[igraph] Merging two graphs

[Kuu]

Hello!

I have two graphs and each of them have vertices with the "id" attribute.

I want to merge both of them by this attribute but I cannot find a method for doing so (I'm on Python btw). I saw union and compose methods, but they seem to join vertices by index, I'm not sure about this as it is not explained in the docs, but my tests seem to point that.

So my questions: how do compose/union methods work?  Is there a way of doing the merge by id/name attribute instead of the vertex index?

Thank you

Regards,
Javier