How to use the base58.alphabet.decode function in base58

class LedgerIdField(ChooseField):
    _base_types = (int,)
    ledger_ids = VALID_LEDGER_IDS

    def __init__(self, **kwargs):
        self.ledger_ids = self.update_with_plugin_ledger_ids()
        super().__init__(self.ledger_ids, **kwargs)

    @staticmethod
    def update_with_plugin_ledger_ids():
        return VALID_LEDGER_IDS + tuple(PLUGIN_LEDGER_IDS)


class Base58Field(FieldBase):
    _base_types = (str,)
    _alphabet = set(base58.alphabet.decode("utf-8"))

    def __init__(self, byte_lengths=None, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.byte_lengths = byte_lengths

    def _specific_validation(self, val):
        invalid_chars = set(val) - self._alphabet
        if invalid_chars:
            # only 10 chars to shorten the output
            # TODO: Why does it need to be sorted
            to_print = sorted(invalid_chars)[:10]
            return 'should not contain the following chars {}{}'.format(
                to_print, ' (truncated)' if len(to_print) < len(invalid_chars) else '')
        if self.byte_lengths is not None:
            # TODO could impact performance, need to check
            b58len = len(base58.b58decode(val))

"""Validators for schema fields."""

import json

from datetime import datetime

from base58 import alphabet
from marshmallow.validate import OneOf, Range, Regexp
from marshmallow.exceptions import ValidationError

from .util import epoch_to_str

B58 = alphabet if isinstance(alphabet, str) else alphabet.decode("ascii")


class IntEpoch(Range):
    """Validate value against (integer) epoch format."""

    EXAMPLE = int(datetime.now().timestamp())

    def __init__(self):
        """Initializer."""

        super().__init__(  # use 64-bit for Aries RFC compatibility
            min=-9223372036854775808,
            max=9223372036854775807,
            error="Value {input} is not a valid integer epoch time",
        )