## Constructors in Vyper In this lesson, we'll discuss what a constructor is in Vyper. We'll focus on the mechanics of how the constructor function works and create a constructor to initialize our `favorites` smart contract. ### Constructors Explained A constructor is a special function that's called only once when a smart contract is deployed. The most common use for a constructor is to initialize state variables or to call other functions needed to set up the contract. ### Defining Constructors Let's set up a constructor in a Vyper contract. First, we'll create a state variable named `owner` which will be a public address: ```python owner: public(address) ``` To define a constructor, we start with the `@deploy` decorator, followed by the definition of a function named `__init__`. Inside the parentheses, we can add any arguments we want to pass to the constructor. For now, we'll leave it empty: ```python @deploy def __init__(): pass ``` ### Setting State Variables in Constructors One of the most common things we do inside a constructor is to set the `owner` of the contract. We can do this by setting the `owner` state variable to `msg.sender`. The `msg.sender` variable refers to the account that deployed the contract. ```python @deploy def __init__(): self.owner = msg.sender ``` ### Passing Parameters to Constructors Let's look at some examples of passing parameters to a constructor. First, we'll add a state variable named `name` that is a public string with a maximum length of 100 characters: ```python name: public(string[100]) ``` Now we will pass the parameter `name` to our constructor function: ```python @deploy def __init__(name: string[100]): self.owner = msg.sender ``` We will then use the `name` parameter to set the `name` state variable: ```python @deploy def __init__(name: string[100]): self.owner = msg.sender self.name = name ``` Now, let's create another state variable named `expiresAt` that will be a public `uint256`: ```python expiresAt: public(uint256) ``` We will now add a second parameter, `duration`, to our constructor function: ```python @deploy def __init__(name: string[100], duration: uint256): self.owner = msg.sender self.name = name ``` Finally, we will set the `expiresAt` variable equal to the current block timestamp plus the `duration`: ```python @deploy def __init__(name: string[100], duration: uint256): self.owner = msg.sender self.name = name self.expiresAt = block.timestamp + duration ``` Now, let's compile and deploy our contract. We'll go to the `Compile` tab. We will click the `Compile` button. We will go to the `Deploy & Run Transactions` tab. We will click on the `Deploy` button. We will then enter the `name`, which is `Vyper`, and the `duration`, which will be `10`. We'll click the `Transact` button. We can then check the state variables. Our `owner` address will be set to the current account. Our `name` will be set to `Vyper`. Our `expiresAt` will be set to the current block timestamp plus 10. ### Another Example Let's create a constructor for our `favorites` smart contract to initialize a `my_favorite_number` state variable. Our state variable `my_favorite_number` will be a public `uint256`. ```python my_favorite_number: public(uint256) ``` Now let's create a constructor: ```python @deploy def __init__(): self.my_favorite_number = 7 ``` To compile and deploy this contract, we will click the `Compile` button. We will then go to the `Deploy & Run Transactions` tab. We will click the `Deploy` button. We can then interact with our contract by clicking on the `my_favorite_number - call` button, which will return the value 7. Great work!