Say in the Element Data documentation what is safe with threads

pipeline/fiftyone_pipeline_core/src/fiftyone_pipeline_core/elementdata.py

@@ -28,6 +28,13 @@ class ElementData(object):
     Stores information created by a FlowElement based on FlowData.
     Stored in FlowData
 
+    Only the Flow Element that creates an instance writes it, so an instance
+    does not need to be safe to write from several threads. A finished
+    instance, once the Flow Element that writes it has finished, is safe to
+    read from several threads at once, such as the caller's own threads. An
+    instance that is written from several threads, or read while another
+    thread writes it, is not catered for.
+
     """
 
     def __init__(self, flow_element):

pipeline/fiftyone_pipeline_core/tests/test_elementdata_documentation.py

@@ -0,0 +1,26 @@
+
+"""
+The documentation of Element Data says it is safe to read from several threads
+once the Flow Element that writes it has finished, and is not catered for when
+written from several threads or read while another thread writes, as the
+thread safety page of the specification requires.
+"""
+
+import unittest
+
+from fiftyone_pipeline_core.elementdata import ElementData
+
+
+class ElementDataDocumentationTests(unittest.TestCase):
+
+    def test_the_documentation_says_what_is_safe_with_threads(self):
+        text = " ".join(ElementData.__doc__.split())
+        self.assertIn("safe to read from several threads", text)
+        self.assertIn("once the Flow Element that writes it has finished", text)
+        self.assertIn("not catered for", text)
+        self.assertIn("written from several threads", text)
+        self.assertIn("read while another thread writes", text)
+
+
+if __name__ == "__main__":
+    unittest.main()