String Method 'find(...)'.

[buran]

for index, ch in enumerate('si si si no yes si'):
    print(f'{index} --> {ch}')
print('si si si no yes si'.find('i', 8, 10))

Output:
0 --> s
1 --> i
2 -->
3 --> s
4 --> i
5 -->
6 --> s
7 --> i
8 -->
9 --> n
10 --> o
11 -->
12 --> y
13 --> e
14 --> s
15 -->
16 --> s
17 --> i
-1
>>>

as you can see, 'i' is on index 1, 4, 7 and 17. with start=6 and end=10 you catch the 'i' on index 7