base64.py 文件源码

def b64decode(s, altchars=None, validate=False):
    """Decode the Base64 encoded bytes-like object or ASCII string s.

    Optional altchars must be a bytes-like object or ASCII string of length 2
    which specifies the alternative alphabet used instead of the '+' and '/'
    characters.

    The result is returned as a bytes object.  A binascii.Error is raised if
    s is incorrectly padded.

    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 binascii.Error.
    """
    s = _bytes_from_decode_data(s)
    if altchars is not None:
        altchars = _bytes_from_decode_data(altchars)
        assert len(altchars) == 2, repr(altchars)
        s = s.translate(bytes.maketrans(altchars, b'+/'))
    if validate and not re.match(b'^[A-Za-z0-9+/]*={0,2}$', s):
        raise binascii.Error('Non-base64 digit found')
    return binascii.a2b_base64(s)