msg-sender.md

Chapter 2: Msg.sender

Now that we have our mappings to keep track of which trainer owns which pokemon, we'll want to update the contract functions to use them.

In order to do this, we need to use something called msg.sender.

msg.sender

In Vyper, there are certain global variables that are available to all functions. One of these is msg.sender, which refers to the address of the person (or smart contract) who called the current function.

Note: In Vyper, function execution always needs to start with an external caller. A contract will just sit on the blockchain doing nothing until someone calls one of its functions. So there will always be a msg.sender.

Here's an example of using msg.sender and updating a mapping:

favoriteNumber: HashMap[address, uint256]

@external
def setMyNumber(myNumber: uint256):
    # Update our `favoriteNumber` mapping to store `myNumber` under `msg.sender`
    favoriteNumber[msg.sender] = myNumber

@view
@external
def whatIsMyNumber() -> uint256:
    # Retrieve the value stored in the sender's address
    # Will be `0` if the sender hasn't called `setMyNumber` yet
    return favoriteNumber[msg.sender]

In this trivial example, anyone could call setMyNumber and store a uint256 in our contract, which would be tied to their address. Then when they called whatIsMyNumber, they would be returned the uint256 that they stored.

Using msg.sender gives you the security of the Ethereum blockchain — the only way someone can modify someone else's data would be to steal the private key associated with their Ethereum address.

Put it to the test

We need to update the contract functions so that we can create a trainer using the Trainer struct and trainerList mapping. This function should also call _createPokemon function to create a pokemon and then use the trainerToPokemon mapping to assign the ownership of a pokemon to a trainer. Finally, we would increment the trainerPokemonCount.

1. Create an event, NewTrainerCreated with a single property: name (String[32]).

2. Create an external function named createTrainer which takes 2 parameters: trainerName (String[32]) and pokemonName (String[32]).

3. Inside the createTrainer body, call the _createPokemon by passing pokemonName as the _name parameter. This will return a Pokemon. Create a variable newPokemon of type Pokemon and assign its value as the Pokemon returned from _createPokemon.

4. Inside the createTrainer body, create a Trainer using Trainer struct with the name value as trainerName. Create a variable newTrainer of type Trainer and assign its value as the Trainer created.

5. Add the new trainer to the trainerList mapping by assigning msg.sender to newTrainer.

6. Add the new trainer and pokemon to the trainerToPokemon nested mapping by adding 2 keys: first key as msg.sender and second key as self.trainerPokemonCount[msg.sender] (which acts as a unique key for each pokemon of a trainer). The value assigned is newPokemon.

7. Increment the trainerPokemonCount for our trainer address msg.sender by 1.

8. Fire the NewTrainerCreated event with the parameter trainerName.

** Template **

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** Solution **

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** Previous Chapter Solution **

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