Python string recursion, string index out of range

Question

I am new at python and very bad at thinking in a recursive way. This code gives me an IndexError: string index out of range. And I have no idea how to correct it.

def get_permutations(sequence):

    def permutationhelp(sequence,chosen):

        if sequence=="":
            print(chosen)

        else:

            for i in range(len(sequence)):
                c= sequence[i]
                chosen +=c
                sequence=sequence[i+1:]
                permutationhelp(sequence,chosen)
                sequence=c+sequence
                chosen=chosen[:-1]

    def permutation(sequence):
        permutationhelp(sequence,"")


    return permutation(sequence)

Example:

get_permutations('abc')
['abc', 'acb', 'bac', 'bca', 'cab', 'cba']

The Traceback is this:

Traceback (most recent call last):
  File "soRecursivePermutations.py", line 25, in <module>
    get_permutations('abc')
  File "soRecursivePermutations.py", line 23, in get_permutations
    return permutation(sequence)
  File "soRecursivePermutations.py", line 20, in permutation
    permutationhelp(sequence,"")
  File "soRecursivePermutations.py", line 12, in permutationhelp
    c= sequence[i]
IndexError: string index out of range

Answer 1

The reason for your Traceback is here:

sequence=sequence[i+1:]
permutationhelp(sequence,chosen)
sequence=c+sequence

The first line leaves sequence with just the end of the string. and the third line only adds one character back onto sequence, so sequence gets shorter as you go through the loop.

However, this is probably the program you were looking for:

# https://stackoverflow.com/a/53088155/4834
def remove_at(i, s):
    return s[:i] + s[i + 1:]

def permutationhelp(sequence, chosen, collect):
    if sequence == "":
        collect.append(chosen)
    else:
        for i,c in enumerate(sequence):
            permutationhelp(remove_at(i, sequence), chosen + c, collect)

def get_permutations(sequence):
    collect = []
    permutationhelp(sequence, "", collect)
    return collect

print(get_permutations('abc'))

Output:

['abc', 'acb', 'bac', 'bca', 'cab', 'cba']