MockerFixture) -> N
     assert tester.status_code == 0
 
 
+def test_shell_without_virtualenv_shows_error(
+    command_tester_factory: CommandTesterFactory,
+    mocker: MockerFixture,
+    tmp_path: Path,
+) -> None:
+    env = MockEnv(path=tmp_path / "system", is_venv=False)
+    tester = command_tester_factory("shell", environment=env)
+    shell_activate = mocker.patch("poetry_plugin_shell.shell.Shell.activate")
+
+    tester.execute()
+
+    error = tester.io.fetch_error()
+    shell_activate.assert_not_called()
+    assert "Spawning shell" not in tester.io.fetch_output()
+    assert "could not find a virtual environment" in error
+    assert "virtualenvs.create" in error
+    assert tester.status_code == 1
+
+
 @pytest.mark.parametrize(
     ("poetry_active", "real_prefix", "prefix", "expected"),
     [