column_from = {}
for i, bank in enumerate(participants):
   temp = []
   [temp.append(bank + "_to_" + other bank) for otherbank in participants if (otherbank != bank)]
   preface = ["key", "date", "processed_time"]
   column_from[bank] = preface + temp

{'Bank1' : ['key', 'date', 'processed_time', 'Bank1_to_Bank2', 'Bank1_to_Bank3', 'Bank1_to_Bank4']....}

error: can only concatenate str (not "tuple") to str