If you are using the element of interest you can define on a diagram, you can use the following method:
pc.get_element_of_interest_for_diagrams()
That returns the list of Diagram where this element is an element of interest.
You also have contextual element:
pc.get_contextual_element_for_diagrams()
if you navigate from the diagram:
myDiagram.get_elements_of_interest()
myDiagram.get_contextual_elements()
if the element is not on a reference above and just in the diagram, you can use:
myDiagram.get_represented_elements()
You can check this post for more details on the last method.