tests: enforce lazy markdown_it import

Author: hukkin

tests/test_performance.py

@@ -0,0 +1,29 @@
+import subprocess
+import sys
+
+import pytest
+
+
+def test_importtime__no_mdit_import():
+    """Test that `markdown_it` isn't imported when `mdformat` and
+    `mdformat._cli` are.
+
+    Do this in a subprocess to have a clean environment separate from
+    pytest and other tests.
+    """
+    test_script = """\
+import sys
+import mdformat
+import mdformat._cli
+assert 'markdown_it' not in sys.modules, 'markdown_it was imported'
+"""
+    result = subprocess.run(
+        [sys.executable, "-c", test_script],
+        stdin=subprocess.DEVNULL,
+        stderr=subprocess.PIPE,
+    )
+    if result.returncode:
+        assert (
+            b"markdown_it was imported" in result.stderr
+        ), "unexpected error in subprocess"
+        pytest.fail("markdown_it was imported")