def __init__(self, salt, input_key_material, hash=hashlib.sha256):
self._hash = hash<EOL>self._prk = hkdf_extract(salt, input_key_material, self._hash)<EOL>
Extract a pseudorandom key from `salt` and `input_key_material` arguments.

See the HKDF draft RFC for guidance on setting these values. The constructor
optionally takes a `hash` arugment defining the hash function use,
defaulting to hashlib.sha256.