Python recursively search printout multiple files in a single directory

So here is my dir structure

Project
 -Sub
   --401.nii.gz
   --xx.bval
 -Sub2
   --401.nii.gz
   --xx.bval
 -Sub3
   --401.json
   --xx.bval
 -Sub4
   --401.json
   --xx.bval

etc

Here is the code

import os
PATH = "/Users/diabetesgp/Desktop/Analysis/HV"
for root, subFolder, files in os.walk(PATH):
    for item in files:
        if item.endswith("401.json") or item.endswith("401.nii.gz"):
            jsonnii = os.path.join(root,item)
        if item.endswith(".bval"):
            bval = os.path.join(root, item)

     print(jsonnii+bval)

I want to concatenate these two string in a linux command. As you can see i'm trying to get a variable out of the first loop so i can use two different searches and later run a command line. How can i do this? In c# which i'm most familiar with we do jsonnii = null before the if statement. I tried setting jsonnii =None and =0 still no joy. Please help. Thank you.