python recursive function calls

Question

I'm trying to implement a recursive function and have run into some difficulties, would appreciate your thoughts. As an example, let's try to create a function called sliding that does this

sliding("python", 2)
["py", "yt", "th", "ho", "on"]

That is, for the chosen integer, we slide along the string, grabbing substrings of the appropriate length, and then return them all in a list.

Now here's how I might (foolishly) try to define this recursively:

def sliding(string,k):
  return s if len(string)==k else [string[:k]].append(sliding(string[1:],k))

This will not work, mainly because list.append() happens in place and returns a None. So my question is - Is there a way to do this kind of recursive function even though lots of Python methods occurs in place?

Here's the best I've got so far,

def sliding(s,k):
    if len(s)==k:
        return s
    else:
        temp = [s[:k]]
        temp.append(sliding(s[1:],k) ) 
        return temp

This results in

sliding("python",k=2)
['py', ['yt', ['th', ['ho', 'on']]]]

which is obviously not quite the desired output, but is in the right direction. What other ways might there be to do this? Thanks for your thoughts.

Answer 1

Use the + operator to get a new concatenated list:

def sliding(s, k):
    if len(s) < k: return []
    else: return [s[:k]] + sliding(s[1:], k)