def quoteStringArgument(argument):
    """
    Quote an argument to L{serverFromString} and L{clientFromString}.  Since
    arguments are separated with colons and colons are escaped with
    backslashes, some care is necessary if, for example, you have a pathname,
    you may be tempted to interpolate into a string like this::

        serverFromString(reactor, "ssl:443:privateKey=%s" % (myPathName,))

    This may appear to work, but will have portability issues (Windows
    pathnames, for example).  Usually you should just construct the appropriate
    endpoint type rather than interpolating strings, which in this case would
    be L{SSL4ServerEndpoint}.  There are some use-cases where you may need to
    generate such a string, though; for example, a tool to manipulate a
    configuration file which has strports descriptions in it.  To be correct in
    those cases, do this instead::

        serverFromString(reactor, "ssl:443:privateKey=%s" %
                         (quoteStringArgument(myPathName),))

    @param argument: The part of the endpoint description string you want to
        pass through.

    @type argument: C{str}

    @return: The quoted argument.

    @rtype: C{str}
    """
    backslash, colon = '\\:'
    for c in backslash, colon:
        argument = argument.replace(c, backslash + c)
    return argument