How to search XML? Are there special libs?

[Sullivan WxPyQtKinter]

I am now using XML to record my lab records in quite a complex way, in
which about 80 tags are used to identify different types of data. I
think it is  a good idea to search for a popular and mature XML search
engine before I started to program it myself:

I need the following functions:
Search and get all the records that share the same tree structures.
Search records containing a specific node or sub tree structres.

Does anyone know about something suitable? Or other powerful XML search
engine in Python?
Thank you so much for help.

PS: I do not care about the learning curve, as long as it is not  a
cliff to climb.