So until one has not edited this section, training_dataset doesn’t become itself training_datasetdataset
Also I see in the next line again some typo mistake in the for statement, so be careful while writing the codes, chances are while use of key board space+enter, you might unlocked or edited the cell which you weren’t suppose to do.
Get a fresh copy and re-start your assignment, making sure only to replace None place between the markers ###START AND END CODE HERE###
here is a link explaining how to get a fresh copy, refer Topic 1a