Implement eth_chainId endpoint

docs/web3.eth.rst

@@ -153,6 +153,18 @@ The following properties are available on the ``web3.eth`` namespace.
        '63'
 
 
+.. py:attribute:: Eth.chainId
+
+    * Delegates to ``eth_chainId`` RPC Method
+
+    Returns a hex-encoded integer value for the currently configured "Chain Id" value introduced in `EIP-155 <https://github.com/ethereum/EIPs/blob/master/EIPS/eip-155.md>`_. Returns ``None`` if no Chain Id is available.
+
+    .. code-block:: python
+
+       >>> web3.eth.chainId
+       '0x3d'
+
+
 Methods
 -------
 

tests/core/eth-module/test_eth_protocolVersion.py → tests/core/eth-module/test_eth_properties.py

@@ -1,2 +1,6 @@
 def test_eth_protocolVersion(web3):
     assert web3.eth.protocolVersion == '63'
+
+
+def test_eth_chainId(web3):
+    assert web3.eth.chainId == '0x3d'

tests/integration/go_ethereum/common.py

@@ -64,6 +64,10 @@ def test_eth_estimateGas_with_block(self,
     def test_eth_signTransaction(self, web3, unlocked_account):
         super().test_eth_signTransaction(web3, unlocked_account, GETH_SIGNED_TX)
 
+    def test_eth_chainId(self, web3):
+        pytest.xfail('eth_chainId has not been implemented in geth')
+        super().test_eth_chainId(web3)
+
 
 class GoEthereumVersionModuleTest(VersionModuleTest):
     pass

tests/integration/test_ethereum_tester.py

@@ -7,9 +7,13 @@
 from eth_utils import (
     is_checksum_address,
     is_dict,
+    is_hex,
 )
 
 from web3 import Web3
+from web3._utils.formatters import (
+    hex_to_integer,
+)
 from web3._utils.module_testing import (
     EthModuleTest,
     GoEthereumPersonalModuleTest,
@@ -291,6 +295,11 @@ def test_eth_estimateGas_with_block(self,
             web3, unlocked_account_dual_type
         )
 
+    def test_eth_chainId(self, web3):
+        chain_id = web3.eth.chainId
+        assert is_hex(chain_id)
+        assert hex_to_integer(chain_id) is 61
+
 
 class TestEthereumTesterVersionModule(VersionModuleTest):
     pass

web3/_utils/module_testing/eth_module.py

@@ -65,6 +65,10 @@ def test_eth_hashrate(self, web3):
         assert is_integer(hashrate)
         assert hashrate >= 0
 
+    def test_eth_chainId(self, web3):
+        chain_id = web3.eth.chainId
+        assert chain_id is None
+
     def test_eth_gasPrice(self, web3):
         gas_price = web3.eth.gasPrice
         assert is_integer(gas_price)

web3/eth.py

@@ -101,6 +101,10 @@ def accounts(self):
     def blockNumber(self):
         return self.web3.manager.request_blocking("eth_blockNumber", [])
 
+    @property
+    def chainId(self):
+        return self.web3.manager.request_blocking("eth_chainId", [])
+
     def getBalance(self, account, block_identifier=None):
         if block_identifier is None:
             block_identifier = self.defaultBlock

web3/providers/eth_tester/defaults.py

@@ -170,6 +170,7 @@ def personal_send_transaction(eth_tester, params):
         ),
         'mining': static_return(False),
         'hashrate': static_return(0),
+        'chainId': static_return('0x3d'),
         'gasPrice': static_return(1),
         'accounts': call_eth_tester('get_accounts'),
         'blockNumber': compose(