How to calculate ethash in python?

Question

I am trying to use ethash(hash(blockHeader),nonce) < TARGET to prove that the block was mined with at least a minimum difficulty.

Is the formula above correct?

When I try to use ehtereum.pow.ethash from pyethereum:

from ethereum.pow import ethash
difficulty = 3963642329
block_number = 4156209
nonce = int('0xf245822d3412da7f', 16)
block_hash = 0x47b8f62c1400dae65105d2f8e03824bfc58481c0b32f45788ad3378fbc05e9f6
cache = ethash.mkcache(block_number)
ethash = hashimoto_light(block_number, cache, block_hash, nonce)
print(ethash)

I get this error:

Traceback (most recent call last):
  File "test.py", line 10, in <module>
    cache = ethash.mkcache(block_number)
  File "(environment_path)/lib/python3.6/site-packages/ethereum/pow/ethash.py", line 15, in mkcache
    while len(cache_seeds) <= block_number // EPOCH_LENGTH:
NameError: name 'EPOCH_LENGTH' is not defined

Am I doing it wrong? Or is there a bug in ethereum.pow.ethash?

Answer 1

The ethereum module is not maintained. Check out pyethash for the same purpose.

You can import like:

from pyethash import (
    EPOCH_LENGTH,
    hashimoto_light,
    mkcache_bytes,
)

py-evm uses it to check proof of work like so:

def check_pow(block_number: int,
              mining_hash: Hash32,
              mix_hash: Hash32,
              nonce: bytes,
              difficulty: int) -> None:
    validate_length(mix_hash, 32, title="Mix Hash")
    validate_length(mining_hash, 32, title="Mining Hash")
    validate_length(nonce, 8, title="POW Nonce")
    cache = get_cache(block_number)
    mining_output = hashimoto_light(
        block_number, cache, mining_hash, big_endian_to_int(nonce))
    if mining_output[b'mix digest'] != mix_hash:
        raise ValidationError("mix hash mismatch; {0} != {1}".format(
            encode_hex(mining_output[b'mix digest']), encode_hex(mix_hash)))
    result = big_endian_to_int(mining_output[b'result'])
    validate_lte(result, 2**256 // difficulty, title="POW Difficulty")

Where the cache is generated and stored with:

# Type annotation here is to ensure we don't accidentally use strings instead of bytes.
cache_seeds = [b'\x00' * 32]  # type: List[bytes]
cache_by_seed = OrderedDict()  # type: OrderedDict[bytes, bytearray]
CACHE_MAX_ITEMS = 10


def get_cache(block_number: int) -> bytes:
    while len(cache_seeds) <= block_number // EPOCH_LENGTH:
        cache_seeds.append(keccak(cache_seeds[-1]))
    seed = cache_seeds[block_number // EPOCH_LENGTH]
    if seed in cache_by_seed:
        c = cache_by_seed.pop(seed)  # pop and append at end
        cache_by_seed[seed] = c
        return c
    c = mkcache_bytes(block_number)
    cache_by_seed[seed] = c
    if len(cache_by_seed) > CACHE_MAX_ITEMS:
        cache_by_seed.popitem(last=False)  # remove last recently accessed
    return c