In this lesson we will continue working with our _buy-me-a-coffee_ smart contract. We will introduce a few more key fundamentals to help us understand the basics of smart contract development in Vyper. We'll then move on to more complex aspects later in the course. Here is the code we are working with so far: ```python #pragma version 0.4.1 @license MIT @title Buy Me A Coffee! @author You! @notice This contract is for creating a sample funding contract # We'll learn a new way to do interfaces later... interface AggregatorV3Interface: def decimals() -> uint8: view def description() -> String[1000]: view def version() -> uint256: view def latestAnswer() -> int256: view # Constants & Immutables MINIMUM_USD: public(constant(uint256)) = as_wei(value=5, "ether") PRICE_FEED: public(immutable(AggregatorV3Interface)) = 0x6944AA1769357215DE4FAC081f113094dC325306 sep0lia # ABI Address OWNER: public(immutable(address)) PRECISION: constant(uint256) = 1 * (10 ** 18) # Storage funders: public(DynArray[address, 1000]) funder_to_amount_funded: public(HashMap[address, uint256]) # With constants: 262, 853 @deploy def __init__(price_feed: address): PRICE_FEED = AggregatorV3Interface(price_feed) OWNER = msg.sender @external @payable def fund(): """Allows users to send $ to this contract Have a minimum $ amount to send """ usd_value_of_eth: uint256 = self.get_eth_to_usd_rate(msg.value) assert usd_value_of_eth >= MINIMUM_USD, "You must spend more ETH!!" self.funders.append(msg.sender) self.funder_to_amount_funded[msg.sender] += msg.value ``` In this code, we have: - A _decimals_ function which returns a uint8 value. - A _description_ function which returns a String value. - A _version_ function which returns a uint256 value. - A _latestAnswer_ function which returns an int256 value. - We have declared the constants _MINIMUM_USD_, _PRICE_FEED_, _OWNER_, and _PRECISION_. - We have declared the storage variables _funders_ and _funder_to_amount_funded_. - We have declared the initializer function **_init_**. - We have declared the _fund_ function which is payable and allows users to send ETH to the contract. The topics we've covered so far are: - Interfaces - Constants - Immutables - Storage - Initializers - Payable Decorators - Function Documentation We have also seen how to use `msg.sender`, `msg.value`, and `self` in our code. We will cover more advanced topics later in the course.