test: --formatter=none with Black installed or not [drop-black-depend…

…ency]

src/darker/tests/test_main.py

@@ -22,6 +22,7 @@
 from darker.help import LINTING_GUIDE
 from darker.terminal import output
 from darker.tests.examples import A_PY, A_PY_BLACK, A_PY_BLACK_FLYNT, A_PY_BLACK_ISORT
+from darker.tests.helpers import black_present
 from darker.tests.test_fstring import FLYNTED_SOURCE, MODIFIED_SOURCE, ORIGINAL_SOURCE
 from darkgraylib.git import RevisionRange
 from darkgraylib.testtools.highlighting_helpers import BLUE, CYAN, RESET, WHITE, YELLOW
@@ -630,3 +631,32 @@ def test_long_command_length(git_repo):
     git_repo.add(files, commit="Add all the files")
     result = darker.__main__.main(["--diff", "--check", "src"])
     assert result == 0
+
+
+@pytest.fixture(scope="module")
+def formatter_none_repo(git_repo_m):
+    """Create a Git repository with a single file and a formatter that does nothing."""
+    files = git_repo_m.add({"file1.py": "# old content\n"}, commit="Initial")
+    files["file1.py"].write_text(
+        "import sys, os\n"
+        "print ( 'untouched unformatted code' )\n"
+    )
+    return files
+
+
+@pytest.mark.parametrize("has_black", [False, True])
+def test_formatter_none(has_black, formatter_none_repo):
+    """The dummy formatter works regardless of whether Black is installed or not."""
+    with black_present(present=has_black):
+        argv = ["--formatter=none", "--isort", "file1.py"]
+
+        result = darker.__main__.main(argv)
+
+    assert result == 0
+    expect = (
+        "import os\n"
+        "import sys\n"
+        "\n"
+        "print ( 'untouched unformatted code' )\n"
+    )
+    assert formatter_none_repo["file1.py"].read_text() == expect