Don't overwrite the user-provided dict when converting to a message

[rickwalder]

rosidl_runtime_py/rosidl_runtime_py/set_message.py

Line 99 in e6c860f

value[n] = submsg

instead, make a list before entering the loop, get all the values and place them in the list, and then use the list to assign the message

[fujitatomoya]

@rickwalder is this a bug or enhancement? can you elaborate a bit about this report? and that would be appreciated if you can consider the pull request.

[sloretz]

@rickwalder friendly ping. Mind explaining a little bit more about what this would fix? 🧇

[rickwalder]

sorry, this one fell off the map!

looks to be limited to messages that contain a list of messages - the dict the user provides will have the list of "not-messages" overwritten with messages, which is a problem if you want to use the same dict template for a bunch of messages

This example demonstrates it:

from rosidl_runtime_py.set_message import set_message_fields
from sensor_msgs.msg import PointCloud
c = {"points": [{"x": 1, "y": 2, "z": 3}, {"x": 4, "y": 5, "z": 6}]}
set_message_fields(PointCloud(), c)

print(c)

c is now

{'points': [geometry_msgs.msg.Point32(x=1.0, y=2.0, z=3.0), geometry_msgs.msg.Point32(x=4.0, y=5.0, z=6.0)]}

and the next time I try to set_message_fields(PointCloud(), c), I run into trouble:

>>> set_message_fields(PointCloud(), c)
Traceback (most recent call last):
    ...

(or do anything else with c)

Not a major issue, but wanted to jot it down

[fujitatomoya]

@rickwalder that sounds reasonable, and it is kinda surprising for user application that values are updated silently.

i created the PR for #34, it would be appreciated if you can review.

[rickwalder]

your PR looks good. I won't comment there so I don't throw off the actual review :)