Hi Mark,
I presume the python scripts extracts all the visible elements in the view? If so, try connecting the output of the “Transaction.End” node to a dummy input on the python node. In Dynamo a node will not initiate until all inputs are computed.
I can’t remember the last time I needed a “Transaction.Start” node because Dynamo’s global transaction manager has become very adequate and can predict very well if a node needs to be run inside a transaction.