Vyper by Example

Simple Open Auction

As an introductory example of a smart contract written in Vyper, we will begin with a simple open auction contract. As we dive into the code, it is important to remember that all Vyper syntax is valid Python3 syntax, however not all Python3 functionality is available in Vyper.

In this contract, we will be looking at a simple open auction contract where participants can submit bids during a limited time period. When the auction period ends, a predetermined beneficiary will receive the amount of the highest bid.

# Open Auction

# Auction params
# Beneficiary receives money from the highest bidder
beneficiary: public(address)
auction_start: public(timestamp)
auction_end: public(timestamp)

# Current state of auction
highest_bidder: public(address)
highest_bid: public(wei_value)

# Set to true at the end, disallows any change
ended: public(bool)

# Create a simple auction with `_bidding_time`
# seconds bidding time on behalf of the
# beneficiary address `_beneficiary`.
@public
def __init__(_beneficiary: address, _bidding_time: timedelta):
    self.beneficiary = _beneficiary
    self.auction_start = block.timestamp
    self.auction_end = self.auction_start + _bidding_time

# Bid on the auction with the value sent
# together with this transaction.
# The value will only be refunded if the
# auction is not won.
@public
@payable
def bid():
    # Check if bidding period is over.
    assert block.timestamp < self.auction_end
    # Check if bid is high enough
    assert msg.value > self.highest_bid
    if not self.highest_bid == 0:
        # Sends money back to the previous highest bidder
        send(self.highest_bidder, self.highest_bid)
    self.highest_bidder = msg.sender
    self.highest_bid = msg.value


# End the auction and send the highest bid
# to the beneficiary.
@public
def end_auction():
    # It is a good guideline to structure functions that interact
    # with other contracts (i.e. they call functions or send Ether)
    # into three phases:
    # 1. checking conditions
    # 2. performing actions (potentially changing conditions)
    # 3. interacting with other contracts
    # If these phases are mixed up, the other contract could call
    # back into the current contract and modify the state or cause
    # effects (Ether payout) to be performed multiple times.
    # If functions called internally include interaction with external
    # contracts, they also have to be considered interaction with
    # external contracts.

    # 1. Conditions
    # Check if auction endtime has been reached
    assert block.timestamp >= self.auction_end
    # Check if this function has already been called
    assert not self.ended

    # 2. Effects
    self.ended = True

    # 3. Interaction
    send(self.beneficiary, self.highest_bid)

As you can see, this example only has a constructor, two methods to call, and a few variables to manage the contract state. Believe it or not, this is all we need for a basic implementation of an auction smart contract.

Let’s get started!

# Auction params
# Beneficiary receives money from the highest bidder
beneficiary: public(address)
auction_start: public(timestamp)
auction_end: public(timestamp)

# Current state of auction
highest_bidder: public(address)
highest_bid: public(wei_value)

# Set to true at the end, disallows any change
ended: public(bool)

We begin by declaring a few variables to keep track of our contract state. We initialize a global variable beneficiary by calling public on the datatype address. The beneficiary will be the receiver of money from the highest bidder. We also initialize the variables auction_start and auction_end with the datatype timestamp to manage the open auction period and highest_bid with datatype wei_value, the smallest denomination of ether, to manage auction state. The variable ended is a boolean to determine whether the auction is officially over.

You may notice all of the variables being passed into the public function. By declaring the variable public, the variable is callable by external contracts. Initializing the variables without the public function defaults to a private declaration and thus only accessible to methods within the same contract. The public function additionally creates a ‘getter’ function for the variable, accessible with a call such as self.get_beneficiary(some_address).

Now, the constructor.

The contract is initialized with two arguments: _beneficiary of type address and _bidding_time with type timedelta, the time difference between the start and end of the auction. We then store these two pieces of information into the contract variables self.beneficiary and self.auction_end. Notice that we have access to the current time by calling block.timestamp. block is an object available within any Vyper contract and provides information about the block at the time of calling. Similar to block, another important object available to us within the contract is msg, which provides information on the method caller as we will soon see.

With initial setup out of the way, lets look at how our users can make bids.

The @payable decorator will allow a user to send some ether to the contract in order to call the decorated method. In this case, a user wanting to make a bid would call the bid() method while sending an amount equal to their desired bid (not including gas fees). When calling any method within a contract, we are provided with a built-in variable msg and we can access the public address of any method caller with msg.sender. Similarly, the amount of ether a user sends can be accessed by calling msg.value.

Warning

msg.sender will change between internal function calls so that if you’re calling a function from the outside, it’s correct for the first function call. But then, for the function calls after, msg.sender will reference the contract itself as opposed to the sender of the transaction.

Here, we first check whether the current time is before the auction’s end time using the assert function which takes any boolean statement. We also check to see if the new bid is greater than the highest bid. If the two assert statements pass, we can safely continue to the next lines; otherwise, the bid() method will throw an error and revert the transaction. If the two assert statements and the check that the previous bid is not equal to zero pass, we can safely conclude that we have a valid new highest bid. We will send back the previous highest_bid to the previous highest_bidder and set our new highest_bid and highest_bidder.

With the auction_end() method, we check whether our current time is past the auction_end time we set upon initialization of the contract. We also check that self.ended had not previously been set to True. We do this to prevent any calls to the method if the auction had already ended, which could potentially be malicious if the check had not been made. We then officially end the auction by setting self.ended to True and sending the highest bid amount to the beneficiary.

And there you have it - an open auction contract. Of course, this is a simplified example with barebones functionality and can be improved. Hopefully, this has provided some insight to the possibilities of Vyper. As we move on to exploring more complex examples, we will encounter more design patterns and features of the Vyper language.

And of course, no smart contract tutorial is complete without a note on security.

Note

It’s always important to keep security in mind when designing a smart contract. As any application becomes more complex, the greater the potential for introducing new risks. Thus, it’s always good practice to keep contracts as readable and simple as possible.

Whenever you’re ready, let’s turn it up a notch in the next example.

Safe Remote Purchases

In this example, we have an escrow contract implementing a system for a trustless transaction between a buyer and a seller. In this system, a seller posts an item for sale and makes a deposit to the contract of twice the item’s value. At this moment, the contract has a balance of 2 * value. The seller can reclaim the deposit and close the sale as long as a buyer has not yet made a purchase. If a buyer is interested in making a purchase, they would make a payment and submit an equal amount for deposit (totaling 2 * value) into the contract and locking the contract from further modification. At this moment, the contract has a balance of 4 * value and the seller would send the item to buyer. Upon the buyer’s receipt of the item, the buyer will mark the item as received in the contract, thereby returning the buyer’s deposit (not payment), releasing the remaining funds to the seller, and completing the transaction.

There are certainly others ways of designing a secure escrow system with less overhead for both the buyer and seller, but for the purpose of this example, we want to explore one way how an escrow system can be implemented trustlessly.

Let’s go!

#Safe Remote Purchase
#Originally from
#https://github.com/ethereum/solidity/blob/develop/docs/solidity-by-example.rst
#ported to vyper and optimized

#Rundown of the transaction:
#1. Seller posts item for sale and posts safety deposit of double the item value.
# Balance is 2*value.
#(1.1. Seller can reclaim deposit and close the sale as long as nothing was purchased.)
#2. Buyer purchases item (value) plus posts an additional safety deposit (Item value).
# Balance is 4*value.
#3. Seller ships item.
#4. Buyer confirms receiving the item. Buyer's deposit (value) is returned.
#Seller's deposit (2*value) + items value is returned. Balance is 0.

value: public(wei_value) #Value of the item
seller: public(address)
buyer: public(address)
unlocked: public(bool)
#@constant
#def unlocked() -> bool: #Is a refund possible for the seller?
#    return (self.balance == self.value*2)

@public
@payable
def __init__():
    assert (msg.value % 2) == 0
    self.value = msg.value / 2  #The seller initializes the contract by
        #posting a safety deposit of 2*value of the item up for sale.
    self.seller = msg.sender
    self.unlocked = True

@public
def abort():
    assert self.unlocked #Is the contract still refundable?
    assert msg.sender == self.seller #Only the seller can refund
        # his deposit before any buyer purchases the item.
    selfdestruct(self.seller) #Refunds the seller and deletes the contract.

@public
@payable
def purchase():
    assert self.unlocked #Is the contract still open (is the item still up for sale)?
    assert msg.value == (2 * self.value) #Is the deposit the correct value?
    self.buyer = msg.sender
    self.unlocked = False

@public
def received():
    assert not self.unlocked #Is the item already purchased and pending confirmation
        # from the buyer?
    assert msg.sender == self.buyer
    send(self.buyer, self.value) #Return the buyer's deposit (=value) to the buyer.
    selfdestruct(self.seller) #Return the seller's deposit (=2*value)
        # and the purchase price (=value) to the seller.

This is also a moderately short contract, however a little more complex in logic. Let’s break down this contract bit by bit.

value: public(wei_value) #Value of the item
seller: public(address)
buyer: public(address)
unlocked: public(bool)

Like the other contracts, we begin by declaring our global variables public with their respective datatypes. Remember that the public function allows the variables to be readable by an external caller, but not writeable.

With a @payable decorator on the constructor, the contract creator will be required to make an initial deposit equal to twice the item’s value to initialize the contract, which will be later returned. This is in addition to the gas fees needed to deploy the contract on the blockchain, which is not returned. We assert that the deposit is divisible by 2 to ensure that the seller deposited a valid amount. The constructor stores the item’s value in the contract variable self.value and saves the contract creator into self.seller. The contract variable self.unlocked is initialized to True.

The abort() method is a method only callable by the seller and while the contract is still unlocked—meaning it is callable only prior to any buyer making a purchase. As we will see in the purchase() method that when a buyer calls the purchase() method and sends a valid amount to the contract, the contract will be locked and the seller will no longer be able to call abort().

When the seller calls abort() and if the assert statements pass, the contract will call the selfdestruct() function and refunds the seller and subsequently destroys the contract.

Like the constructor, the purchase() method has a @payable decorator, meaning it can be called with a payment. For the buyer to make a valid purchase, we must first assert that the contract’s unlocked property is True and that the amount sent is equal to twice the item’s value. We then set the buyer to the msg.sender and lock the contract. At this point, the contract has a balance equal to 4 times the item value and the seller must send the item to the buyer.

Finally, upon the buyer’s receipt of the item, the buyer can confirm their receipt by calling the received() method to distribute the funds as intended—where the seller receives 3/4 of the contract balance and the buyer receives 1/4.

By calling received(), we begin by checking that the contract is indeed locked, ensuring that a buyer had previously paid. We also ensure that this method is only callable by the buyer. If these two assert statements pass, we refund the buyer their initial deposit and send the seller the remaining funds. The contract is finally destroyed and the transaction is complete.

Whenever we’re ready, let’s move on to the next example.

Crowdfund

Now, let’s explore a straightforward example for a crowdfunding contract where prospective participants can contribute funds to a campaign. If the total contribution to the campaign reaches or surpasses a predetermined funding goal, the funds will be sent to the beneficiary at the end of the campaign deadline. Participants will be refunded their respective contributions if the total funding does not reach its target goal.

#Setup private variables (only callable from within the contract)
funders: {sender: address, value: wei_value}[int128]
nextFunderIndex: int128
beneficiary: address
deadline: timestamp
goal: wei_value
refundIndex: int128
timelimit: timedelta

# Setup global variables
@public
def __init__(_beneficiary: address, _goal: wei_value, _timelimit: timedelta):
    self.beneficiary = _beneficiary
    self.deadline = block.timestamp + _timelimit
    self.timelimit = _timelimit
    self.goal = _goal

# Participate in this crowdfunding campaign
@public
@payable
def participate():
    assert block.timestamp < self.deadline
    nfi: int128 = self.nextFunderIndex
    self.funders[nfi] = {sender: msg.sender, value: msg.value}
    self.nextFunderIndex = nfi + 1

# Enough money was raised! Send funds to the beneficiary
@public
def finalize():
    assert block.timestamp >= self.deadline and self.balance >= self.goal
    selfdestruct(self.beneficiary)

# Not enough money was raised! Refund everyone (max 30 people at a time
# to avoid gas limit issues)
@public
def refund():
    assert block.timestamp >= self.deadline and self.balance < self.goal
    ind: int128 = self.refundIndex
    for i in range(ind, ind + 30):
        if i >= self.nextFunderIndex:
            self.refundIndex = self.nextFunderIndex
            return
        send(self.funders[i].sender, self.funders[i].value)
        self.funders[i] = None
    self.refundIndex = ind + 30

Most of this code should be relatively straightforward after going through our previous examples. Let’s dive right in.

#Setup private variables (only callable from within the contract)
funders: {sender: address, value: wei_value}[int128]
nextFunderIndex: int128
beneficiary: address
deadline: timestamp
goal: wei_value
refundIndex: int128
timelimit: timedelta

Like other examples, we begin by initiating our variables - except this time, we’re not calling them with the public function. Variables initiated this way are, by default, private.

..note :: Unlike the existence of the function public(), there is no equivalent private() function. Variables simply default to private if initiated without the public() function.

The funders variable is initiated as a mapping where the key is a number, and the value is a struct representing the contribution of each participant. This struct contains each participant’s public address and their respective value contributed to the fund. The key corresponding to each struct in the mapping will be represented by the variable nextFunderIndex which is incremented with each additional contributing participant. Variables initialized with the int128 type without an explicit value, such as nextFunderIndex, defaults to 0. The beneficiary will be the final receiver of the funds once the crowdfunding period is over—as determined by the deadline and timelimit variables. The goal variable is the target total contribution of all participants. refundIndex is a variable for bookkeeping purposes in order to avoid gas limit issues in the scenario of a refund.

Our constructor function takes 3 arguments: the beneficiary’s address, the goal in wei value, and the difference in time from start to finish of the crowdfunding. We initialize the arguments as contract variables with their corresponding names. Additionally, a self.deadline is initialized to set a definitive end time for the crowdfunding period.

Now lets take a look at how a person can participate in the crowdfund.

Once again, we see the @payable decorator on a method, which allows a person to send some ether along with a call to the method. In this case, the participate() method accesses the sender’s address with msg.sender and the corresponding amount sent with msg.value. This information is stored into a struct and then saved into the funders mapping with self.nextFunderIndex as the key. As more participants are added to the mapping, self.nextFunderIndex increments appropriately to properly index each participant.

The finalize() method is used to complete the crowdfunding process. However, to complete the crowdfunding, the method first checks to see if the crowdfunding period is over and that the balance has reached/passed its set goal. If those two conditions pass, the contract calls the selfdestruct() function and sends the collected funds to the beneficiary.

Note

Notice that we have access to the total amount sent to the contract by calling self.balance, a variable we never explicitly set. Similar to msg and block, self.balance is a built-in variable thats available in all Vyper contracts.

We can finalize the campaign if all goes well, but what happens if the crowdfunding campaign isn’t successful? We’re going to need a way to refund all the participants.

In the refund() method, we first check that the crowdfunding period is indeed over and that the total collected balance is less than the goal with the assert statement . If those two conditions pass, we then loop through every participant and call send() to send each participant their respective contribution. For the sake of gas limits, we group the number of contributors in batches of 30 and refund them one at a time. Unfortunately, if there’s a large number of of participants, multiple calls to refund() may be necessary.

Voting

In this contract, we will implement a system for participants to vote on a list of proposals. The chairperson of the contract will be able to give each participant the right to vote, and each participant may choose to vote, or delegate their vote to another voter. Finally, a winning proposal will be determined upon calling the winning_proposals() method, which iterates through all the proposals and returns the one with the greatest number of votes.

# Voting with delegation.

# Information about voters
voters: public({
    # weight is accumulated by delegation
    weight: int128,
    # if true, that person already voted (which includes voting by delegating)
    voted: bool,
    # person delegated to
    delegate: address,
    # index of the voted proposal, which is not meaningful unless `voted` is True.
    vote: int128
}[address])

# This is a type for a list of proposals.
proposals: public({
    # short name (up to 32 bytes)
    name: bytes32,
    # int128ber of accumulated votes
    vote_count: int128
}[int128])

voter_count: public(int128)
chairperson: public(address)
int128_proposals: public(int128)

@public
@constant
def delegated(addr: address) -> bool:
    # equivalent to
        # self.voters[addr].delegate != 0x0000000000000000000000000000000000000000
    return not not self.voters[addr].delegate

@public
@constant
def directly_voted(addr: address) -> bool:
    # not <address> equivalent to
        # <address> == 0x0000000000000000000000000000000000000000
    return self.voters[addr].voted and not self.voters[addr].delegate

# Setup global variables
@public
def __init__(_proposalNames: bytes32[2]):
    self.chairperson = msg.sender
    self.voter_count = 0
    for i in range(2):
        self.proposals[i] = {
            name: _proposalNames[i],
            vote_count: 0
        }
        self.int128_proposals += 1

# Give a `voter` the right to vote on this ballot.
# This may only be called by the `chairperson`.
@public
def give_right_to_vote(voter: address):
    # Throws if the sender is not the chairperson.
    assert msg.sender == self.chairperson
    # Throws if the voter has already voted.
    assert not self.voters[voter].voted
    # Throws if the voter's voting weight isn't 0.
    assert self.voters[voter].weight == 0
    self.voters[voter].weight = 1
    self.voter_count += 1

# Used by `delegate` below, and can be called by anyone.
@public
def forward_weight(delegate_with_weight_to_forward: address):
    assert self.delegated(delegate_with_weight_to_forward)
    # Throw if there is nothing to do:
    assert self.voters[delegate_with_weight_to_forward].weight > 0

    target: address = self.voters[delegate_with_weight_to_forward].delegate
    for i in range(4):
        if self.delegated(target):
            target = self.voters[target].delegate
            # The following effectively detects cycles of length <= 5,
            # in which the delegation is given back to the delegator.
            # This could be done for any int128ber of loops,
            # or even infinitely with a while loop.
            # However, cycles aren't actually problematic for correctness;
            # they just result in spoiled votes.
            # So, in the production version, this should instead be
            # the responsibility of the contract's client, and this
            # check should be removed.
            assert target != delegate_with_weight_to_forward
        else:
            # Weight will be moved to someone who directly voted or
            # hasn't voted.
            break

    weight_to_forward: int128 = self.voters[delegate_with_weight_to_forward].weight
    self.voters[delegate_with_weight_to_forward].weight = 0
    self.voters[target].weight += weight_to_forward

    if self.directly_voted(target):
        self.proposals[self.voters[target].vote].vote_count += weight_to_forward
        self.voters[target].weight = 0

    # To reiterate: if target is also a delegate, this function will need
    # to be called again, similarly to as above.

# Delegate your vote to the voter `to`.
@public
def delegate(to: address):
    # Throws if the sender has already voted
    assert not self.voters[msg.sender].voted
    # Throws if the sender tries to delegate their vote to themselves or to
    # the default address value of 0x0000000000000000000000000000000000000000
    # (the latter might not be problematic, but I don't want to think about it).
    assert to != msg.sender and not not to

    self.voters[msg.sender].voted = True
    self.voters[msg.sender].delegate = to

    # This call will throw if and only if this delegation would cause a loop
        # of length <= 5 that ends up delegating back to the delegator.
    self.forward_weight(msg.sender)

# Give your vote (including votes delegated to you)
# to proposal `proposals[proposal].name`.
@public
def vote(proposal: int128):
    # can't vote twice
    assert not self.voters[msg.sender].voted
    # can only vote on legitimate proposals
    assert proposal < self.int128_proposals

    self.voters[msg.sender].vote = proposal
    self.voters[msg.sender].voted = True

    # transfer msg.sender's weight to proposal
    self.proposals[proposal].vote_count += self.voters[msg.sender].weight
    self.voters[msg.sender].weight = 0

# Computes the winning proposal taking all
# previous votes into account.
@public
@constant
def winning_proposal() -> int128:
    winning_vote_count: int128 = 0
    winning_proposal: int128 = 0
    for i in range(2):
        if self.proposals[i].vote_count > winning_vote_count:
            winning_vote_count = self.proposals[i].vote_count
            winning_proposal = i
    return winning_proposal

# Calls winning_proposal() function to get the index
# of the winner contained in the proposals array and then
# returns the name of the winner
@public
@constant
def winner_name() -> bytes32:
    return self.proposals[self.winning_proposal()].name

As we can see, this is contract of moderate length which we will dissect section by section. Let’s begin!

# Information about voters
voters: public({
    # weight is accumulated by delegation
    weight: int128,
    # if true, that person already voted (which includes voting by delegating)
    voted: bool,
    # person delegated to
    delegate: address,
    # index of the voted proposal, which is not meaningful unless `voted` is True.
    vote: int128
}[address])

# This is a type for a list of proposals.
proposals: public({
    # short name (up to 32 bytes)
    name: bytes32,
    # int128ber of accumulated votes
    vote_count: int128
}[int128])

voter_count: public(int128)
chairperson: public(address)
int128_proposals: public(int128)

The variable voters is initialized as a mapping where the key is the voter’s public address and the value is a struct describing the voter’s properties: weight, voted, delegate, and vote, along with their respective datatypes.

Similarly, the proposals variable is initialized as a public mapping with int128 as the key’s datatype and a struct to represent each proposal with the properties name and vote_count. Like our last example, we can access any value by key’ing into the mapping with a number just as one would with an index in an array.

Then, voter_count and chairperson are initialized as public with their respective datatypes.

Let’s move onto the constructor.

Warning

Both msg.sender and msg.balance change between internal function calls so that if you’re calling a function from the outside, it’s correct for the first function call. But then, for the function calls after, msg.sender and msg.balance reference the contract itself as opposed to the sender of the transaction.

In the constructor, we hard-coded the contract to accept an array argument of exactly two proposal names of type bytes32 for the contracts initialization. Because upon initialization, the __init__() method is called by the contract creator, we have access to the contract creator’s address with msg.sender and store it in the contract variable self.chairperson. We also initialize the contract variable self.voter_count to zero to initially represent the number of votes allowed. This value will be incremented as each participant in the contract is given the right to vote by the method give_right_to_vote(), which we will explore next. We loop through the two proposals from the argument and insert them into proposals mapping with their respective index in the original array as its key.

Now that the initial setup is done, lets take a look at the functionality.

We need a way to control who has the ability to vote. The method give_right_to_vote() is a method callable by only the chairperson by taking a voter address and granting it the right to vote by incrementing the voter’s weight property. We sequentially check for 3 conditions using assert. The assert not function will check for falsy boolean values - in this case, we want to know that the voter has not already voted. To represent voting power, we will set their weight to 1 and we will keep track of the total number of voters by incrementing voter_count.

In the method delegate, firstly, we check to see that msg.sender has not already voted and secondly, that the target delegate and the msg.sender are not the same. Voters shouldn’t be able to delegate votes to themselves. We, then, loop through all the voters to determine whether the person delegate to had further delegated their vote to someone else in order to follow the chain of delegation. We then mark the msg.sender as having voted if they delegated their vote. We increment the proposal’s vote_count directly if the delegate had already voted or increase the delegate’s vote weight if the delegate has not yet voted.

Now, let’s take a look at the logic inside the vote() method, which is surprisingly simple. The method takes the key of the proposal in the proposals mapping as an argument, check that the method caller had not already voted, sets the voter’s vote property to the proposal key, and increments the proposals vote_count by the voter’s weight.

With all the basic functionality complete, what’s left is simply returning the winning proposal. To do this, we have two methods: winning_proposal(), which returns the key of the proposal, and winner_name(), returning the name of the proposal. Notice the @constant decorator on these two methods. We do this because the two methods only read the blockchain state and do not modify it. Remember, reading the blockchain state is free; modifying the state costs gas. By having the @constant decorator, we let the EVM know that this is a read-only function and we benefit by saving gas fees.

The winning_proposal() method returns the key of proposal in the proposals mapping. We will keep track of greatest number of votes and the winning proposal with the variables winning_vote_count and winning_proposal, respectively by looping through all the proposals.

And finally, the winner_name() method returns the name of the proposal by key’ing into the proposals mapping with the return result of the winning_proposal() method.

And there you have it - a voting contract. Currently, many transactions are needed to assign the rights to vote to all participants. As an exercise, can we try to optimize this?

Now that we’re familiar with basic contracts. Let’s step up the difficulty.

Company Stock

This contract is just a tad bit more thorough than the ones we’ve previously encountered. In this example, we are going to look at a comprehensive contract that manages the holdings of all shares of a company. The contract allows for a person to buy, sell, and transfer shares of a company as well as allowing for the company to pay a person in ether. The company, upon initialization of the contract, holds all shares of the company at first but can sell them all.

Let’s get started.

units: {
    currency_value: "Currency Value"
}

# Financial events the contract logs
Transfer: event({_from: indexed(address), _to: indexed(address), _value: uint256(currency_value)})
Buy: event({_buyer: indexed(address), _buy_order: uint256(currency_value)})
Sell: event({_seller: indexed(address), _sell_order: uint256(currency_value)})
Pay: event({_vendor: indexed(address), _amount: wei_value})

# Initiate the variables for the company and it's own shares.
company: public(address)
total_shares: public(uint256(currency_value))
price: public(uint256 (wei / currency_value))

# Store a ledger of stockholder holdings.
holdings: uint256(currency_value)[address]

# Set up the company.
@public
def __init__(_company: address, _total_shares: uint256(currency_value),
        initial_price: uint256(wei / currency_value) ):
    assert _total_shares > 0
    assert initial_price > 0

    self.company = _company
    self.total_shares = _total_shares
    self.price = initial_price

    # The company holds all the shares at first, but can sell them all.
    self.holdings[self.company] = _total_shares

@public
@constant
def stock_available() -> uint256(currency_value):
    return self.holdings[self.company]

# Give some value to the company and get stock in return.
@public
@payable
def buy_stock():
    # Note: full amount is given to company (no fractional shares),
    #       so be sure to send exact amount to buy shares
    buy_order: uint256(currency_value) = msg.value / self.price # rounds down

    # Check that there are enough shares to buy.
    assert self.stock_available() >= buy_order

    # Take the shares off the market and give them to the stockholder.
    self.holdings[self.company] -= buy_order
    self.holdings[msg.sender] += buy_order

    # Log the buy event.
    log.Buy(msg.sender, buy_order)

# Find out how much stock any address (that's owned by someone) has.
@public
@constant
def get_holding(_stockholder: address) -> uint256(currency_value):
    return self.holdings[_stockholder]

# Return the amount the company has on hand in cash.
@public
@constant
def cash() -> wei_value:
    return self.balance

# Give stock back to the company and get money back as ETH.
@public
def sell_stock(sell_order: uint256(currency_value)):
    assert sell_order > 0 # Otherwise, this would fail at send() below,
        # due to an OOG error (there would be zero value available for gas).
    # You can only sell as much stock as you own.
    assert self.get_holding(msg.sender) >= sell_order
    # Check that the company can pay you.
    assert self.cash() >= (sell_order * self.price)

    # Sell the stock, send the proceeds to the user
    # and put the stock back on the market.
    self.holdings[msg.sender] -= sell_order
    self.holdings[self.company] += sell_order
    send(msg.sender, sell_order * self.price)

    # Log the sell event.
    log.Sell(msg.sender, sell_order)

# Transfer stock from one stockholder to another. (Assume that the
# receiver is given some compensation, but this is not enforced.)
@public
def transfer_stock(receiver: address, transfer_order: uint256(currency_value)):
    assert transfer_order > 0 # This is similar to sell_stock above.
    # Similarly, you can only trade as much stock as you own.
    assert self.get_holding(msg.sender) >= transfer_order

    # Debit the sender's stock and add to the receiver's address.
    self.holdings[msg.sender] -= transfer_order
    self.holdings[receiver] += transfer_order

    # Log the transfer event.
    log.Transfer(msg.sender, receiver, transfer_order)

# Allow the company to pay someone for services rendered.
@public
def pay_bill(vendor: address, amount: wei_value):
    # Only the company can pay people.
    assert msg.sender == self.company
    # Also, it can pay only if there's enough to pay them with.
    assert self.cash() >= amount

    # Pay the bill!
    send(vendor, amount)

    # Log the payment event.
    log.Pay(vendor, amount)

# Return the amount in wei that a company has raised in stock offerings.
@public
@constant
def debt() -> wei_value:
    return (self.total_shares - self.holdings[self.company]) * self.price

# Return the cash holdings minus the debt of the company.
# The share debt or liability only is included here,
# but of course all other liabilities can be included.
@public
@constant
def worth() -> wei_value:
    return self.cash() - self.debt()

The contract contains a number of methods that modify the contract state as well as a few ‘getter’ methods to read it. We first declare several events that the contract logs. We then declare our global variables, followed by function defintions.

Buy: event({_buyer: indexed(address), _buy_order: uint256(currency_value)})
Sell: event({_seller: indexed(address), _sell_order: uint256(currency_value)})
Pay: event({_vendor: indexed(address), _amount: wei_value})

# Initiate the variables for the company and it's own shares.
company: public(address)
total_shares: public(uint256(currency_value))

We initiate the company variable to be of type address that’s public. The total_shares variable is of type currency_value, which in this case represents the total available shares of the company. The price variable represents the wei value of a share and holdings is a mapping that maps an address to the number of shares the address owns.

In the constructor, we set up the contract to check for valid inputs during the initialization of the contract via the two assert statements. If the inputs are valid, the contract variables are set accordingly and the company’s address is initialized to hold all shares of the company in the holdings mapping.

We will be seeing a few @constant decorators in this contract—which is used to decorate methods that simply read the contract state or return a simple calculation on the contract state without modifying it. Remember, reading the blockchain is free, writing on it is not. Since Vyper is a statically typed language, we see an arrow following the definition of the stock_available() method, which simply represents the data type which the function is expected to return. In the method, we simply key into self.holdings with the company’s address and check it’s holdings.

Now, lets take a look at a method that lets a person buy stock from the company’s holding.

The buy_stock() method is a @payable method which takes an amount of ether sent and calculates the buy_order (the stock value equivalence at the time of call). The number of shares is deducted from the company’s holdings and transferred to the sender’s in the holdings mapping.

Now that people can buy shares, how do we check someone’s holdings?

The get_holdings() is another @constant method that takes an address and returns its corresponding stock holdings by keying into self.holdings.

To check the ether balance of the company, we can simply call the getter method cash().

To sell a stock, we have the sell_stock() method which takes a number of stocks a person wishes to sell, and sends the equivalent value in ether to the seller’s address. We first assert that the number of stocks the person wishes to sell is a value greater than 0. We also assert to see that the user can only sell as much as the user owns and that the company has enough ether to complete the sale. If all conditions are met, the holdings are deducted from the seller and given to the company. The ethers are then sent to the seller.

A stockholder can also transfer their stock to another stockholder with the transfer_stock() method. The method takes a receiver address and the number of shares to send. It first asserts that the amount being sent is greater than 0 and asserts whether the sender has enough stocks to send. If both conditions are satisfied, the transfer is made.

The company is also allowed to pay out an amount in ether to an address by calling the pay_bill() method. This method should only be callable by the company and thus first checks whether the method caller’s address matches that of the company. Another important condition to check is that the company has enough funds to pay the amount. If both conditions satisfy, the contract sends its ether to an address.

We can also check how much the company has raised by multiplying the number of shares the company has sold and the price of each share. We can get this value by calling the debt() method.

Finally, in this worth() method, we can check the worth of a company by subtracting its debt from its ether balance.

This contract has been the most thorough example so far in terms of its functionality and features. Yet despite the thoroughness of such a contract, the logic remained simple. Hopefully, by now, the Vyper language has convinced you of its capabilities and readability in writing smart contracts.