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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def b64decode(s, altchars=None, validate=False):
 """Decode bytes encoded with the standard Base64 alphabet. Argument ``s`` is a :term:`bytes-like object` or ASCII string to decode. Optional ``altchars`` must be a :term:`bytes-like object` or ASCII string of length 2 which specifies the alternative alphabet used instead of the '+' and '/' characters. If ``validate`` is ``False`` (the default), characters that are neither in the normal base-64 alphabet nor the alternative alphabet are discarded prior to the padding check. If ``validate`` is ``True``, these non-alphabet characters in the input result in a :exc:`binascii.Error`. The result is returned as a :class:`bytes` object. A :exc:`binascii.Error` is raised if ``s`` is incorrectly padded. """

    if version_info < (3, 0) or validate:
        if validate and len(s) % 4 != 0:
            raise BinAsciiError('Incorrect padding')
        s = _get_bytes(s)
        if altchars is not None:
            altchars = _get_bytes(altchars)
            assert len(altchars) == 2, repr(altchars)
            if version_info < (3, 0):
                map = maketrans(altchars, b'+/')
            else:
                map = bytes.maketrans(altchars, b'+/')
            s = s.translate(map)
        try:
            result = builtin_decode(s, altchars)
        except TypeError as e:
            raise BinAsciiError(str(e))
        if validate:
            # check length of result vs length of input
            padding = 0
            if len(s) > 1 and s[-2] in (b'=', 61):
                padding = padding + 1
            if len(s) > 0 and s[-1] in (b'=', 61):
                padding = padding + 1
            if 3 * (len(s) / 4) - padding != len(result):
                raise BinAsciiError('Non-base64 digit found')
        return result
    return builtin_decode(s, altchars)