Comment by elandybarr

Failing to use join is a big one.

I have seen countless instances of people writing the logic to output commas in between items (like for CSV export) that they want to concatenate into a string.

    header_line = ','.join( header for header in headers )
    csv_line    = ','.join( str(dataset[key]) for key in dataset.keys() )

Example for a case of a dictionary mapping a string to a bunch of numbers.