ABI Types

The Web3 library follows the following conventions.

Bytes vs Text

  • The term bytes is used to refer to the binary representation of a string.

  • The term text is used to refer to unicode representations of strings.

Hexadecimal Representations

  • All hexadecimal values will be returned as text.

  • All hexadecimal values will be 0x prefixed.

Ethereum Addresses

All addresses must be supplied in one of three ways:

  • While connected to mainnet, an Ethereum Name Service name (often in the form myname.eth)

  • A 20-byte hexadecimal that is checksummed using the EIP-55 spec.

  • A 20-byte binary address.

Strict Bytes Type Checking


In version 6, this will be the default behavior

There is a method on web3 that will enable stricter bytes type checking. The default is to allow Python strings, and to allow bytestrings less than the specified byte size. To enable stricter checks, use w3.enable_strict_bytes_type_checking(). This method will cause the web3 instance to raise an error if a Python string is passed in without a “0x” prefix. It will also raise an error if the byte string or hex string is not the exact number of bytes specified by the ABI type. See the Enabling Strict Checks for Bytes Types section for an example and more details.

Types by Example

Let’s use a contrived contract to demonstrate input types in Web3.py:

contract ManyTypes {
    // booleans
    bool public b;

    // unsigned ints
    uint8 public u8;
    uint256 public u256;
    uint256[] public u256s;

    // signed ints
    int8 public i8;

    // addresses
    address public addr;
    address[] public addrs;

    // bytes
    bytes1 public b1;

    // structs
    struct S {
      address sa;
      bytes32 sb;
    mapping(address => S) addrStructs;

    function updateBool(bool x) public { b = x; }
    function updateUint8(uint8 x) public { u8 = x; }
    function updateUint256(uint256 x) public { u256 = x; }
    function updateUintArray(uint256[] memory x) public { u256s = x; }
    function updateInt8(int8 x) public { i8 = x; }
    function updateAddr(address x) public { addr = x; }
    function updateBytes1(bytes1 x) public { b1 = x; }
    function updateMapping(S memory x) public { addrStructs[x.sa] = x; }



Unsigned Integers

contract_instance.functions.updateUint256(2**256 - 1).transact()
contract_instance.functions.updateUintArray([1, 2, 3]).transact()

Signed Integers







contract_instance.functions.updateMapping({"sa": "0x0000000000000000000000000000000000000000", "sb": HexBytes(123)}).transact()