def b64encode(s, altchars=None):
<EOL>encoded = binascii.b2a_base64(s)[:-<NUM_LIT:1>]<EOL>if altchars is not None:<EOL><INDENT>return encoded.translate(string.maketrans(b'<STR_LIT>', altchars[:<NUM_LIT:2>]))<EOL><DEDENT>return encoded<EOL>
Encode a string using Base64.

    s is the string to encode.  Optional altchars must be a string of at least
    length 2 (additional characters are ignored) which specifies an
    alternative alphabet for the '+' and '/' characters.  This allows an
    application to e.g. generate url or filesystem safe Base64 strings.

    The encoded string is returned.