# Tutorial: Controlling Ethereum with Python

Ethereum makes it easy to run simple computations on blockchains while keeping the value of decentralized networks– but real applications will need access to off-blockchain resources, whether for heavy computations or pulling data from the internet. This tutorial walks through how to deploy a smart contract on a private test blockchain and use Python to read data from the blockchain or send data to the blockchain. We’ll review the relevant components of the Ethereum network, walk through how to interact with the system using Python, and deploy example smart contracts.

Note that this was written in July 2016 using the Ethereum Homestead release, Python 2.7.12, and geth 1.6.6. More resources are found on the official Ethereum website and the Ethereum Homestead documentation. Because these projects are open-source, there is a lot of outdated information (probably including this guide, by the time you read it). This notebook is also available on github here.

## Background: Ethereum is not a computer¶

For background, it’s helpful to keep in mind that “Ethereum” is a communication protocol- not a specific network, computer, or application. Just as the main Internet exists alongside many private Intranets and local networks, there is a main community Ethereum network (which supports the Ether cryptocurrency) alongside a community test network (called “Ropsten”) and many private networks. For this tutorial we’ll be working on implementing a test server that just serves data on your local computer- just as you might run a localhost:// server for developing a web page, but later deploy it to a wider network.

For my research, I don’t want to deploy on the full Ethereum network, where my code is subject to attack and operations cost real Ether- instead ther are a variety of different options for prototyping:

• Private node in development mode: Just run an ethereum node on your own machine, ignoring all connections- the equivalent of running on localhost.
• Private network: Networking a set of computers into a private network that isn’t discoverable by others.
• Public test network: Open for anyone to access, but configured to quickly generate Ether for testing software.
• Full Ethereum network: This is the real deal, where Ether costs money and hackers may roam.

So Ethereum is a communication protocol- but how do we interact with it? There are different open-source software packages which connect with Ethereum networks for mining or development purposes- these are equivalent to competing internet browsers. Common implementations are eth (the C++ implementation, not to be confused with the cryptocurrency), and geth (the GoLang implementation, which we’ll be using here).

Ultimately, we care about the scripts which are run on the Ethereum Virtual Machine (the EVM)- the decentralized, consensus-driven computer which distinguishes Ethereum from earlier blockchains. This Virtual Machine runs its own language of bytecode, which we’ll generate with a compiler. We’ll be writing scripts in Solidity, a Javascript-derived language, and compiling them into bytecode using the sol-c compiler. We’ll then use our Ethereum client (i.e geth) to push this compiled bytecode to the Ethereum network, and to run the bytecode which others have written.

From now on, we’ll assume that we’re working with the geth implementation and will work on creating a private test network on our own computer in order to develop and deploy smart contracts which we’ll be interacting with using Python.

## Big concepts: Navigating the software¶

For development, there are 3 pieces of software that you’ll interact with: the Ethereum node (the server), the console (where you can type code to interact directly with the server), and the Ethereum Wallet (a GUI which provides graphical interaction with the server. These are configured with two important directories: the datadir which stores the important configuration, and the location of your .ipc (inter-process communication) file which lets the pieces talk with each other.

### Important software pieces:¶

• Ethereum Node (e.g. Geth server): This runs your actual Ethereum node and maintains a connection with the rest of the Ethereum network. This can be created by running geth (starts server only), by running geth console (starts server and then console), or by opening the Ethereum Wallet. However you start it, the ethereum node is discoverable to related software through
• Console: Allows Javascript command-line interface to the server. Can be used for changing settings or deploying contracts. Run by geth console (will attempt to spin up a node) or geth attach (will look for geth.ipc file)
• Ethereum Wallet (aka Mist): GUI for interfacing with wallets and deploying contracts. Will look for /Users/emunsing/Library/Ethereum/geth.ipc and if it does not find geth.ipc the Wallet will start up a server for the node.

Configuration
The Ethereum environment is dictated by two data sources: the data directory (called ‘datadir’ here) and the geth.ipc file. The datadir stores the blockchain, account keys, and other personal settings. The geth.ipc file is created when a node starts up, and allows other services (geth console, ethereum wallet) to attach to that server. In addition, there are a number of command line options that you can use to direct how and where your Ethereum node should look for peers.

• By default, the Ethereum Wallet or geth attach looks for the geth.ipc file in ~/Library/Ethereum/geth.ipc (i.e. /Users/username/Library/Ethereum/geth.ipc on Mac OSX).
• Specify the datadir location with the command line options --datadir ~/data and specify the ipc location with --ipcpath ~/Ethereum/geth.ipc
• Generally avoid using geth.ipc locations other than the default- there is no easy way to configure the Wallet to connect to these
If you’re using a computer for developing new ethereum apps, you might want to have a number of different blockchains for different public and private implementations. I find it helpful to keep these separate by running one node per computer, using the default .ipc location, and using separate datadir locations for each project.

### Back up the /keystore!¶

Each account’s private keys are encrypted and stored in the /keystore folder of the datadir. These should be backed up off of the hard drive in case of removal- whether accidental or malicious.

• To restore an account after /keystore was removed, simply paste the backed-up files back to /keystore under the data directory
• If trying to just clean up the blockchain, can do so with geth upgradedb or geth removedb – these will not affect the keystore

## Installing Geth¶

Geth, or go-ethereum, is the main software platform for running an ethereum node on your computer. You can use this to connect to the main ethereum network, the public test network, or to create your own private test network.

We’ll install Geth using Homebrew, following the Geth installation instructions for Mac. This tutorial was written using

• Geth v1.6.6
• Python v2.7.12
• ethjsonrpc v0.3.0
• py-solc v.1.2.0

First, make sure that homebrew is installed and up-to-date by running brew update …if it isn’t installed, follow these instructions.

brew update
brew tap ethereum/ethereum
brew install ethereum



Check that Geth is now installed by running geth account new which will prompt you to create a new account.

To upgrade Geth, run brew update && brew upgrade ethereum. You can check your version with geth version

## Installing Sol-c (compiler)¶

• Install the solidity compiler: npm install -g solc or npm install solc –global then check that the installation succeeded: solcjs –help
• Install solC using homebrew

# Getting Up and Running¶

From here on, we’ll use the dollar sign to indicate Unix/bash command line entries like $geth attach and use the angle bracket to indicate commands entered into the geth console like > miner.start(1) Setting up a private test network: 1. Run a server node with $ geth --dev --rpc --ipcpath ~/Library/Ethereum/geth.ipc --datadir ~/Library/Ethereum/pyEthTutorial
2. In a second terminal window, attach a geth console with \$ geth attach
3. If you haven’t created an account yet, you’ll need to create a new account with > personal.newAccount() and follow the prompt to create a password. If everything is running right, you should see your geth server output announce the creation of a new account, and should also see a new file appear in your /keystore directory.
4. You can get a list of all the accounts associated with your keystore by running > eth.accounts. We’ll assume that we want to work with the first of these, i.e. > eth.accounts[0]
5. In the geth console, unlock the coinbase account with > personal.unlockAccount( eth.accounts[0], 'passwd', 300) where 300 is the number of seconds for which it should be unlocked (set to 0 for indefinite). The console should output true
6. Start mining with > miner.start(1) which should trigger a stream of text on your first terminal window. You can now see your balance in Ether by checking web3.fromWei( eth.getBalance(eth.coinbase), "ether")
In [1]:
import sys, os, time
from solc import compile_source, compile_files, link_code
from ethjsonrpc import EthJsonRpc

print("Using environment in "+sys.prefix)
print("Python version "+sys.version)

Using environment in /Users/emunsing/Documents/BlogPosts/env/bin/..
Python version 2.7.12 (default, Aug 23 2016, 08:54:23)
[GCC 4.2.1 Compatible Apple LLVM 7.3.0 (clang-703.0.31)]

In [2]:
# Initiate connection to ethereum node
#   Requires a node running with an RPC connection available at port 8545
c = EthJsonRpc('127.0.0.1', 8545)
c.web3_clientVersion()

Out[2]:
u'Geth/v1.6.6-stable-10a45cb5/darwin-amd64/go1.8.3'

We can also check the current block number with c.eth_blockNumber() which should match the most recent blocks mined in our miner window.

## Solidity Source¶

We can load the Solidity code from file or work with it as direct text. We’ll take the Solidity code, compile it with Solc, and then send it to our private development network. The following is similar to the Greeter example from the Ethereum docs, but any Solidity code can be used.

In [3]:
source = """
pragma solidity ^0.4.2;

contract Example {

string s="Hello World!";

function set_s(string new_s) {
s = new_s;
}

function get_s() returns (string) {
return s;
}
}"""


### Compile the contract and send to network¶

Once this code is compiled, it will be submitted to the network’s transaction queue. In order for the code to become a valid smart contract, we need to mine it into the blockchain- othewise it’s just stuck in the queue. Accordingly, make sure you’re mining on your private network (or you’re connected to a network where somebody is mining!

In [4]:
# Basic contract compiling process.
#   Requires that the creating account be unlocked.
#   Note that by default, the account will only be unlocked for 5 minutes (300s).
#   Specify a different duration in the geth personal.unlockAccount('acct','passwd',300) call, or 0 for no limit

compiled = compile_source(source)
# compiled = compile_files(['Solidity/ethjsonrpc_tutorial.sol'])  #Note: Use this to compile from a file
compiledCode = compiled['Example']['bin']
compiledCode = '0x'+compiledCode # This is a hack which makes the system work

# Put the contract in the pool for mining, with a gas reward for processing
contractTx = c.create_contract(c.eth_coinbase(), compiledCode, gas=300000)
print("Contract transaction id is "+contractTx)

print("Waiting for the contract to be mined into the blockchain...")
while c.eth_getTransactionReceipt(contractTx) is None:
time.sleep(1)

# There is no cost or delay for reading the state of the blockchain, as this is held on our node
results = c.call(contractAddr, 'get_s()', [], ['string'])

Contract transaction id is 0xcb51f4a37526e65f1285d429f8fa50186c1b2290af83d48af5d7e7bafcf0765e
Waiting for the contract to be mined into the blockchain...

In [5]:
# Send a transaction to interact with the contract
#   We are targeting the set_s() function, which accepts 1 argument (a string)
#   In the contact definition, this was   function set_s(string new_s){s = new_s;}
params = ['Hello, fair parishioner!']
tx = c.call_with_transaction(c.eth_coinbase(), contractAddr, 'set_s(string)', params)

print("Sending these parameters to the function: "+str(params))
print("Waiting for the state changes to be mined and take effect...")
while c.eth_getTransactionReceipt(tx) is None:
time.sleep(1)

results = c.call(contractAddr, 'get_s()', [], ['string'])

Sending these parameters to the function: ['Hello, fair parishioner!']
Waiting for the state changes to be mined and take effect...
The message now reads 'Hello, fair parishioner!'


# That’s it- start working on Dapps!¶

We now have the ability to use all the power of Python to handle complex computations, and the slexibility of Ethereum for decentralized, trustless transactions. We’ve seen how to read the state of smart contracts on the blockchain, and how to change their state. This means that we could use Python to listen for a change on the blockchain (like the change in an account balance, or a message sent to a smart contract) and then use Python to run complex computations, control hardware, or interface with resources on the Internet and beyond.