ERC-20 Token Standard

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Introduction

What is a Token?

Tokens can represent virtually anything in Ethereum:

reputation points in an online platform
skills of a character in a game
lottery tickets
financial assets like a share in a company
a fiat currency like USD
an ounce of gold
and more...

Such a powerful feature of Ethereum must be handled by a robust standard, right? That's exactly where the ERC-20 plays its role! This standard allows developers to build token applications that are interoperable with other products and services.

What is ERC-20?

The ERC-20 introduces a standard for Fungible Tokens, in other words, they have a property that makes each Token be exactly the same (in type and value) of another Token. For example, an ERC-20 Token acts just like the ETH, meaning that 1 Token is and will always be equal to all the other Tokens.

Prerequisites

Body

The ERC-20 (Ethereum Request for Comments 20), proposed by Fabian Vogelsteller in November 2015, is a Token Standard that implements an API for tokens within Smart Contracts.

It provides functionalities like to transfer tokens from one account to another, to get the current token balance of an account and also the total supply of the token available on the network. Besides these it also has some other functionalities like to approve that an amount of token from an account can be spent by a third party account.

If a Smart Contract implements the following methods and events it can be called an ERC-20 Token Contract and, once deployed, it will be responsible to keep track of the created tokens on Ethereum.

From EIP-20:

Methods

1function name() public view returns (string)
2function symbol() public view returns (string)
3function decimals() public view returns (uint8)
4function totalSupply() public view returns (uint256)
5function balanceOf(address _owner) public view returns (uint256 balance)
6function transfer(address _to, uint256 _value) public returns (bool success)
7function transferFrom(address _from, address _to, uint256 _value) public returns (bool success)
8function approve(address _spender, uint256 _value) public returns (bool success)
9function allowance(address _owner, address _spender) public view returns (uint256 remaining)
10
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Events

1event Transfer(address indexed _from, address indexed _to, uint256 _value)
2event Approval(address indexed _owner, address indexed _spender, uint256 _value)
3
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Examples

Let's see how a Standard is so important to make things simple for us to inspect any ERC-20 Token Contract on Ethereum. We just need the Contract Application Binary Interface (ABI) to create an interface to any ERC-20 Token. As you can see below we will use a simplified ABI, to make it a low friction example.

Web3.py Example

First, make sure you have installed Web3.py Python library:

1$ pip install web3
2

1from web3 import Web3
2
3
4w3 = Web3(Web3.HTTPProvider("https://cloudflare-eth.com"))
5
6dai_token_addr = "0x6B175474E89094C44Da98b954EedeAC495271d0F"     # DAI
7weth_token_addr = "0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2"    # Wrapped ether (WETH)
8
9acc_address = "0xA478c2975Ab1Ea89e8196811F51A7B7Ade33eB11"        # Uniswap V2: DAI 2
10
11# This is a simplified Contract Application Binary Interface (ABI) of an ERC-20 Token Contract.
12# It will expose only the methods: balanceOf(address), decimals(), symbol() and totalSupply()
13simplified_abi = [
14    {
15        'inputs': [{'internalType': 'address', 'name': 'account', 'type': 'address'}],
16        'name': 'balanceOf',
17        'outputs': [{'internalType': 'uint256', 'name': '', 'type': 'uint256'}],
18        'stateMutability': 'view', 'type': 'function', 'constant': True
19    },
20    {
21        'inputs': [],
22        'name': 'decimals',
23        'outputs': [{'internalType': 'uint8', 'name': '', 'type': 'uint8'}],
24        'stateMutability': 'view', 'type': 'function', 'constant': True
25    },
26    {
27        'inputs': [],
28        'name': 'symbol',
29        'outputs': [{'internalType': 'string', 'name': '', 'type': 'string'}],
30        'stateMutability': 'view', 'type': 'function', 'constant': True
31    },
32    {
33        'inputs': [],
34        'name': 'totalSupply',
35        'outputs': [{'internalType': 'uint256', 'name': '', 'type': 'uint256'}],
36        'stateMutability': 'view', 'type': 'function', 'constant': True
37    }
38]
39
40dai_contract = w3.eth.contract(address=w3.toChecksumAddress(dai_token_addr), abi=simplified_abi)
41symbol = dai_contract.functions.symbol().call()
42decimals = dai_contract.functions.decimals().call()
43totalSupply = dai_contract.functions.totalSupply().call() / 10**decimals
44addr_balance = dai_contract.functions.balanceOf(acc_address).call() / 10**decimals
45
46#  DAI
47print("===== %s =====" % symbol)
48print("Total Supply:", totalSupply)
49print("Addr Balance:", addr_balance)
50
51weth_contract = w3.eth.contract(address=w3.toChecksumAddress(weth_token_addr), abi=simplified_abi)
52symbol = weth_contract.functions.symbol().call()
53decimals = weth_contract.functions.decimals().call()
54totalSupply = weth_contract.functions.totalSupply().call() / 10**decimals
55addr_balance = weth_contract.functions.balanceOf(acc_address).call() / 10**decimals
56
57#  WETH
58print("===== %s =====" % symbol)
59print("Total Supply:", totalSupply)
60print("Addr Balance:", addr_balance)
61
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