Use index() to find multiple identical strings in Python

I have a string. The letter 'o' appears twice in the whole string, but when I try to use the index() function to locate the position where the 'o' appears, it just traverses from left to right and stops when it finds the first 'o'.

Why can’t index() print all the locations of 'o'?

If possible, how can I use index() to print all the strings that meet the conditions?

a = 'HaloPython!'
print(a.index('o'))

Answer

index returns exactly one index per call. It’s not going to try to shove all indices into the result, changing return type based on the values.

Two obvious ways to handle it:

1. A listcomp with enumerate:

   allindices = [i for i, x in enumerate(a) if x == 'o']
   

2. A loop that calls index with an explicit start argument:

   idx = -1  # -1 means first search will begin at beginning of string
   try:
       while True:
           idx = a.index('o', idx + 1)  # Resume search after last index found
           print(idx)
   except ValueError:
       pass  # Ran out of 'o's, we're done
   

The listcomp solution is by far the most common and obvious solution (and can be made to work for length > 1 substrings by using start modified calls to str.startswith), but if you must use index to do it, option #2 exists.