title: Using int() after concatenation of digits returns an error
body: <p>I'm using <code>itertools.combinations_with_replacement</code> to list all possible numbers with certain digits. They have to be strings to concatenate them, but when I turn them back to integers, it produces an error.</p>
  
  <pre><code>Traceback (most recent call last):
    File "p35.py", line 16, in &lt;module&gt;
      print int(num)
  ValueError: invalid literal for int() with base 10: ''
  </code></pre>
  
  <p>This is the code that I used.</p>
  
  <pre><code>from itertools import combinations_with_replacement
  for i in range(6):
      for j in combinations_with_replacement(['1','3','7','9'],i):
          num =''.join(j)
          print int(num)
  </code></pre>
  
  <p>Note that the goal is not to print <code>num</code>. I just need to convert them to integers because I need to do some numerical operations on them. This is just what reproduces the error.</p>
  
  <p>How can I join the numbers while still being able to convert them back to integers?</p>