Blockchain tutorials for double spending and signature verification

Question

I am not new to Blockchain technology and have been studying the TPCoin python codes line by line. However, when I was looking into the code, I found that there is no validation method that will prevent the double spending issues. Have shared my code below:-

class Transaction: 

        def __init__( self, sender, recipient, value ): 
              self.sender = sender 
              self.recipient = recipient 
              self.value = value 
              self.time = datetime.datetime.now() 
              self.signer = "" 

      def to_dict( self ): 

            if self.sender == "Genesis": 
                  identity = "Genesis" 
            else: 
                    identity = self.sender.identity 

            return collections.OrderedDict( { 'sender': identity, 'recipient': self.recipient, 'value': self.value, 'time' : self.time } ) 

            def sign_transaction( self ): 

                  private_key = self.sender._private_key 
                  signer = PKCS1_v1_5.new( private_key ) 
                  h = SHA.new( str( self.to_dict( ) ).encode( 'utf8' ) ) 
                  self.signer = binascii.hexlify( signer.sign( h ) ).decode( 'ascii' ) 
                  return self.signer 

            def display_transaction( self ): 
                  dict = self.to_dict( ) 
                  print ("sender: " + dict['sender']) 
                  print ('-----') 
                  print ("recipient: " + dict['recipient']) 
                  print ('-----') 
                  print ("value: " + str(dict['value'])) 
                  print ('-----') 
                  print ("time: " + str(dict['time'])) 
                  print ('-----') 
                  print ("signature: " + self.signer) 
                  print ('-----') 

        def validate_transaction( self ): 
                ### Double spending? Signature Verification? 
                return

I think there should be a sort of validation function within the Transaction class...but not quite sure what to do. I want a few ideas on how to handle this which will be well appreciated!!

Answer 1

To answer your question, regardless of how many ever coins are spent, only a single one will be included to the block while the rest will be invalid and also, the invalid transactions cannot be included to the new blocks due to the UTXO’s non-acceptance of the output. However, for instance, when a single or double miner snatch two different types of transactions which are from the same output excluding the latency of the network, then both blocks are added to their own chains. Observing the largest concept of the chain and clarifying the concept of orphaned chains is one of the best ways to solve your problem. Hope this answers your question!!