title: How add dig in a string by index without deleting anything
body: <p>How adding a dig <code>/</code> in a string that looks like this: </p>
  
  <p><code>012019</code> </p>
  
  <p>so it will look like this: </p>
  
  <p><code>01/2019</code></p>
  
  <p>Also add maybe day like </p>
  
  <p><code>01/01/2019</code></p>
  
  <p>The data:</p>
  
  <pre class="lang-py prettyprint-override"><code>import pandas as pd
  
  df= pd.DataFrame({ "month": ["012019","152019","222019","142019","302019","012020"]})
  </code></pre>
  
  <p>My code:</p>
  
  <pre class="lang-py prettyprint-override"><code>df.month = df.month.apply(lambda x: '{:0&gt;2}'.format(x.split('/')[0]))
  
  </code></pre>
  
  <p>But it does not work.</p>