YANQ: Yet Another Newbie Question !

[nilamo]

Without knowing what service is, we should assume it's an object, and ref is just a property of that object.  We can fake it like so:

>>> class Service:
...   def __init__(self, tag):
...     self.ref = tag
...
>>> service = Service("4097:1:5:23")
>>> service.ref
'4097:1:5:23'

If that's true (...and it might not be), then we could take the fact that strings are immutable in python, and work with it directly...

>>> service.ref.startswith("4097:")
True
>>> new_tag = service.ref.replace("4097:", "1:", 1)
>>> service.ref
'4097:1:5:23'
>>> new_tag
'1:1:5:23'

Because strings are immutable, we have the new tag that we wanted (with 4097 replaced with 1), while service.ref is left unchanged.

Also, I added a count parameter to service.ref.replace, so that only the first 4097 found will be replaced.  That way, if the tag happens to look like "4097:1:4097:23", it won't be changed to "1:1:1:23".