title: Converting a number to binary with a fixed length
body: <p>Say I get a random number between 1 and 127. I change the number to binary and remove the <code>0b</code> from it with the following code:</p>
  <pre><code>key_one= int(raw_input(&quot;Enter key (0 &lt;= key &lt;= 127): &quot;))
  
  if key_one in range(128):
      bin_key_one=bin(key_one)[2:]
  print bin_key_one
  else:
      print &quot;You have to enter key (0 &lt;= key &lt;= 127)&quot;
  </code></pre>
  <p>Now I want to make it 7-characters long by padding the beginning with zeros as necessary. I think I need to use a for loop, but can someone show me how to do it?</p>