Python Recursion function, index out of bound

First of all, rename your string and char. Right now you're overloading str() and chr()

Your error comes from the fact that once you have removed things from the string, indexes matching the latter values in its old length will no longer be availible

def find_combinations(strz, remaining_char=""):
    print("find_combinations(%s, %s)" % (strz, remaining_char))

    if not strz:
        print(remaining_char)
    else:

        for i in range(len(strz)):
            print(i)
            print(strz)
            chrz = strz[i] # at this line it shows index out of bound
            remaining_char = remaining_char+chrz
            strz = strz[1:]

            find_combinations(strz, remaining_char)

find_combinations("ABCD")

outputs

find_combinations(ABCD, )
0
find_combinations(BCD, A)
0
find_combinations(CD, AB)
0
find_combinations(D, ABC)
0
find_combinations(, ABCD)
ABCD
1

As you can see, list index 1 is out of range

Look at what you're doing: consider a string of 4 chars.

    for i in range(len(str)):
        chr = str[i] 
        str = str[1:]

i str

0 ABCD 1 ABC 2 AB ... and the next time you try to access str[i], the string is too short.

Keep the original string intact; send just the truncated one into the recursion:

    for i in range(len(s)):
        chr = s[i] 
        remaining_char += chr
        find_combinations(s[1:], remaining_char)

You still have logic problems in your program, but this will get you through the next step. I strongly recommend that you do three things:

1. See this lovely debug blog for help.
2. Use incremental programming -- write just 2-4 lines at a time, one small chunk of code. Make sure that they work before continuing. Part of the reason that you're needing so many SO questions is that you've introduced multiple errors into your function before testing. You can't easily isolate the cause of trouble when you have several causes waiting to bite you
3. Research on line the ways that others have solved this problem; their guidance will likely be helpful in smoothing the remaining problems in your code.

You don't need the for line. You can remove it.

Just replace the chr = str[i] with chr = str[0], and rename your variables as said in previous posts (chr and str).

It will do what you want.

You can always use itertools.permutations which is a python provided function. The example below generates permutations taking all 4 characters in account

import itertools as it
words = list('ABCD')
print(list(it.permutations(words, 4)))
'''[('A', 'B', 'C', 'D'), ('A', 'B', 'D', 'C'), ('A', 'C', 'B', 'D'), 
('A', 'C', 'D', 'B'), ('A', 'D', 'B', 'C'), ('A', 'D', 'C', 'B'), 
('B', 'A', 'C', 'D'), ('B', 'A', 'D', 'C'), ('B', 'C', 'A', 'D'), 
('B', 'C', 'D', 'A'), ('B', 'D', 'A', 'C'), ('B', 'D', 'C', 'A'),
('C', 'A', 'B', 'D'), ('C', 'A', 'D', 'B'), ('C', 'B', 'A', 'D'),
 ('C', 'B', 'D', 'A'), ('C', 'D', 'A', 'B'), ('C', 'D', 'B', 'A'), 
('D', 'A', 'B', 'C'), ('D', 'A', 'C', 'B'), ('D', 'B', 'A', 'C'), 
('D', 'B', 'C', 'A'), ('D', 'C', 'A', 'B'), ('D', 'C', 'B', 'A')]'''

print(list(it.permutations(words, 2)))
#[('A', 'B'), ('A', 'C'), ('A', 'D'), ('B', 'A'), ('B', 'C'), 
#('B', 'D'), ('C', 'A'), ('C', 'B'), ('C', 'D'), ('D', 'A'), ('D', 'B'), ('D', 'C')]