How to replace the some characters from the end of a string?

Question

I want to replace characters at the end of a python string. I have this string:

 s = \"123123\"

I want to replace the last 2 with

Answer 1

I got a tricky answer, but it is not efficient enough

    >>> fname = '12345.png.pngasdfg.png'
    >>> suffix = '.png'
    >>> fname_rev = fname[::-1]
    >>> suffix_rev = suffix[::-1]
    >>> fullname_rev = fname_rev.replace(suffix_rev, '', 1)
    >>> fullname = fullname_rev[::-1]
    >>> fullname '12345.png.pngasdfg'

Built-in function replace() takes three arguments str.replace(old, new, max_time) So you can delete the last mached string from the origional string