Complete section: Preparing the Blockchain for Collaboration

Author: 15Dkatz

backend/blockchain/block.py

@@ -37,6 +37,9 @@ def __repr__(self):
             f'nonce: {self.nonce})'
         )
 
+    def __eq__(self, other):
+        return self.__dict__ == other.__dict__
+
     @staticmethod
     def mine_block(last_block, data):
         """

backend/blockchain/blockchain.py

@@ -14,6 +14,39 @@ def add_block(self, data):
     def __repr__(self):
         return f'Blockchain: {self.chain}'
 
+    def replace_chain(self, chain):
+        """
+        Replace the local chain with the incoming one if the following applies:
+          - The incoming chain is longer than the local one.
+          - The incoming chain is formatted properly.
+        """
+        if len(chain) <= len(self.chain):
+            raise Exception('Cannot replace. The incoming chain must be longer.')
+
+        try:
+            Blockchain.is_valid_chain(chain)
+        except Exception as e:
+            raise Exception(f'Cannot replace. The incoming chain is invalid: {e}')
+
+        self.chain = chain
+
+    @staticmethod
+    def is_valid_chain(chain):
+        """
+        Validate the incoming chain.
+        Enforce the following rules of the blockchain:
+          - the chain must start with the genesis block
+          - blocks must be formatted correctly
+        """
+        if chain[0] != Block.genesis():
+            raise Exception('The genesis block must be valid')
+
+        for i in range(1, len(chain)):
+            block = chain[i]
+            last_block = chain[i-1]
+            Block.is_valid_block(last_block, block)
+
+
 def main():
     blockchain = Blockchain()
     blockchain.add_block('one')

backend/tests/blockchain/test_blockchain.py

@@ -1,3 +1,5 @@
+import pytest
+
 from backend.blockchain.blockchain import Blockchain
 from backend.blockchain.block import GENESIS_DATA
 
@@ -10,4 +12,39 @@ def test_add_block():
     data = 'test-data'
     blockchain.add_block(data)
 
-    assert blockchain.chain[-1].data == data
+    assert blockchain.chain[-1].data == data
+
+@pytest.fixture
+def blockchain_three_blocks():
+    blockchain = Blockchain()
+    for i in range(3):
+        blockchain.add_block(i)
+    return blockchain
+
+def test_is_valid_chain(blockchain_three_blocks):
+    Blockchain.is_valid_chain(blockchain_three_blocks.chain)
+
+def test_is_valid_chain_bad_genesis(blockchain_three_blocks):
+    blockchain_three_blocks.chain[0].hash = 'evil_hash'
+
+    with pytest.raises(Exception, match='genesis block must be valid'):
+        Blockchain.is_valid_chain(blockchain_three_blocks.chain)
+
+def test_replace_chain(blockchain_three_blocks):
+    blockchain = Blockchain()
+    blockchain.replace_chain(blockchain_three_blocks.chain)
+
+    assert blockchain.chain == blockchain_three_blocks.chain
+
+def test_replace_chain_not_longer(blockchain_three_blocks):
+    blockchain = Blockchain()
+
+    with pytest.raises(Exception, match='The incoming chain must be longer'):
+        blockchain_three_blocks.replace_chain(blockchain.chain)
+
+def test_replace_chain_bad_chain(blockchain_three_blocks):
+    blockchain = Blockchain()
+    blockchain_three_blocks.chain[1].hash = 'evil_hash'
+
+    with pytest.raises(Exception, match='The incoming chain is invalid'):
+        blockchain.replace_chain(blockchain_three_blocks.chain)