fix: skip pip invocation when resolved groups are empty

CHANGELOG.rst

@@ -11,6 +11,9 @@ Unreleased
 - Raise a clear ``TypeError`` when an ``include-group`` value is not a string
   (was a cryptic ``TypeError`` from name normalization), and accept any
   ``Mapping`` for include items, not only ``dict``.
+- Fix a bug in which ``pip-install-dependency-groups`` failed when the
+  resolved groups were empty. It now prints ``Nothing to install`` and exits
+  normally.
 
 1.3.1
 -----

src/dependency_groups/_pip_wrapper.py

@@ -55,6 +55,10 @@ def main(*, argv: list[str] | None = None) -> None:
             print(f"  {msg}")
         sys.exit(1)
 
+    if not resolved:
+        print("Nothing to install")
+        return
+
     _invoke_pip(resolved)
 
 

tests/test_pip_wrapper_cli.py

@@ -0,0 +1,49 @@
+import dataclasses
+
+import pytest
+
+
+@dataclasses.dataclass
+class CLIResult:
+    code: int
+    stdout: str
+    stderr: str
+
+
+@pytest.fixture
+def invoked_pip_args(monkeypatch):
+    calls: list[list[str]] = []
+    monkeypatch.setattr("dependency_groups._pip_wrapper._invoke_pip", calls.append)
+    return calls
+
+
+@pytest.fixture
+def run(capsys, invoked_pip_args):
+    from dependency_groups._pip_wrapper import main as cli_main
+
+    def _run(*argv):
+        try:
+            cli_main(argv=[str(arg) for arg in argv])
+            rc = 0
+        except SystemExit as e:
+            rc = e.code
+
+        stdio = capsys.readouterr()
+        return CLIResult(rc, stdio.out, stdio.err)
+
+    return _run
+
+
+def test_empty_group_skips_pip(run, invoked_pip_args, tmp_path):
+    tomlfile = tmp_path / "pyproject.toml"
+    tomlfile.write_text(
+        """\
+[dependency-groups]
+empty = []
+"""
+    )
+
+    res = run("-f", tomlfile, "empty")
+    assert res.code == 0
+    assert invoked_pip_args == []
+    assert "Nothing to install" in res.stdout