'The read policy to use for any relevent RPC.
if unset STRONG_CONSISTENCY will be used.
Raises:
BadArgumentError if value is not a known read policy.'

@ConfigOption
def read_policy(value):

    if (value not in Configuration.ALL_READ_POLICIES):
        raise datastore_errors.BadArgumentError(('read_policy    argument    invalid    (%r)' % (value,)))
    return value
