title: how to assign to slice of slice in pandas
body: <p>I have a pandas dataframe <em>df</em> as shown.</p>
  
  <pre><code>   col1  col2
  0   NaN     a
  1     2     b
  2   NaN     c
  3   NaN     d
  4     5     e
  5     6     f
  </code></pre>
  
  <p>I want to find the first NaN value in col1 and assign a new value to it. I've tried both of the following methods but none of them works.</p>
  
  <pre><code>df.loc[df['col'].isna(), 'col1'][0] = 1
  
  df.loc[df['col'].isna(), 'col1'].iloc[0] = 1
  </code></pre>
  
  <p>Both of them don't show any error or warning. But when I check the value of the original dataframe, it doesn't change. </p>
  
  <p>What is the correct way to do this?</p>