title: pexpect child.before is empty when output is more than 2000 lines
body: <p>I'm using the pexpect module with Python3.4 to automate a few commands in a junos router.
  I'm successfully able to get into the router and execute the commands. However I want to save the output of the command <code>show configuration | display sets | no more</code> in a <code>child.before</code> variable, but it's empty. <code>Child.before</code> is getting populated properly if the output is only a few lines. But here, the output is around 2000 lines long.</p>
  
  <p>The code I used is:</p>
  
  <pre><code>self.child.sendline('show configuration | display set | no-more')
  self.child.expect('&gt;')
  output = self.child.before
  print(output)
  </code></pre>
  
  <p>Output is returning empty if the command returns a large number of lines.
  I checked if there is a max read which I can set but even that didn't help me out.</p>
  
  <p>Can someone show me what can be done here?</p>