Create a new string from an old string using python(1)

My function has three string parameters, string, search and replace. If the search argument is an empty string, the function should insert replaceable arguments before the first argument, between each character of the old string, and after the last character of the old string. Here's what I've done so far:

def main():
    s='I am going go'
    x=""
    y='aa'

    print(rep_str(s,x,y))

def rep_str(s, x, y):

    if x in s:
        result = ''
        i = 0
        while i < len(s):
            if s[i : i + len(x)] == x:
                result += y
                i += len(x)
            else:
                result += s[i]
                i += 1


    elif x not in s:
        result= s
    else:
        if x=="":
            result=y        
            for ch in s:
                result+=(ch+y)   

    return result
main()  

I developed each condition separately and put them together in a function when I got a satisfactory result. My last other condition is that it works fine in a separate run, but not in a function module. I don't know what's wrong with the code. I would appreciate if someone could give me some guidance. thanks

The output of the last else condition should be:

aaIaa aaaaamaa aagaaoaaiaanaagaa aagaaoaa