SmartPy NFT FA2 Contract with reveal feature

FA2_REVEAL.py

import smartpy as sp
FA2 = sp.io.import_script_from_url("https://smartpy.io/templates/fa2_lib.py")

class MyProject(
    FA2.Admin,
    FA2.BurnNft,
    FA2.ChangeMetadata,
    FA2.WithdrawMutez,
    FA2.MintNft,
    FA2.OnchainviewBalanceOf,
    FA2.Fa2Nft):
    def __init__(self, admin, metadata, token_metadata = {}, ledger = {}, policy = FA2.OwnerOrOperatorTransfer(), metadata_base = None):
        FA2.Fa2Nft.__init__(self, metadata, token_metadata = token_metadata, ledger = ledger, policy = policy, metadata_base = metadata_base)
        FA2.Admin.__init__(self, admin)        
        self.update_initial_storage(      
            last_token_id_revealed = 0)
   
    @sp.private_lambda(with_storage=None, with_operations=False, wrap_call=False)
    def nat_stringify(self,from_nat):
        sp.set_type(from_nat,sp.TNat)
        digits = sp.map(l={x : str(x) for x in range(0, 10)}, tkey = sp.TNat, tvalue=sp.TString) 
        my_nat   = sp.local('my_nat', from_nat)
        my_string = sp.local("my_string", [])
        sp.if my_nat.value == 0:
            my_string.value.push("0")
        sp.while 0 < my_nat.value:
            my_string.value.push(digits[my_nat.value % 10])
            my_nat.value //= 10
        sp.result(sp.concat(my_string.value))   

    def called_by_owner(self):        
        sp.if self.data.administrator != sp.sender:
            sp.failwith("FA2_NOT_OWNER")    

    @sp.entry_point
    def reveal(self, params): 
        sp.set_type(params.revealed_folder_hash, sp.TString)
        self.called_by_owner()
        with sp.for_("token_id", sp.range(self.data.last_token_id_revealed,self.data.last_token_id,step=1)) as token_id:
            token_id = params.token_id
            token_id_stringify = self.nat_stringify(token_id)
            token_info = "ipfs://"+params.revealed_folder_hash+"/"+token_id_stringify+".json"
            self.data.token_metadata[token_id].token_info[""] = sp.pack(token_info)
            self.data.last_token_id_revealed = self.data.last_token_id
            

sp.add_compilation_target(
    "My Project Name",
    MyProject(
        admin=sp.address("tz1LgP5DCcgBZ6AaHQw5iwDynCi21G9MKXxy"),
        metadata = sp.utils.metadata_of_url("ipfs://hash")
    )
)