create rosmsg content from string

edit

You know that 'rostopic pub' has this functionality, so your first step should be to check its code.

It is implemented in rostopic [Line 1707]. It's python so that there has to be a way to fill a message from such a string representation. The user input is stored in pub_args, so the next function to check is argv_publish, which leads to publish_message [L 1651], there, we find _fillMessageArgs [L. 1687], which then uses the final 'genpy.message.fill_message_args(msg, pub_args, keys=keys)' to fill the message. So the search continues to genpy.message and we finally find:

def fill_message_args(msg, msg_args, keys={}):
    """
    Populate message with specified args. Args are assumed to be a
    list of arguments from a command-line YAML parser. See
    http://www.ros.org/wiki/ROS/YAMLCommandLine for specification on
    how messages are filled. (...)

This should be the function you are looking for.