'Calculate longest line for each column.
:returns: dictionary where key is column number and value longest value'

def _longest_rows(self):

    longest = {c: 0 for c in range(self._columns)}
    for row in self.table_data:
        for (index, value) in enumerate(row):
            if (len(str(value)) > longest[index]):
                longest[index] = len(str(value))
    return longest
