title: Looping through a list of pandas dataframes
body: <p>Two quick pandas questions for you.</p>
  
  <ol>
  <li><p>I have a list of dataframes I would like to apply a filter to.</p>
  
  <pre><code>countries = [us, uk, france]
  for df in countries:
      df = df[(df["Send Date"] &gt; '2016-11-01') &amp; (df["Send Date"] &lt; '2016-11-30')] 
  </code></pre>
  
  <p>When I run this, the df's don't change afterwards. Why is that?
  If I loop through the dataframes to create a new column, as below, this works fine, and changes each df in the list. </p>
  
  <pre><code> for df in countries:
        df["Continent"] = "Europe"
  </code></pre></li>
  <li><p>As a follow up question, I noticed something strange when I created a list of dataframes for different countries. I defined the list then applied transformations to each df in the list. After I transformed these different dfs, I called the list again. I was surprised to see that the list still pointed to the unchanged dataframes, and I had to redefine the list to update the results. Could anybody shed any light on why that is?</p></li>
  </ol>