def consume_netstring(s):
    """If s is a bytestring beginning with a netstring, returns (value, rest)
    where value is the contents of the netstring, and rest is the part of s
    after the netstring.
    
    Raises ValueError if s does not begin with a netstring.
    
    """
    (length, sep, rest) = s.partition(b':')
    if sep != b':':
        raise ValueError("No colon found in s")
    if not length.isdigit():
        raise ValueError("Length is not numeric")
    length = int(length)
    if len(rest) <= length:
        raise ValueError("String not long enough")
    if rest[length] != 0x2c:
        raise ValueError("String not terminated with comma")
    return (rest[:length], rest[length+1:])