title: what does {:02d} mean in Python
body: <p>it's very hard to find information about <code>{:}</code> online
  I saw some code below:</p>
  
  <pre><code>def dateformat(date):
      day, month, year=date.split('/')
      return "{:4d}{:02d}{:02d}".format(int(year),int(month),int(day))
  </code></pre>
  
  <p>I kinda of know it is filling leading <code>0</code> in the format, but I don't know what do <code>'02'</code> and <code>'d'</code> in <code>{:02d}</code> do?</p>