At our latest meeting, I promised to look up online services to help us select locations in the city based on convenience. “The 19 communes” is not precise enough, and might lead to frustration as someone suggests a place and someone else rejects it on the basis of a criterion that we had not considered before.
A criterion that did come up a lot is public transport. A concept that helps here is that of isochrone maps: the idea is to draw a curve on a map. Each point on the curve takes the same time to get to from some point chosen.
There are some API services to compute isochrones based on OpenStreetMap, the Wikipedia of geography. Here is one for walking distances from Gare du Midi. The area shaded in purple is reachable on foot in 10 minutes from the station; that shaded in red (which includes my current home, yay!) in 20.
You can make your own tests here. Just enter the address that you are interested, your mode of transport, and the maximum length of travel time.
This particular website covers walking, biking and driving, but not public transport. There are services that do like Iso4app, but it is a little more complicated, because it requires registration, generating an API key and maybe some fooling around with the software. Happy to do it, though, on prompts from the group.