Better error message for test_output.py

python/ray/tests/test_output.py

@@ -6,11 +6,12 @@
 
 
 def test_output():
+    # Use subprocess to execute the __main__ below.
     outputs = subprocess.check_output(
         [sys.executable, __file__, "_ray_instance"],
         stderr=subprocess.STDOUT).decode()
     lines = outputs.split("\n")
-    assert len(lines) == 3
+    assert len(lines) == 3, lines
     logging_header = r"\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2},\d{3}\sINFO\s"
     assert re.match(
         logging_header + r"resource_spec.py:\d+ -- Starting Ray with [0-9\.]+ "
@@ -24,7 +25,7 @@ def test_output():
 
 if __name__ == "__main__":
     if len(sys.argv) > 1 and sys.argv[1] == "_ray_instance":
-        ray.init()
+        ray.init(num_cpus=1)
         ray.shutdown()
     else:
         sys.exit(pytest.main(["-v", __file__]))