title: Counting multiple letter groups in a string
body: <p>I've been trying to adapt my python function to count groups of letters instead of single letters and I'm having a bit of trouble. Here's the code I have to count individual letters:</p>
  <pre><code>my_seq = &quot;CTAAAGTCAACCTTCGGTTGACCTTGAAAGGGCCTTGGGAACCTTCGGTTGACCTTGAGGGTTCCCTAAGGGTT&quot;
  
  def count_letters(str):
      counts = {}
      for c in str:
          if c in counts:
              counts[c]+=1
          else:
              counts[c]=1
      return counts
  
  counts = count_letters(my_seq)
  print(counts)
  </code></pre>
  <p>The function currently spits out counts for each individual letter. Right now it prints this:</p>
  <pre><code>{'C': 23, 'T': 30, 'G': 30, 'A': 20}
  </code></pre>
  <p>Ideally, I'd like it to print something like this:</p>
  <pre><code>{'CTA': 2, 'TAG': 3, 'CGC': 1, 'GAG': 2 ... }
  </code></pre>
  <p>I'm very new to python and this is proving to be difficult.</p>