fix: make --list respect the --output option

CHANGELOG.rst

@@ -14,6 +14,7 @@ Unreleased
 - 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.
+- `dependency-groups --list` now respects the `-o`/`--output` option.
 
 1.3.1
 -----

src/dependency_groups/__main__.py

@@ -1,11 +1,13 @@
+from __future__ import annotations
+
 import sys
 
 from ._argparse_compat import ArgumentParser
 from ._implementation import resolve
 from ._toml_compat import tomllib
 
 
-def main() -> None:
+def main(*, argv: list[str] | None = None) -> None:
     if tomllib is None:
         print(
             "Usage error: dependency-groups CLI requires tomli or Python 3.11+",
@@ -38,21 +40,20 @@ def main() -> None:
         action="store_true",
         help="List the available dependency groups",
     )
-    args = parser.parse_args()
+    args = parser.parse_args(argv if argv is not None else sys.argv[1:])
 
     with open(args.pyproject_file, "rb") as fp:
         pyproject = tomllib.load(fp)
 
     dependency_groups_raw = pyproject.get("dependency-groups", {})
 
     if args.list:
-        print(*dependency_groups_raw.keys())
-        return
-    if not args.GROUP_NAME:
+        content = " ".join(dependency_groups_raw.keys())
+    elif not args.GROUP_NAME:
         print("A GROUP_NAME is required", file=sys.stderr)
         raise SystemExit(3)
-
-    content = "\n".join(resolve(dependency_groups_raw, *args.GROUP_NAME))
+    else:
+        content = "\n".join(resolve(dependency_groups_raw, *args.GROUP_NAME))
 
     if args.output is None or args.output == "-":
         print(content)

tests/test_main_cli.py

@@ -0,0 +1,53 @@
+import dataclasses
+
+import pytest
+
+PYPROJECT = """\
+[dependency-groups]
+test = ["pytest"]
+docs = ["sphinx"]
+"""
+
+
+@dataclasses.dataclass
+class CLIResult:
+    code: int
+    stdout: str
+    stderr: str
+
+
+@pytest.fixture
+def run(capsys):
+    from dependency_groups.__main__ 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_list_to_stdout(run, tmp_path):
+    tomlfile = tmp_path / "pyproject.toml"
+    tomlfile.write_text(PYPROJECT)
+
+    res = run("-f", tomlfile, "--list")
+    assert res.code == 0
+    assert res.stdout == "test docs\n"
+
+
+def test_list_respects_output_file(run, tmp_path):
+    tomlfile = tmp_path / "pyproject.toml"
+    tomlfile.write_text(PYPROJECT)
+    outfile = tmp_path / "out.txt"
+
+    res = run("-f", tomlfile, "--list", "-o", outfile)
+    assert res.code == 0
+    assert res.stdout == ""
+    assert outfile.read_text() == "test docs\n"