title: Apply fillna as lambda to entire df with swifter
body: <p>I have a line of code to fillna in a pandas dataframe:</p>
  
  <pre><code>sessions_combined.fillna('na', inplace = True)
  </code></pre>
  
  <p>This works fine, any null values are replaced with the string 'na' which is what I desire.</p>
  
  <p>However, it's slow. Elsewhere in my code I've been using a lambda function with swifter which processes in parallel using available cores, e.g:</p>
  
  <pre><code>import swifter
  pages['dimension3'] = pages['dimension3'].swifter.apply(lambda x: my_custom_fun(x))
  </code></pre>
  
  <p>This works fine on a specific column, dimension3.</p>
  
  <p>Is it possible to use swifter on the entire dataframe? Tried:</p>
  
  <pre><code>sessions_combined.swifter.apply(lambda x: fillna(x, 'na', inplace = True))
  </code></pre>
  
  <blockquote>
    <p>NameError: name 'fillna' is not defined</p>
  </blockquote>