NaNs disappear after saving to excel file
<p>Missing files in my data was represented at 999, 99, 9999, so i used <code>df.replace()</code> to change them to NaNs however, NaNs disappear after saving to excel.</p>

<pre><code>df4 = df3.replace('99', np.nan)

df4.to_excel('42003hnew.xlsx', sheet_name = 'Sheet1')
</code></pre>