title: Column assigned into a pd.DataFrame is duplicated in another pd.DataFrame
body: <p>I have the following peace of code:</p>
  <pre><code>predictions_dict['AE'] = predictions_df
  errors_dict['AE'] = (train_with_nan_df.iloc[1:] - predictions_dict['AE'])
  errors_dict['AE']['MSE'] = np.nanmean(np.power(errors_dict['AE'], 2), axis=1)
  </code></pre>
  <p>I am using dictionaries to store the predictions and errors from some different models. When I run the peace of code below, I get that both <strong>errors_dict['AE']['MSE']</strong> and <strong>predictions_dict['AE']['MSE']</strong> have values assigned, when that should be only the case for <strong>errors_dict</strong>.</p>
  <p>As additional information I, the original <strong>predictions_df</strong> shows no MSE entry, as expected. Could you explain the logic inside python making that happens? and how to solve it?</p>
  <p>I tried using .copy() with the same results.</p>