You can assign your variable to None:

>>> a = range(20)
>>> a[15:None:-1]
[15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> 

I think you're overthinking this:

First, reverse the list:

inverselist = k1[::-1]

Then, replace the first nonzero element:

for i, item in enumerate(inverselist):
    if item:
        inverselist[i] += 100
        break

If I understood your question properly then I think you're looking for enumerate():

>>> for ind, char in enumerate("mystring"):
...     print ind,char
...     
0 m
1 y
2 s
3 t
4 r
5 i
6 n
7 g

help on enumerate:

>>> enumerate?
Docstring:
enumerate(iterable[, start]) -> iterator for index, value of iterable

Return an enumerate object.  iterable must be another object that supports
iteration.  The enumerate object yields pairs containing a count (from
start, which defaults to zero) and a value yielded by the iterable argument.
enumerate is useful for obtaining an indexed list:
    (0, seq[0]), (1, seq[1]), (2, seq[2]), ...

Edit:

To get the index of first match of any substring you can use str.index or str.find.

str.index will raise ValueError if the item is not found and str.find will return -1:

>>> strs = "hello"
>>> strs.index("h")
0
>>> strs.find("h")
0
>>> strs.find("m")
-1
>>> strs.index("m")
Traceback (most recent call last):
  File "<ipython-input-9-5f19ab4b0632>", line 1, in <module>
    strs.index("m")
ValueError: substring not found