title: if looping over a dataframe and creating new columns inside that loop, will it be endless?
body: <p>I want to loop over a dataframe and manipulate each column. Say I do so by:</p>
  <pre><code>for feature in df:
          df[feature] = df[feature].apply(lambda x: manipulate(x))
          print (str(feature) + ' ready!')
  </code></pre>
  <p>Will this make me end up in an endless loop because python will iterate over all columns, including those that are newly created, or only the ones from my initial input-df?</p>