I am effectively iterating over columns. "schema_full" is a list of dictionaries containing information about a column in the target table (in this case, the user is "appending", so we can derive the schema from the target table). You're right, the variable name "row" is a bit ambiguous - I meant row in "schema_full" which corresponds to column information in the final result set. Here  am building up a string representation of the table schema in the format which bigquery likes:

"fieldname1 : fieldtype1, fieldname2 : fieldtype2, ...."