Quickstart

Note

This quickstart is aspirational. The code examples may not work yet.

Installation

pip install py-evm

Sync and interact with the Ropsten chain

Currently we only provide a light client that will sync only block headers, although it can fetch block bodies on demand. The easiest way to try it is by running the lightchain_shell, which will run the LightChain in the background and let you use the python interpreter to interact with it:

$ python -i -m evm.lightchain_shell -db /tmp/testnet.db

That will immediately give you a python shell, with a chain variable that you can use even before it has finished syncing:

>>> chain.get_canonical_head()
<BlockHeader #2200794 e3f9c6bb>

Some LightChain methods (e.g. those that need data from block bodies) are coroutines that need to be executed by asyncio’s event loop, so for those we provide a helper that will schedule their execution and wait for the result:

>>> wait_for_result(chain.get_canonical_block_by_number(42))
<FrontierBlock(#Block #42)>

Accessing an existing chain database

The Chain object manages the series of fork rules contained in every blockchain. It requires that you define the VM ranges. Some pre-built chains are available for your convenience. To access the Mainnet chain you can use:

from evm import MainnetChain
from evm.chains.mainnet import MAINNET_GENESIS_HEADER
from evm.db.backends.level import LevelDB
from evm.db.chain import BaseChainDB

# Read the previously saved chain database
chaindb = BaseChainDB(LevelDB('/tmp/mainnet.db'))

# Load the saved database into a mainnet chain object
chain = MainnetChain(chaindb)

Then you can read data about the chain that you already downloaded. For example:

highest_block_num = chain.get_canonical_head().block_number

block1 = chain.get_canonical_block_by_number(1)
assert block1.number() == 1

blockhash = block1.hash()
blockgas = block1.get_cumulative_gas_used()

The methods available on the block are variable. They depend on what fork you’re on. The mainnet follows “Frontier” rules at the beginning, then Homestead, and so on. To see block features for Frontier, see the API for FrontierBlock.

The JSON-RPC API

Like all ethereum clients, Py-EVM will eventually provide a JSON-RPC API with all the methods defined in https://github.com/ethereum/wiki/wiki/JSON-RPC, but for now only a few of them are supported. To start the JSON-RPC server, simply run:

$ python -i -m evm.rpc.server -db /tmp/testnet.db

That will start a server listening on port 8080, with a LightChain syncing block headers on the Ropsten network. You can then use curl as described on the wikipage above to interact with it.