Recursive function going infinite and I can't see why.

[Terry Reedy]

>Would anyone be able to tell me why my function below is getting stuck
>in infinite recusion?

>def replace_within_node(node,oldnode,newnode):
>    if node is oldnode:
>        return newnode

Without looking further, the most likely reason is that the base case is 
never true: ie, node is never oldnode.  I would probably add
  print node, oldnode, newnode
at the top (and maybe another print later) and then run the code on a small 
tree where I thought I knew what should be printed and then see where my 
expectation diverges from the actual.

Terry Jan Reedy