title: What does var,_ = something mean in Python? String concatenation?
body: <p>I am learning Python and am reading through an example script that includes some variable definitions that look like:</p>
  
  <pre><code>output,_ = call_command('git status')
  output,_ = call_command('pwd')
  
  def call_command(command):
      process = subprocess.Popen(command.split(' '),
          stdout=subprocess.PIPE,
          stderr=subprocess.PIPE)
      return process.communicate()
  </code></pre>
  
  <p>If I print output I get the resulting shell output strung together, so I know it's concatenating the variables. But I can't find any reference to the ,_ convention in any of the docs. Can someone explain it to me so that I know for sure I am using it correctly?</p>