def __init__(self,db_file):
''' (Bank,str)-> NoneType
initialize the Bank database from a file given as argument
'''
self._database = set()
handle = open(db_file,"r")
lines = handle.readlines()
for line in lines:
data = line.split(",")
p = Person(data[0],data[1],data[2])
a = BankAccount(p)
a.credit(int(data[3]))
self._database.add(a)
