title: why should I make a copy of a data frame in pandas
body: <p>When selecting a sub dataframe from a parent dataframe, I noticed that some programmers make a copy of the data frame using the <code>.copy()</code> method. For example,</p>
  <pre class="lang-py prettyprint-override"><code>X = my_dataframe[features_list].copy()
  </code></pre>
  <p>...instead of just</p>
  <pre class="lang-py prettyprint-override"><code>X = my_dataframe[features_list]
  </code></pre>
  <p>Why are they making a copy of the data frame? What will happen if I don't make a copy?</p>