Compiling and deploying a smart contract with geth and Python

In our last post, we have been cheating a bit – I have shown you how to use the web3 Python library to access an existing smart contract, but in order to compile and deploy, we have still been relying on Brownie. Time to learn how this can be done with web3 and the Python-Solidity compiler interface as well. Today, we will also use the Go-Ethereum client for the first time. This will be a short post and the last one about development tools before we then turn our attention to token standards.

Preparations

To follow this post, there is again a couple of preparational steps. If you have read my previous posts, you might already have completed some of them, but I have decided to list them here once more, in case you are just joining us or start over with a fresh setup. First, you will have to install the web3 library (unless, of course, you have already done this before).

sudo apt-get install python3-pip python3-dev gcc
pip3 install web3

The next step is to install the Go-Ethereum (geth) client. As the client is written in Go, it comes as a single binary file, which you can simply extract from the distribution archive (which also contains the license) and copy to a location on your path. As we have already put the Brownie binary into .local/bin, I have decided to go with this as well.

cd /tmp
wget https://gethstore.blob.core.windows.net/builds/geth-linux-amd64-1.10.6-576681f2.tar.gz
gzip -d geth-linux-amd64-1.10.6-576681f2.tar.gz
tar -xvf  geth-linux-amd64-1.10.6-576681f2.tar
cp geth-linux-amd64-1.10.6-576681f2/geth ~/.local/bin/
chmod 700 ~/.local/bin/geth
export PATH=$PATH:$HOME/.local/bin

Once this has been done, it is time to start the client. We will talk more about the various options and switches in a later post, when we will actually use the client to connect to the Rinkeby testnet. For today, you can use the following command to start geth in development mode.

geth --dev --datadir=~/.ethereum --http

In this mode, geth will be listening on port 8545 of your local PC and bring up local, single-node blockchain, quite similar to Ganache. New blocks will automatically be mined as needed, regardless of the gas price of your transactions, and one account will be created which is unlocked and at the same time the beneficiary of newly mined blocks (so do not worry, you have plenty of Ether at your disposal).

Compiling the contract

Next, we need to compile the contract. Of course, this comes down to running the Solidity compiler, so we could go ahead, download the compiler and run it. To do this with Python, we could of course invoke the compiler as a subprocess and collect its output, thus effectively wrapping the compiler into a Python class. Fortunately, someone else has already done all of the hard work and created such a wrapper – the py-solc-x library (a fork of a previous library called py-solc). To install it and to instruct it to download a specific version of the compiler, run the following commands (this will install the compiler in ~/.solcx)

pip3 install py-solc-x
python3 -m solcx.install v0.8.6
~/.solcx/solc-v0.8.6 --version

If the last command spits out the correct version, the binary is installed and we are ready to use it. Let us try this out. Of course, we need a contract – we will use the Counter contract from the previous posts again. So go ahead, grab a copy of my repository and bring up an interactive Python session.

git clone https://github.com/christianb93/nft-bootcamp
cd nft-bootcamp
iypthon3

How do we actually use solcx? The wrapper offers a few functions to invoke the Solidity compiler. We will use the so-called JSON input-output interface. With this approach, we need to feed a JSON structure into the compiler, which contains information like the code we want to compile and the output we want the compiler to produce, and the compiler will spit out a similar structure containing the results. The solcx package offers a function compile_standard which wraps this interface. So we need to prepare the input (consult the Solidity documentation to better understand what the individual fields mean), call the wrapper and collect the output.

import solcx
source = "contracts/Counter.sol"
file = "Counter.sol"
spec = {
        "language": "Solidity",
        "sources": {
            file: {
                "urls": [
                    source
                ]
            }
        },
        "settings": {
            "optimizer": {
               "enabled": True
            },
            "outputSelection": {
                "*": {
                    "*": [
                        "metadata", "evm.bytecode", "abi"
                    ]
                }
            }
        }
    };
out = solcx.compile_standard(spec, allow_paths=".");

The output is actually a rather complex data structures. It is a dictionary that contains the contracts created as result of the compilation as well as a reference to the source code. The contracts are again structured by source file and contract name. For each contract, we have the ABI, a structure called evm that contains the bytecode as well as the corresponding opcodes, and some metadata like the details of the used compiler version. Let us grab the ABI and the bytecode that we will need.

abi = out['contracts']['Counter.sol']['Counter']['abi']
bytecode = out['contracts']['Counter.sol']['Counter']['evm']['bytecode']['object']

Deploying the contract

Let us now deploy the contract. First, we will have to import web3 and establish a connection to our geth instance. We have done this before for Ganache, but there is a subtlety explained here – the PoA implementation that geth uses has extended the length of the extra data field of a block. Fortunately, web3 ships with a middleware that we can use to perform a mapping between this block layout and the standard.

import web3
w3 = web3.Web3(web3.HTTPProvider("http://127.0.0.1:8545"))
from web3.middleware import geth_poa_middleware
w3.middleware_onion.inject(geth_poa_middleware, layer=0)

Once the middleware is installed, we first get an account that we will use – this is the first and only account managed by geth in our setup, and is the coinbase account with plenty of Ether in it. Now, we want to create a transaction that deploys the smart contract. Theoretically, we know how to do this. We need a transaction that has the bytecode as data and the zero address as to address. We could probably prepare this manually, but things are a bit more tricky if the contract has a constructor which takes arguments (we will need this later when implementing our NFT). Instead of going through the process of encoding the arguments manually, there is a trick – we first build a local copy of the contract which is not yet deployed (and therefore has no address so that calls to it will fail – try it) then call its constructor() method to obtain a ContractConstructor (this is were the arguments would go) and then invoke its method buildTransaction to get a transaction that we can use. We can then send this transaction (if, as in our case, the account we want to use is managed by the node) or sign and send it as demonstrated in the last post.

me = w3.eth.get_accounts()[0];
temp = w3.eth.contract(bytecode=bytecode, abi=abi)
txn = temp.constructor().buildTransaction({"from": me}); 
txn_hash = w3.eth.send_transaction(txn)
txn_receipt = w3.eth.wait_for_transaction_receipt(txn_hash)
address = txn_receipt['contractAddress']

Now we can interact with our contract. As the temp contract is of course not the deployed contract, we first need to get a reference to the actual contract as demonstrated in the previous post – which we can do, as we have the ABI and the address in our hands – and can then invoke its methods as usual. Here is an example.

counter = w3.eth.contract(address=address, abi=abi)
counter.functions.read().call()
txn_hash = counter.functions.increment().transact({"from": me});
w3.eth.wait_for_transaction_receipt(txn_hash)
counter.functions.read().call()

This completes our post for today. Looking back at what we have achieved in the last few posts, we are now proud owner of an entire arsenal of tools and methods to compile and deploy smart contracts and to interact with them. Time to turn our attention away from the simple counter that we used so far to demonstrate this and to more complex contracts. With the next post, we will actually get into one the most exciting use cases of smart contracts – token. Hope to see you soon.

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