is returned.  A TypeError is raised if s is
    incorrectly padded.  Characters that are neither in the normal base-64
    alphabet nor the alternative alphabet are discarded prior to the padding
    check.
    """
    if altchars is not None:
        s = s.translate(string.maketrans(altchars[:2], '+/'))
    try:
        return binascii.a2b_base64(s)
    except binascii.Error, msg:
        # Transform this exception for consistency
        raise TypeError(msg)


def standard_b64encode(s):
    """Encode a string using the standard Base64 alphabet.

    s is the string to encode.  The encoded string is returned.
    """
    return b64encode(s)

def standard_b64decode(s):
    """Decode a string encoded with the standard Base64 alphabet.

    Argument s is the string to decode.  The decoded string is returned.  A
    TypeError is raised if the string is incorrectly padded.  Characters that
    are not in the standard alphabet are discarded prior to the padding
    check.
    """
    return b64decode(s)

_urlsafe_encode_translation = string.maketrans(b'+/', b'-_')
_urlsafe_decode_translation = string.maketrans(b'-_', b'+/')

def urlsafe_b64encode(s):
    """Encode a string using the URL- and filesystem-safe Base64 alphabet.

    Argument s is the string to encode.  The encoded string is returned.  The
    alphabet uses '-' instead of '+' and '_' instead of '/'.
    """
    return b64encode(s).translate(_urlsafe_encode_translation)

def urlsafe_b64decode(s):
    """Decode a string using the URL- and filesystem-safe Base64 alphabet.

    Argument s is the string to decode.  The decoded string is returned.  A
    TypeError is raised if the string is incorrectly padded.  Characters that
    are not in the URL-safe base-64 alphabet, and are not a plus '+' or slash
    '/', are discarded prior to the padding check.

    The alphabet uses '-' instead of '+' and '_' instead of '/'.
    """
    return b64decode(s.translate(_urlsafe_decode_translation))