Immutable variables are like constants except value are assigned when the contract is deployed.

When to use immutable variables?

• You have a variable that needs to be set when the contract is deployed, for example like setting contract owner to msg.sender
• and this variable will never change after deployment

Why declare variables as immutable?

Like constants, immutable variables save run time gas

# pragma version ^0.4.0

OWNER: public(immutable(address))
MY_IMMUTABLE: public(immutable(uint256))

@deploy
def __init__(val: uint256):
    OWNER = msg.sender
    MY_IMMUTABLE = val


@external
@view
def get_my_immutable() -> uint256:
  return MY_IMMUTABLE