title: python return statement with strings and functions
body: <pre><code>def intron_comparison(retained_introns_file, non_retained_introns_file):
      with open(retained_introns_file, 'r') as r, open(non_retained_introns_file, 'r') as nr:
          r_gc = avg_gc(r)
          nr_gc = avg_gc(nr)
          r_avglen = intlength(r)
          nr_avglen = intlength(nr)
          r_percdiv = intdiv(r)
          nr_percdiv = intdiv(nr)
      return ('The GC content for non retained introns is ' + nr_gc + '\n' +
          'The GC content for retained introns is ' + r_gc + '\n' +
          'The average length of non retained introns is ' + nr_avglen + '\n' +
          'The average length of retained introns is ' + r_avglen + '\n' +
          'The percent of non retained introns divisible by 3 is ' + nr_percdiv + '\n' +
          'The percent of retained introns divisible by 3 is ' + r_percdiv + '.')
  </code></pre>
  
  <p>Above is a function I've written that pulls together 3 other functions and compares 2 files across them. I'd like the return statement to denote a descriptor before each value, as you can see, but I am receiving the following error:</p>
  
  <pre><code>TypeError: Can't convert 'NoneType' object to str implicitly
  </code></pre>
  
  <p>regarding that last line. What am I missing?</p>
  
  <p>EDIT: The code works wonderfully, I'm just having problems with that return statement - I assume it's some small thing I am missing? Thank you in advance for your help! </p>