PostgreSQL: Python UDF not shown in query plan obtained by EXPLAIN ANALYZE

I have a question regarding the execution of a Python UDF. Let's suppose I have a UDF named testUDF(...) and I apply it to table "testtable". When I run the query SELECT testUDF(...) from testtable, I get the correct result (I omitted the params of the UDF on purpose). However, when I want to check the query plan for this query, the UDF is not included in there. I used EXPLAIN ANALYZE SELECT testUDF(...) from testtable and EXPLAIN SELECT testUDF(...) from testtable. In both cases, the output only shows a seq. scan but the UDF is not mentioned anywhere. For more complex queries, I am interested in how to see at which position of the query plan the UDF is executed. How is this possible? Many thanks in advance for your help!