Here are some common things you might want to do with web3.
Blocks can be looked up by either their number or hash using the
web3.eth.getBlock
API. Block hashes should be in their hexadecimal
representation. Block numbers
# get a block by number
>>> web3.eth.getBlock(12345)
{
'author': '0xad5C1768e5974C231b2148169da064e61910f31a',
'difficulty': 735512610763,
'extraData': '0x476574682f76312e302e302f6c696e75782f676f312e342e32',
'gasLimit': 5000,
'gasUsed': 0,
'hash': '0x767c2bfb3bdee3f78676c1285cd757bcd5d8c272cef2eb30d9733800a78c0b6d',
'logsBloom': '0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000',
'miner': '0xad5c1768e5974c231b2148169da064e61910f31a',
'mixHash': '0x31d9ec7e3855aeba37fd92aa1639845e70b360a60f77f12eff530429ef8cfcba',
'nonce': '0x549f882c5f356f85',
'number': 12345,
'parentHash': '0x4b3c1d7e65a507b62734feca1ee9f27a5379e318bd52ae62de7ba67dbeac66a3',
'receiptsRoot': '0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421',
'sealFields': ['0x31d9ec7e3855aeba37fd92aa1639845e70b360a60f77f12eff530429ef8cfcba',
'0x549f882c5f356f85'],
'sha3Uncles': '0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347',
'size': 539,
'stateRoot': '0xca495e22ed6b88c61714d129dbc8c94f5bf966ac581c09a57c0a72d0e55e7286',
'timestamp': 1438367030,
'totalDifficulty': 3862140487204603,
'transactions': [],
'transactionsRoot': '0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421',
'uncles': [],
}
# get a block by it's hash
>>> web3.eth.getBlock('0x767c2bfb3bdee3f78676c1285cd757bcd5d8c272cef2eb30d9733800a78c0b6d')
{...}
You can also retrieve the latest block using the string 'latest'
in the
web3.eth.getBlock
API.
>>> web3.eth.getBlock('latest')
{...}
If you want to know the latest block number you can use the
web3.eth.blockNumber
property.
>>> web3.eth.blockNumber
4194803
Web3 can help you convert between denominations. The following denominations are supported.
denomination | amount in wei |
wei | 1 |
kwei | 1000 |
babbage | 1000 |
femtoether | 1000 |
mwei | 1000000 |
lovelace | 1000000 |
picoether | 1000000 |
gwei | 1000000000 |
shannon | 1000000000 |
nanoether | 1000000000 |
nano | 1000000000 |
szabo | 1000000000000 |
microether | 1000000000000 |
micro | 1000000000000 |
finney | 1000000000000000 |
milliether | 1000000000000000 |
milli | 1000000000000000 |
ether | 1000000000000000000 |
kether | 1000000000000000000000 |
grand | 1000000000000000000000 |
mether | 1000000000000000000000000 |
gether | 1000000000000000000000000000 |
tether | 1000000000000000000000000000000 |
>>> web3.toWei('1', 'ether')
1000000000000000000
>>> web3.fromWei('1000000000000000000', 'ether')
Decimal('1')
>>> from_wei(123456789, 'ether')
Decimal('1.23456789E-10')
You can look up transactions using the web3.eth.getTransaction
function.
>>> web3.eth.getTransaction('0x5c504ed432cb51138bcf09aa5e8a410dd4a1e204ef84bfed1be16dfba1b22060')
{
'blockHash': '0x4e3a3754410177e6937ef1f84bba68ea139e8d1a2258c5f85db9f1cd715a1bdd',
'blockNumber': 46147,
'condition': None,
'creates': None,
'from': '0xA1E4380A3B1f749673E270229993eE55F35663b4',
'gas': 21000,
'gasPrice': 50000000000000,
'hash': '0x5c504ed432cb51138bcf09aa5e8a410dd4a1e204ef84bfed1be16dfba1b22060',
'input': '0x',
'networkId': None,
'nonce': 0,
'publicKey': '0x376fc429acc35e610f75b14bc96242b13623833569a5bb3d72c17be7e51da0bb58e48e2462a59897cead8ab88e78709f9d24fd6ec24d1456f43aae407a8970e4',
'r': '0x88ff6cf0fefd94db46111149ae4bfc179e9b94721fffd821d38d16464b3f71d0',
'raw': '0xf86780862d79883d2000825208945df9b87991262f6ba471f09758cde1c0fc1de734827a69801ca088ff6cf0fefd94db46111149ae4bfc179e9b94721fffd821d38d16464b3f71d0a045e0aff800961cfce805daef7016b9b675c137a6a41a548f7b60a3484c06a33a',
's': '0x45e0aff800961cfce805daef7016b9b675c137a6a41a548f7b60a3484c06a33a',
'standardV': '0x1',
'to': '0x5DF9B87991262F6BA471F09758CDE1c0FC1De734',
'transactionIndex': 0,
'v': '0x1c',
'value': 31337,
}
If no transaction for the given hash can be found, then this function will
instead return None
.
Transaction receipts can be retrieved using the web3.eth.getTransactionReceipt
API.
>>> web3.eth.getTransactionReceipt('0x5c504ed432cb51138bcf09aa5e8a410dd4a1e204ef84bfed1be16dfba1b22060')
{
'blockHash': '0x4e3a3754410177e6937ef1f84bba68ea139e8d1a2258c5f85db9f1cd715a1bdd',
'blockNumber': 46147,
'contractAddress': None,
'cumulativeGasUsed': 21000,
'gasUsed': 21000,
'logs': [],
'logsBloom': '0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000',
'root': '0x96a8e009d2b88b1483e6941e6812e32263b05683fac202abc622a3e31aed1957',
'transactionHash': '0x5c504ed432cb51138bcf09aa5e8a410dd4a1e204ef84bfed1be16dfba1b22060',
'transactionIndex': 0,
}
If the transaction has not yet been mined then this method will return None
.
Given the following solidity source file stored at contract.sol
.
contract StoreVar {
uint8 public _myVar;
event MyEvent(uint indexed _var);
function setVar(uint8 _var) public {
_myVar = _var;
MyEvent(_var);
}
function getVar() public view returns (uint8) {
return _myVar;
}
}
The following example demonstrates a few things:
- Compiling a contract from a sol file.
- Estimating gas costs of a transaction.
- Transacting with a contract function.
- Waiting for a transaction receipt to be mined.
import sys
import time
import pprint
from web3.providers.eth_tester import EthereumTesterProvider
from web3 import Web3
from solc import compile_source
def compile_source_file(file_path):
with open(file_path, 'r') as f:
source = f.read()
return compile_source(source)
def deploy_contract(w3, contract_interface):
tx_hash = w3.eth.contract(
abi=contract_interface['abi'],
bytecode=contract_interface['bin']).deploy()
address = w3.eth.getTransactionReceipt(tx_hash)['contractAddress']
return address
w3 = Web3(EthereumTesterProvider())
contract_source_path = 'contract.sol'
compiled_sol = compile_source_file('contract.sol')
contract_id, contract_interface = compiled_sol.popitem()
address = deploy_contract(w3, contract_interface)
print("Deployed {0} to: {1}\n".format(contract_id, address))
store_var_contract = w3.eth.contract(
address=address,
abi=contract_interface['abi'])
gas_estimate = store_var_contract.functions.setVar(255).estimateGas()
print("Gas estimate to transact with setVar: {0}\n".format(gas_estimate))
if gas_estimate < 100000:
print("Sending transaction to setVar(255)\n")
tx_hash = store_var_contract.functions.setVar(255).transact()
receipt = w3.eth.waitForTransactionReceipt(tx_hash)
print("Transaction receipt mined: \n")
pprint.pprint(dict(receipt))
print("Was transaction successful? \n")
pprint.pprint(receipt['status'])
else:
print("Gas cost exceeds 100000")
Output:
Deployed <stdin>:StoreVar to: 0xF2E246BB76DF876Cef8b38ae84130F4F55De395b
Gas estimate to transact with setVar: 32463
Sending transaction to setVar(255)
Transaction receipt mined:
{'blockHash': HexBytes('0x94e07b0b88667da284e914fa44b87d4e7fec39761be51245ef94632a3b5ab9f0'),
'blockNumber': 2,
'contractAddress': None,
'cumulativeGasUsed': 43106,
'gasUsed': 43106,
'logs': [AttributeDict({'type': 'mined', 'logIndex': 0, 'transactionIndex': 0, 'transactionHash': HexBytes('0x3ac3518cc59d1698aa03a0bab7fb8191a4ef017aeda7429b11e8c6462b20a62a'), 'blockHash': HexBytes('0x94e07b0b88667da284e914fa44b87d4e7fec39761be51245ef94632a3b5ab9f0'), 'blockNumber': 2, 'address': '0xF2E246BB76DF876Cef8b38ae84130F4F55De395b', 'data': '0x', 'topics': [HexBytes('0x6c2b4666ba8da5a95717621d879a77de725f3d816709b9cbe9f059b8f875e284'), HexBytes('0x00000000000000000000000000000000000000000000000000000000000000ff')]})],
'transactionHash': HexBytes('0x3ac3518cc59d1698aa03a0bab7fb8191a4ef017aeda7429b11e8c6462b20a62a'),
'transactionIndex': 0}
Most fungible tokens on the Ethereum blockchain conform to the ERC20 standard. This section of the guide covers interacting with an existing token contract which conforms to this standard.
.. testsetup:: from web3 import Web3 w3 = Web3(Web3.EthereumTesterProvider()) bytecode = '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' ABI = '[{"constant":true,"inputs":[],"name":"name","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function","stateMutability":"view"},{"constant":false,"inputs":[{"name":"_spender","type":"address"},{"name":"_value","type":"uint256"}],"name":"approve","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function","stateMutability":"nonpayable"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function","stateMutability":"view"},{"constant":false,"inputs":[{"name":"_from","type":"address"},{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transferFrom","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function","stateMutability":"nonpayable"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"name":"","type":"uint8"}],"payable":false,"type":"function","stateMutability":"view"},{"constant":true,"inputs":[{"name":"_owner","type":"address"}],"name":"balanceOf","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function","stateMutability":"view"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function","stateMutability":"view"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transfer","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function","stateMutability":"nonpayable"},{"constant":true,"inputs":[{"name":"_owner","type":"address"},{"name":"_spender","type":"address"}],"name":"allowance","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function","stateMutability":"view"},{"inputs":[{"name":"_totalSupply","type":"uint256"}],"payable":false,"type":"constructor","stateMutability":"nonpayable"},{"anonymous":false,"inputs":[{"indexed":true,"name":"from","type":"address"},{"indexed":true,"name":"to","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Transfer","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"owner","type":"address"},{"indexed":true,"name":"spender","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Approval","type":"event"}]' factory = w3.eth.contract(abi=ABI, bytecode=bytecode) alice, bob = w3.eth.accounts[0], w3.eth.accounts[1] assert alice == '0x7E5F4552091A69125d5DfCb7b8C2659029395Bdf', alice assert bob == '0x2B5AD5c4795c026514f8317c7a215E218DcCD6cF', bob tx_hash = factory.constructor(1000000).transact({'from': alice}) assert tx_hash == b'h9\xeb\xdb4\x07\x03y\x92RP`X\xf6\xf7\x9f\xfaT\xed&e\xee*\xc2\rx\xb3\xab\x8c4\xc9\x1f', tx_hash txn_receipt = w3.eth.waitForTransactionReceipt(tx_hash) assert txn_receipt['contractAddress'] == '0xF2E246BB76DF876Cef8b38ae84130F4F55De395b', txn_receipt['contractAddress'] contract_address = txn_receipt['contractAddress'] contract = w3.eth.contract(contract_address, abi=ABI) total_supply = contract.functions.totalSupply().call() decimals = 10 ** 18 assert total_supply == 1000000 * decimals, total_supply
In this guide we will interact with an existing token contract that we have already deployed to a local testing chain. This guide assumes:
1. An existing token contract at a known address.
1. Access to the proper ABI
for the given contract.
1. A ~web3.main.Web3 instance connected to a provider with an unlocked account which can send transactions.
First we need to create a contract instance with the address of our token
contract and the ERC20
ABI.
>>> contract = w3.eth.contract(contract_address, abi=ABI)
>>> contract.address
'0xF2E246BB76DF876Cef8b38ae84130F4F55De395b'
Each token will have a total supply which represents the total number of tokens in circulation. In this example we've initialized the token contract to have 1 million tokens. Since this token contract is setup to have 18 decimal places, the raw total supply returned by the contract is going to have 18 additional decimal places.
>>> contract.functions.name().call()
'TestToken'
>>> contract.functions.symbol().call()
'TEST'
>>> decimals = contract.functions.decimals().call()
>>> decimals
18
>>> DECIMALS = 10 ** decimals
>>> contract.functions.totalSupply().call() // DECIMALS
1000000
Next we can query some account balances using the contract's balanceOf
function. The token contract we are using starts with a single account which
we'll refer to as alice
holding all of the tokens.
>>> alice = '0x7E5F4552091A69125d5DfCb7b8C2659029395Bdf'
>>> bob = '0x2B5AD5c4795c026514f8317c7a215E218DcCD6cF'
>>> raw_balance = contract.functions.balanceOf(alice).call()
>>> raw_balance
1000000000000000000000000
>>> raw_balance // DECIMALS
1000000
>>> contract.functions.balanceOf(bob).call()
0
Next we can transfer some tokens from alice
to bob
using the contract's
transfer
function.
>>> tx_hash = contract.functions.transfer(bob, 100).transact({'from': alice})
>>> tx_receipt = w3.eth.waitForTransactionReceipt(tx_hash)
>>> contract.functions.balanceOf(alice).call()
999999999999999999999900
>>> contract.functions.balanceOf(bob).call()
100
Alice could also approve someone else to spend tokens from her account using
the approve
function. We can also query how many tokens we're approved to
spend using the allowance
function.
>>> contract.functions.allowance(alice, bob).call()
0
>>> tx_hash = contract.functions.approve(bob, 200).transact({'from': alice})
>>> tx_receipt = w3.eth.waitForTransactionReceipt(tx_hash)
>>> contract.functions.allowance(alice, bob).call()
200
When someone has an allowance they can transfer those tokens using the
transferFrom
function.
>>> contract.functions.allowance(alice, bob).call()
200
>>> contract.functions.balanceOf(bob).call()
100
>>> tx_hash = contract.functions.transferFrom(alice, bob, 75).transact({'from': bob})
>>> tx_receipt = w3.eth.waitForTransactionReceipt(tx_hash)
>>> contract.functions.allowance(alice, bob).call()
125
>>> contract.functions.balanceOf(bob).call()
175
Here is an example of how one can use the pytest framework in python, Web3.py, eth-tester, and PyEVM to perform unit tests entirely in python without any additional need for a full featured ethereum node/client. To install needed dependencies you can use the pinned extra for eth_tester in web3 and pytest:
$ pip install web3[tester] pytest
Once you have an environment set up for testing, you can then write your tests like so:
Import your required libraries
from web3 import Web3, HTTPProvider
Initialize a web3 instance with an Infura node
w3 = Web3(Web3.HTTPProvider("https://rinkeby.infura.io/v3/YOUR_INFURA_KEY"))
Inject the middleware into the middleware onion
from web3.middleware import geth_poa_middleware
w3.middleware_onion.inject(geth_poa_middleware, layer=0)
Just remember that you have to sign all transactions locally, as infura does not handle any keys from your wallet ( refer to this )
transaction = contract.functions.function_Name(params).buildTransaction()
transaction.update({ 'gas' : appropriate_gas_amount })
transaction.update({ 'nonce' : web3.eth.getTransactionCount('Your_Wallet_Address') })
signed_tx = w3.eth.account.signTransaction(transaction, private_key)
P.S : the two updates are done to the transaction dictionary, since a raw transaction might not contain gas & nonce amounts, so you have to add them manually.
And finally, send the transaction
txn_hash = w3.eth.sendRawTransaction(signed_tx.rawTransaction)
txn_receipt = w3.eth.waitForTransactionReceipt(txn_hash)
Tip : afterwards you can use the value stored in txn_hash
, in an explorer like etherscan to view the transaction's details