def get_private_key(self, sequence, wallet, password):
     from wallet import pw_decode
     for_change, i = sequence
     assert for_change == 0
     address = self.get_addresses(0)[i]
     pk = pw_decode(self.keypairs[address][1], password)
     # this checks the password
     assert address == address_from_private_key(pk,
         addrtype = self.active_chain.p2pkh_version, 
         wif_version = self.active_chain.wif_version)
     return [pk]

 def update_password(self, old_password, new_password):
     for k, v in self.keypairs.items():
         pubkey, a = v
         b = pw_decode(a, old_password)
         c = pw_encode(b, new_password)
         self.keypairs[k] = (pubkey, c)