Environment Variables and Constants

Environment Variables

Environment variables always exist in the namespace and are primarily used to provide information about the blockchain or current transaction.

Block and Transaction Properties

Name Type Value
block.coinbase address Current block miner’s address
block.difficulty uint256 Current block difficulty
block.number uint256 Current block number
block.prevhash bytes32 Equivalent to blockhash(block.number - 1)
block.timestamp uint256 Current block epoch timestamp
chain.id uint256 Chain ID
msg.gas uint256 Remaining gas
msg.sender address Sender of the message (current call)
msg.value uint256 Number of wei sent with the message
tx.origin address Sender of the transaction (full call chain)

Note

msg.sender and msg.value can only be accessed from public functions. If you require these values within a private function they must be passed as parameters.

The self Variable

self is an environment variable used to reference a contract from within itself. Along with the normal address members, self allows you to read and write to state variables and to call private functions within the contract.

Name Type Value
self address Current contract’s address
self.balance uint256 Current contract’s balance

Accessing State Variables

self is used to access a contract’s state variables, as shown in the following example:

state_var: uint256

@public
def set_var(value: uint256) -> bool:
    self.state_var = value
    return True

@public
@constant
def get_var() -> uint256:
    return self.state_var

Calling Private Functions

self is also used to call private functions within a contract:

@private
def _times_two(amount: uint256) -> uint256:
    return amount * 2

@public
def calculate(amount: uint256) -> uint256:
    return self._times_two(amount)

Built In Constants

Vyper has a few convenience constants builtin.

Name Type Value
ZERO_ADDRESS address 0x0000000000000000000000000000000000000000
EMPTY_BYTES32 bytes32 0x0000000000000000000000000000000000000000000000000000000000000000
MAX_INT128 int128 2**127 - 1
MIN_INT128 int128 -2**127
MAX_DECIMAL decimal (2**127 - 1)
MIN_DECIMAL decimal (-2**127)
MAX_UINT256 uint256 2**256 - 1

Custom Constants

Custom constants can be defined at a global level in Vyper. To define a constant make use of the constant keyword.

TOTAL_SUPPLY: constant(uint256) = 10000000
total_supply: public(uint256)

@public
def __init__():
    self.total_supply = TOTAL_SUPPLY