def b64encode(s, altchars=None):
    """Encode bytes using the standard Base64 alphabet.

    Argument ``s`` is a :term:`bytes-like object` to encode.

    Optional ``altchars`` must be a byte string of length 2 which specifies
    an alternative alphabet for the '+' and '/' characters.  This allows an
    application to e.g. generate url or filesystem safe Base64 strings.

    The result is returned as a :class:`bytes` object.
    """
    if altchars is not None:
        altchars = _get_bytes(altchars)
        assert len(altchars) == 2, repr(altchars)
    if version_info < (3, 0):
        if isinstance(s, text_type):
            raise TypeError('a bytes-like object is required, not \''
                            + type(s).__name__ + '\'')
    return builtin_encode(s, altchars)
Encode bytes using the standard Base64 alphabet.

    Argument ``s`` is a :term:`bytes-like object` to encode.

    Optional ``altchars`` must be a byte string of length 2 which specifies
    an alternative alphabet for the '+' and '/' characters.  This allows an
    application to e.g. generate url or filesystem safe Base64 strings.

    The result is returned as a :class:`bytes` object.