# Quickstart

## Choose a hashing backend

If you’re not sure, choose “pycryptodome” because it supports pypy3.

You can find a full list of each currently supported backend in `eth_hash.backends`.

## Install

Put the backend you would like to use in brackets during install, like:

    python -m pip install "eth-hash[pycryptodome]"

## Compute a Keccak256 Hash

    >>> from eth_hash.auto import keccak
    >>> keccak(b'')
    b"\xc5\xd2F\x01\x86\xf7#<\x92~}\xb2\xdc\xc7\x03\xc0\xe5\x00\xb6S\xca\x82';{\xfa\xd8\x04]\x85\xa4p"

You may also compute hashes incrementally

    >>> from eth_hash.auto import keccak
    >>> preimage = keccak.new(b'part-a')
    >>> preimage.update(b'part-b')
    >>> preimage.digest()
    b'6\x91l\xdd50\xd6[\x7f\xf9B\xff\xc9SW\x98\xc3\xaal\xd9\xde\xdd6I\xb7\x91\x9e\xf4`pl\x08'

The preimage object returned may be copied as well.

    >>> from eth_hash.auto import keccak
    >>> preimage = keccak.new(b'part-a')
    >>> preimage_copy = preimage.copy()
    >>> preimage.update(b'part-b')
    >>> preimage.digest()
    b'6\x91l\xdd50\xd6[\x7f\xf9B\xff\xc9SW\x98\xc3\xaal\xd9\xde\xdd6I\xb7\x91\x9e\xf4`pl\x08'
    >>> preimage_copy.update(b'part-c')
    >>> preimage_copy.digest()
    b'\xffcy45\xea\xdd\xdf\x8e(\x1c\xfcF\xf3\xd4\xa1S\x0f\xdf\xd8\x01!\xb2(\xe1\xc7\xc6\xa3\x08\xc3\n\x0b'

## Select one of many installed backends

If you have several backends installed, you may want to explicitly specify which one to load. You can specify in an environment variable, or at runtime.

### Specify backend by environment variable

    $ ETH_HASH_BACKEND="pysha3" python
    >>> from eth_hash.auto import keccak
    # This runs with the pysha3 backend
    >>> keccak(b'')
    b"\xc5\xd2F\x01\x86\xf7#<\x92~}\xb2\xdc\xc7\x03\xc0\xe5\x00\xb6S\xca\x82';{\xfa\xd8\x04]\x85\xa4p"

### Specify backend at runtime

    >>> from eth_hash.backends import pysha3
    >>> from eth_hash import Keccak256
    >>> keccak = Keccak256(pysha3)
    >>> keccak(b'')
    b"\xc5\xd2F\x01\x86\xf7#<\x92~}\xb2\xdc\xc7\x03\xc0\xe5\x00\xb6S\xca\x82';{\xfa\xd8\x04]\x85\xa4p"