Python local variable referenced before assignment??

im trying to create a bank account data type. i have the following code:

Code:

---

def bankAccount():
        bal=0
        def credit(amount):
                bal=bal+amount
        def debit(amount):
                bal=bal-amount
        def balance():
                return bal
        return (lambda x: {"credit":credit,
                                "debit":debit,
                                "balance":balance
                                }[x])
b=bankAccount()
print b("balance")()
b("credit")(100)

---

at this point it ends when running with the following error: local variable 'bal' referenced before assignment

can anyone help??

thank you

GTJ