title: AttributeError: 'Series' object has no attribute 'notna'
body: <p>i have a csv file with multiple columns containing empty strings. Upon reading the csv into pandas dataframe, the empty strings get converted to <em>NaN</em>.</p>
  
  <p>Now i want to append a string <code>tag-</code> to the strings already present in the columns but to only those that have some values in it and not on those with <em>NaN</em></p>
  
  <p>this is what i was trying to do:</p>
  
  <pre><code>with open('file1.csv','r') as file:
      for chunk in pd.read_csv(file,chunksize=1000, header=0, names=['A','B','C','D'])
          if len(chunk) &gt;=1:
              if chunk['A'].notna:
                  chunk['A'] = "tag-"+chunk['A'].astype(str)
              if chunk['B'].notna:
                  chunk['B'] = "tag-"+chunk['B'].astype(str)
              if chunk['C'].notna:
                  chunk['C'] = "tag-"+chunk['C'].astype(str)
              if chunk['D'].notna:
                  chunk['D'] = "tag-"+chunk['D'].astype(str)
  </code></pre>
  
  <p>and this is the error I'm getting:</p>
  
  <pre><code>AttributeError: 'Series' object has no attribute 'notna'
  </code></pre>
  
  <p>the final output that i want should be something like this:</p>
  
  <pre><code>A,B,C,D
  tag-a,tab-b,tag-c,
  tag-a,tag-b,,
  tag-a,,,
  ,,tag-c,
  ,,,tag-d
  ,tag-b,,tag-d
  </code></pre>