FAQ: Build your own Mini-Blockchain! - Checking for a Broken Chain

Frequently Asked Questions Python FAQ
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This community-built FAQ covers the “Checking for a Broken Chain” exercise from the lesson “Build your own Mini-Blockchain!”.

Paths and Courses
This exercise can be found in the following Codecademy content:

Introduction to Blockchain

FAQs on the exercise Checking for a Broken Chain

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2 
  def validate_chain(self):
    for i in range(1, len(self.chain)):
      current = self.chain[i]
      previous = self.chain[i-1]
      if current.hash != current.generate_hash():
        return False
      if previous.hash != previous.generate_hash():
        return False
      return True

In another class:

class Block:
    def __init__(self, transactions, previous_hash):
        self.time_stamp = datetime.datetime.now()
        self.transactions = transactions
        self.previous_hash = previous_hash
        self.nonce = 0
        self.hash = self.generate_hash()

    def generate_hash(self):
        block_header = str(self.time_stamp) + str(self.transactions) +str(self.previous_hash) + str(self.nonce)
        block_hash = sha256(block_header.encode())
        return block_hash.hexdigest()

According to instruction the function of validate_chain() method is:

In order to validate the entire blockchain, we must loop through each of the blocks stored inside the blockchain itself. Then, we will check through each of the blocks to ensure that the previous hash value matches with the hash value inside our previous block.

My question is:
What is the difference between using .hash and .generate_hash() ?

1 Like
3

.hash, this is a variable that can be changed. .generate_hash () returns a .hash that cannot be changed.

1 Like
4

I believe hash is an instance variable that is created at when the block is instantiated. The method “generate_hash” is called during the comparison and we would expect that these two would not be equivalent, as for one thing the timestamp would be different.

I hope this helps

1 Like
5

Had the same question, these explanations are very helpful. Thank you

6

Why are we not validating the previous block’s hash with the current block’s hash, and instead verifying it with what is being generated right now? I seem to be missing something here…

1 Like
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I am confused why we are using the generate_hash() method to verify blocks.

Can’t we just check if current.previous_hash != previous.hash? The self.hash variable stores the output of generate_hash() method in a class variable.