pandas check if column is null with query function : <p>I have pandas dataframe that I want to execute on it query function with isnull() or not isnull() condition like that:</p>

<pre><code>In [67]: df_data = pd.DataFrame({'a':[1,20,None,40,50]})
In [68]: df_data
Out[68]:       a
         0   1.0
         1  20.0
         2   NaN
         3  40.0
         4  50.0
</code></pre>

<p>if I use this command:</p>

<pre><code>df_data.query('a isnull', engine='python')
</code></pre>

<p>or this command:</p>

<pre><code>df_data.query('a isnull()', engine='python')
</code></pre>

<p>I get an error:</p>

<pre><code>In [75]: df_data.query('a isnull', engine='python')  
File "&lt;unknown&gt;", line 1    a isnull           
SyntaxError: invalid syntax

In [76]: df_data.query('a isnull()', engine='python')  
File "&lt;unknown&gt;", line 1    a isnull ()           
SyntaxError: invalid syntax
</code></pre>

<p>What is the right way to do that?</p>

<p>Thank you.</p>
