Add test for output printed to stdout

Unit-test for the `print_source` function used when the `--stdout`
option is applied. To test the behaviour with/without pygments the
importing of pygments was extracted to a function.

src/darker/__main__.py

@@ -258,10 +258,11 @@ def print_source(new: TextDocument) -> None:
     """Print the reformatted Python source code"""
     if sys.stdout.isatty():
         try:
-            # pylint: disable=import-outside-toplevel
-            from pygments import highlight
-            from pygments.formatters import TerminalFormatter
-            from pygments.lexers.python import PythonLexer
+            (
+                highlight,
+                TerminalFormatter,  # pylint: disable=invalid-name
+                PythonLexer,
+            ) = _import_pygments()  # type: ignore
         except ImportError:
             print(new.string, end="")
         else:
@@ -270,6 +271,21 @@ def print_source(new: TextDocument) -> None:
         print(new.string, end="")
 
 
+def _import_pygments():  # type: ignore
+    """Import within a function to ease mocking the import in unit-tests.
+
+    Cannot be typed as it imports parts of its own return type.
+    """
+    # pylint: disable=import-outside-toplevel
+    from pygments import highlight
+    from pygments.formatters import (  # pylint: disable=no-name-in-module
+        TerminalFormatter,
+    )
+    from pygments.lexers.python import PythonLexer
+
+    return highlight, TerminalFormatter, PythonLexer
+
+
 def main(argv: List[str] = None) -> int:
     """Parse the command line and reformat and optionally lint each source file
 

src/darker/tests/test_main.py

@@ -8,7 +8,7 @@
 from pathlib import Path
 from textwrap import dedent
 from types import SimpleNamespace
-from unittest.mock import call, patch
+from unittest.mock import Mock, call, patch
 
 import pytest
 
@@ -778,6 +778,50 @@ def test_modify_file(tmp_path, new_content, expect):
     assert result == expect
 
 
+@pytest.mark.kwparametrize(
+    dict(
+        new_content=TextDocument(lines=['print("foo")']),
+        tty=False,
+        with_pygments=False,
+        expect=('print("foo")\n',),
+    ),
+    dict(
+        new_content=TextDocument(lines=['print("foo")']),
+        tty=False,
+        with_pygments=True,
+        expect=('print("foo")\n',),
+    ),
+    dict(
+        new_content=TextDocument(lines=['print("foo")']),
+        tty=True,
+        with_pygments=False,
+        expect=('print("foo")\n',),
+    ),
+    dict(
+        new_content=TextDocument(lines=['print("foo")']),
+        tty=True,
+        with_pygments=True,
+        expect=(
+            '\x1b[36mprint\x1b[39;49;00m(\x1b[33m"\x1b[39;49;00mfoo'
+            + '\x1b[33m"\x1b[39;49;00m)\n',
+            '\x1b[36mprint\x1b[39;49;00m(\x1b[33m"\x1b[39;49;00m\x1b[33mfoo'
+            + '\x1b[39;49;00m\x1b[33m"\x1b[39;49;00m)\n',
+        ),
+    ),
+)
+def test_print_source(new_content, tty, with_pygments, expect, capsys):
+    """Highlight is applied only if tty, final newline is handled correctly."""
+    with patch("sys.stdout.isatty", Mock(return_value=tty)), patch(
+        "darker.__main__._import_pygments",
+        Mock(
+            return_value=darker.__main__._import_pygments(),
+            side_effect=None if with_pygments else ImportError(),
+        ),
+    ):
+        darker.__main__.print_source(new_content)
+        assert capsys.readouterr().out in expect
+
+
 def test_stdout_path_resolution(git_repo, capsys):
     """When using ``--stdout``, file paths are resolved correctly"""
     git_repo.add({"src/menu.py": "print ( 'foo' )\n"})