Add test for missing _multiprocessing module

joblib/test/missing_multiprocessing/_multiprocessing.py

@@ -0,0 +1,5 @@
+"""
+Simulate a missing _multiprocessing module by raising an ImportError.
+test_missing_multiprocessing adds this folder to the path.
+"""
+raise ImportError("No _multiprocessing module!")

joblib/test/test_missing_multiprocessing.py

@@ -0,0 +1,28 @@
+"""
+Pyodide and other single-threaded Python builds will be missing the
+_multiprocessing module. Test that joblib still works in this environment.
+"""
+
+import os
+import subprocess
+import sys
+
+
+def test_missing_multiprocessing():
+    """
+    Test that import joblib works even if _multiprocessing is missing.
+
+    pytest has already imported everything from joblib. The most reasonable way
+    to test importing joblib with modified environment is to invoke a separate
+    Python process. This also ensures that we don't break other tests by
+    importing a bad `_multiprocessing` module.
+    """
+    env = dict(os.environ)
+    # For subprocess, use current sys.path with our custom version of
+    # multiprocessing inserted.
+    import joblib
+
+    env["PYTHONPATH"] = ":".join(
+        [joblib.__path__[0] + "/test/missing_multiprocessing"] + sys.path
+    )
+    subprocess.check_call([sys.executable, "-c", "import joblib"], env=env)

setup.cfg

@@ -16,6 +16,7 @@ addopts =
     --doctest-modules
     -p no:warnings
     --ignore joblib/externals
+    --ignore joblib/test/missing_multiprocessing
 testpaths = joblib
 
 [flake8]