Viper by Example¶
Simple Open Auction¶
As an introductory example of a smart contract written in Viper, we will begin with a simple open auction contract. As we dive into the code, it is important to remember that all Viper syntax is valid Python3 syntax, however not all Python3 functionality is available in Viper.
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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 | # Open Auction
# Auction params
# Beneficiary recieves 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`.
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.
@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.
def auction_end():
# 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 recieves 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.
# Create a simple auction with `_bidding_time`
# seconds bidding time on behalf of the
# beneficiary address `_beneficiary`.
def __init__(_beneficiary: address, _bidding_time: timedelta):
self.beneficiary = _beneficiary
self.auction_start = block.timestamp
self.auction_end = self.auction_start + _bidding_time
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 Viper
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.
# Bid on the auction with the value sent
# together with this transaction.
# The value will only be refunded if the
# auction is not won.
@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
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 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.
# End the auction and send the highest bid
# to the beneficiary.
def auction_end():
# 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)
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 Viper. As we move on to exploring more complex examples, we will encounter more design patterns and features of the Viper 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!
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 | #Safe Remote Purchase (https://github.com/ethereum/solidity/blob/develop/docs/solidity-by-example.rst) ported to viper 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)
#
@payable
def __init__():
assert (msg.value % 2) == 0
self.value = msg.value / 2 #Seller initializes contract by posting a safety deposit of 2*value of the item up for sale
self.seller = msg.sender
self.unlocked = true
def abort():
assert self.unlocked #Is the contract still refundable
assert msg.sender == self.seller #Only seller can refund his deposit before any buyer purchases the item
selfdestruct(self.seller) #Refunds seller, deletes contract
@payable
def purchase():
assert self.unlocked #Contract still open (item still up for sale)?
assert msg.value == (2*self.value) #Is the deposit of correct value?
self.buyer = msg.sender
self.unlocked = false
def received():
assert not self.unlocked #Is the item already purchased and pending confirmation of buyer
assert msg.sender == self.buyer
send(self.buyer, self.value) #Return deposit (=value) to buyer
selfdestruct(self.seller) #Returns deposit (=2*value) and the purchase price (=value)
|
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.
@payable
def __init__():
assert (msg.value % 2) == 0
self.value = msg.value / 2 #Seller initializes contract by posting a safety deposit of 2*value of the item up for sale
self.seller = msg.sender
self.unlocked = true
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.
def abort():
assert self.unlocked #Is the contract still refundable
assert msg.sender == self.seller #Only seller can refund his deposit before any buyer purchases the item
selfdestruct(self.seller) #Refunds seller, deletes contract
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.
@payable
def purchase():
assert self.unlocked #Contract still open (item still up for sale)?
assert msg.value == (2*self.value) #Is the deposit of correct value?
self.buyer = msg.sender
self.unlocked = false
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
False 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.
def received():
assert not self.unlocked #Is the item already purchased and pending confirmation of buyer
assert msg.sender == self.buyer
send(self.buyer, self.value) #Return deposit (=value) to buyer
selfdestruct(self.seller) #Returns deposit (=2*value) and the purchase price (=value)
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 - 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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 | funders: {sender: address, value: wei_value}[num]
nextFunderIndex: num
beneficiary: address
deadline: timestamp
goal: wei_value
refundIndex: num
timelimit: timedelta
# Setup global variables
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
@payable
def participate():
assert block.timestamp < self.deadline
nfi = self.nextFunderIndex
self.funders[nfi] = {sender: msg.sender, value: msg.value}
self.nextFunderIndex = nfi + 1
# Enough money was raised! Send funds to the beneficiary
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)
def refund():
assert block.timestamp >= self.deadline and self.balance < self.goal
ind = 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.
funders: {sender: address, value: wei_value}[num]
nextFunderIndex: num
beneficiary: address
deadline: timestamp
goal: wei_value
refundIndex: num
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 num 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.
# Setup global variables
def __init__(_beneficiary: address, _goal: wei_value, _timelimit: timedelta):
self.beneficiary = _beneficiary
self.deadline = block.timestamp + _timelimit
self.timelimit = _timelimit
self.goal = _goal
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.
# Participate in this crowdfunding campaign
@payable
def participate():
assert block.timestamp < self.deadline
nfi = self.nextFunderIndex
self.funders[nfi] = {sender: msg.sender, value: msg.value}
self.nextFunderIndex = nfi + 1
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.
# Enough money was raised! Send funds to the beneficiary
def finalize():
assert block.timestamp >= self.deadline and self.balance >= self.goal
selfdestruct(self.beneficiary)
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
Viper 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.
# Not enough money was raised! Refund everyone (max 30 people at a time
# to avoid gas limit issues)
def refund():
assert block.timestamp >= self.deadline and self.balance < self.goal
ind = 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
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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 | # Voting with delegation.
# Information about voters
voters: public({
# weight is accumulated by delegation
weight: num,
# if true, that person already voted
voted: bool,
# person delegated to
delegate: address,
# index of the voted proposal
vote: num
}[address])
# This is a type for a list of proposals.
proposals: public({
# short name (up to 32 bytes)
name: bytes32,
# number of accumulated votes
vote_count: num
}[num])
voter_count: public(num)
chairperson: public(address)
# Setup global variables
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
}
# Give `voter` the right to vote on this ballot.
# May only be called by `chairperson`.
def give_right_to_vote(voter: address):
# Throws if sender is not chairpers
assert msg.sender == self.chairperson
# Throws if voter has already voted
assert not self.voters[voter].voted
# Throws if voters voting weight isn't 0
assert self.voters[voter].weight == 0
self.voters[voter].weight = 1
self.voter_count += 1
# Delegate your vote to the voter `to`.
def delegate(_to: address):
to = _to
# Throws if sender has already voted
assert not self.voters[msg.sender].voted
# Throws if sender tries to delegate their vote to themselves
assert not msg.sender == to
# loop can delegate votes up to the current voter count
for i in range(self.voter_count, self.voter_count+1):
if self.voters[to].delegate:
# Because there are not while loops, use recursion to forward the delegation
# self.delegate(self.voters[to].delegate)
assert self.voters[to].delegate != msg.sender
to = self.voters[to].delegate
self.voters[msg.sender].voted = True
self.voters[msg.sender].delegate = to
if self.voters[to].voted:
# If the delegate already voted,
# directly add to the number of votes
self.proposals[self.voters[to].vote].vote_count += self.voters[msg.sender].weight
else:
# If the delegate did not vote yet,
# add to her weight.
self.voters[to].weight += self.voters[msg.sender].weight
# Give your vote (including votes delegated to you)
# to proposal `proposals[proposal].name`.
def vote(proposal: num):
assert not self.voters[msg.sender].voted
self.voters[msg.sender].voted = True
self.voters[msg.sender].vote = proposal
# If `proposal` is out of the range of the array,
# this will throw automatically and revert all
# changes.
self.proposals[proposal].vote_count += self.voters[msg.sender].weight
# Computes the winning proposal taking all
# previous votes into account.
@constant
def winning_proposal() -> num:
winning_vote_count = 0
for i in range(5):
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
@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: num,
# if true, that person already voted
voted: bool,
# person delegated to
delegate: address,
# index of the voted proposal
vote: num
}[address])
# This is a type for a list of proposals.
proposals: public({
# short name (up to 32 bytes)
name: bytes32,
# number of accumulated votes
vote_count: num
}[num])
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 num 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.
# Setup global variables
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
}
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.
# Give `voter` the right to vote on this ballot.
# May only be called by `chairperson`.
def give_right_to_vote(voter: address):
# Throws if sender is not chairpers
assert msg.sender == self.chairperson
# Throws if voter has already voted
assert not self.voters[voter].voted
# Throws if voters voting weight isn't 0
assert self.voters[voter].weight == 0
self.voters[voter].weight = 1
self.voter_count += 1
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.
# Delegate your vote to the voter `to`.
def delegate(_to: address):
to = _to
# Throws if sender has already voted
assert not self.voters[msg.sender].voted
# Throws if sender tries to delegate their vote to themselves
assert not msg.sender == to
# loop can delegate votes up to the current voter count
for i in range(self.voter_count, self.voter_count+1):
if self.voters[to].delegate:
# Because there are not while loops, use recursion to forward the delegation
# self.delegate(self.voters[to].delegate)
assert self.voters[to].delegate != msg.sender
to = self.voters[to].delegate
self.voters[msg.sender].voted = True
self.voters[msg.sender].delegate = to
if self.voters[to].voted:
# If the delegate already voted,
# directly add to the number of votes
self.proposals[self.voters[to].vote].vote_count += self.voters[msg.sender].weight
else:
# If the delegate did not vote yet,
# add to her weight.
self.voters[to].weight += self.voters[msg.sender].weight
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.
# Give your vote (including votes delegated to you)
# to proposal `proposals[proposal].name`.
def vote(proposal: num):
assert not self.voters[msg.sender].voted
self.voters[msg.sender].voted = True
self.voters[msg.sender].vote = proposal
# If `proposal` is out of the range of the array,
# this will throw automatically and revert all
# changes.
self.proposals[proposal].vote_count += self.voters[msg.sender].weight
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.
# Computes the winning proposal taking all
# previous votes into account.
@constant
def winning_proposal() -> num:
winning_vote_count = 0
for i in range(5):
if self.proposals[i].vote_count > winning_vote_count:
winning_vote_count = self.proposals[i].vote_count
winning_proposal = i
return winning_proposal
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.
# Calls winning_proposal() function to get the index
# of the winner contained in the proposals array and then
# returns the name of the winner
@constant
def winner_name() -> bytes32:
return self.proposals[self.winning_proposal()].name
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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 | # Own shares of a company!
company: public(address)
total_shares: public(currency_value)
price: public(num (wei / currency))
# Store ledger of stockholder holdings
holdings: currency_value[address]
# Setup company
def __init__(_company: address, _total_shares: currency_value,
initial_price: num(wei / currency) ):
assert _total_shares > 0
assert initial_price > 0
self.company = _company
self.total_shares = _total_shares
self.price = initial_price
# Company holds all the shares at first, but can sell them all
self.holdings[self.company] = _total_shares
@constant
def stock_available() -> currency_value:
return self.holdings[self.company]
# Give value to company and get stock in return
@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 = msg.value / self.price # rounds down
# There are enough shares to buy
assert self.stock_available() >= buy_order
# Take the shares off the market and give to stockholder
self.holdings[self.company] -= buy_order
self.holdings[msg.sender] += buy_order
# So someone can find out how much they have
@constant
def get_holding(_stockholder: address) -> currency_value:
return self.holdings[_stockholder]
# The amount the company has on hand in cash
@constant
def cash() -> wei_value:
return self.balance
# Give stock back to company and get my money back!
def sell_stock(sell_order: currency_value):
assert sell_order > 0 # Otherwise, will fail at send() below
# Can only sell as much stock as you own
assert self.get_holding(msg.sender) >= sell_order
# 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)
# Transfer stock from one stockholder to another
# (Assumes the receiver is given some compensation, but not enforced)
def transfer_stock(receiver: address, transfer_order: currency_value):
assert transfer_order > 0 # AUDIT revealed this!
# Can only trade as much stock as you own
assert self.get_holding(msg.sender) >= transfer_order
# Debit sender's stock and add to receiver's address
self.holdings[msg.sender] -= transfer_order
self.holdings[receiver] += transfer_order
# Allows the company to pay someone for services rendered
def pay_bill(vendor: address, amount: wei_value):
# Only the company can pay people
assert msg.sender == self.company
# And only if there's enough to pay them with
assert self.cash() >= amount
# Pay the bill!
send(vendor, amount)
# The amount a company has raised in the stock offering
@constant
def debt() -> wei_value:
return (self.total_shares - self.holdings[self.company]) * self.price
# The balance sheet of the company
@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. As always, we begin by initiating our variables.
# Own shares of a company!
company: public(address)
total_shares: public(currency_value)
price: public(num (wei / currency))
# Store ledger of stockholder holdings
holdings: currency_value[address]
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.
# Setup company
def __init__(_company: address, _total_shares: currency_value,
initial_price: num(wei / currency) ):
assert _total_shares > 0
assert initial_price > 0
self.company = _company
self.total_shares = _total_shares
self.price = initial_price
# Company holds all the shares at first, but can sell them all
self.holdings[self.company] = _total_shares
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.
@constant
def stock_available() -> currency_value:
return self.holdings[self.company]
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 Viper is a statically typed
language, we see an arrow following the definition of the stock_available()
method, which simply represents the datatype 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.
# Give value to company and get stock in return
@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 = msg.value / self.price # rounds down
# There are enough shares to buy
assert self.stock_available() >= buy_order
# Take the shares off the market and give to stockholder
self.holdings[self.company] -= buy_order
self.holdings[msg.sender] += buy_order
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?
# So someone can find out how much they have
@constant
def get_holding(_stockholder: address) -> currency_value:
return self.holdings[_stockholder]
The get_holdings() is another @constant method that takes an address
and returns its corresponding stock holdings by keying into self.holdings.
# The amount the company has on hand in cash
@constant
def cash() -> wei_value:
return self.balance
To check the ether balance of the company, we can simply call the getter method
cash().
# Give stock back to company and get my money back!
def sell_stock(sell_order: currency_value):
assert sell_order > 0 # Otherwise, will fail at send() below
# Can only sell as much stock as you own
assert self.get_holding(msg.sender) >= sell_order
# 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)
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.
# Transfer stock from one stockholder to another
# (Assumes the receiver is given some compensation, but not enforced)
def transfer_stock(receiver: address, transfer_order: currency_value):
assert transfer_order > 0 # AUDIT revealed this!
# Can only trade as much stock as you own
assert self.get_holding(msg.sender) >= transfer_order
# Debit sender's stock and add to receiver's address
self.holdings[msg.sender] -= transfer_order
self.holdings[receiver] += transfer_order
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.
# Allows the company to pay someone for services rendered
def pay_bill(vendor: address, amount: wei_value):
# Only the company can pay people
assert msg.sender == self.company
# And only if there's enough to pay them with
assert self.cash() >= amount
# Pay the bill!
send(vendor, amount)
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.
# The amount a company has raised in the stock offering
@constant
def debt() -> wei_value:
return (self.total_shares - self.holdings[self.company]) * self.price
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.
# The balance sheet of the company
@constant
def worth() -> wei_value:
return self.cash() - self.debt()
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 Viper language has convinced you of its capabilities and readability in writing smart contracts.