title: Pandas pd.isnull() function
body: <p>I need to replace not null values in my dataframe with 1  and null values with 0.</p>
  
  <p>Here is my dataframe:</p>
  
  <pre><code>my_list= [['a','b','c'],['test1','test2',None],[None,'101','000']]
  
  mydf= pd.DataFrame(my_list,columns=['col1','col2','col3'])
  
  mydf
  
      col1   col2  col3
  0      a      b     c
  1  test1  test2  None
  2   None    101   000
  
  mydf.where((pd.isnull(mydf)),0,inplace=True)
  
  mydf
  
     col1 col2  col3
  0     0    0     0
  1     0    0  None
  2  None    0     0
  </code></pre>
  
  <p>I am not sure why it is replacing not null values with zero. pd.notnull() does the opposite. Can anyone explain what I am missing here?</p>