add proof verification example to docs

Author: paouvrard

docs/web3.eth.rst

@@ -235,6 +235,72 @@ The following methods are available on the ``web3.eth`` namespace.
             ]
         })
 
+    * Merkle proof verification using py-trie.
+    
+    The following example verifies that the values returned in the AttributeDict are included in the state of given trie ``root``.
+
+    .. code-block:: python
+
+        from eth_utils import (
+            keccak,
+        )
+        import rlp
+        from rlp.sedes import (
+            Binary,
+            big_endian_int,
+        )
+        from trie import (
+            HexaryTrie,
+        )
+        from web3._utils.encoding import (
+            pad_bytes,
+        )
+
+        def format_proof_nodes(proof):
+            trie_proof = []
+            for rlp_node in proof:
+                trie_proof.append(rlp.decode(bytes(rlp_node)))
+            return trie_proof
+
+        def verify_eth_getProof(proof, root):
+            trie_root = Binary.fixed_length(32, allow_empty=True)
+            hash32 = Binary.fixed_length(32)
+
+            class _Account(rlp.Serializable):
+                fields = [
+                            ('nonce', big_endian_int),
+                            ('balance', big_endian_int),
+                            ('storage', trie_root),
+                            ('code_hash', hash32)
+                        ]
+            acc = _Account(
+                proof.nonce, proof.balance, proof.storageHash, proof.codeHash
+            )
+            rlp_account = rlp.encode(acc)
+            trie_key = keccak(bytes.fromhex(proof.address[2:]))
+        
+            assert rlp_account == HexaryTrie.get_from_proof(
+                root, trie_key, format_proof_nodes(proof.accountProof)
+            ), "Failed to verify account proof {}".format(proof.address)
+
+            for storage_proof in proof.storageProof:
+                trie_key = keccak(pad_bytes(b'\x00', 32, storage_proof.key))
+                root = proof.storageHash
+                if storage_proof.value == b'\x00':
+                    rlp_value = b''
+                else:
+                    rlp_value = rlp.encode(storage_proof.value)
+
+                assert rlp_value == HexaryTrie.get_from_proof(
+                    root, trie_key, format_proof_nodes(storage_proof.proof)
+                ), "Failed to verify storage proof {}".format(storage_proof.key)
+
+            return True
+
+        block = w3.eth.getBlock(3391)
+        proof = w3.eth.getProof('0x6c8f2a135f6ed072de4503bd7c4999a1a17f824b', [0, 1], 3391)
+        assert verify_eth_getProof(proof, block.stateRoot)
+
 
 .. py:method:: Eth.getCode(account, block_identifier=eth.defaultBlock)