add comment on in-function import

Author: mfeurer

openml/flows/flow.py

@@ -341,6 +341,10 @@ def publish(self):
         self : OpenMLFlow
 
         """
+        # Import at top not possible because of cyclic dependencies. In
+        # particular, flow.py tries to import functions.py in order to call
+        # get_flow(), while functions.py tries to import flow.py in order to
+        # instantiate an OpenMLFlow.
         import openml.flows.functions
 
         xml_description = self._to_xml()