title: Fill in a blank dataframe column with all 0 values using Python
body: <p>I have a Python dataframe with a column called 'avg_snow' that is completely blank when I output the table to a <a href="http://en.wikipedia.org/wiki/Comma-separated_values" rel="noreferrer">CSV</a> file.</p>
  
  <p>I want to fill in the blank rows with the value 0 (datatype float). I have tried the following, but it gives me an error.</p>
  
  <pre><code>merged_left_1 = merged_left_1['avg_snow'].replace('', 0, inplace = True)
  </code></pre>
  
  <p>This is the error message I get:</p>
  
  <blockquote>
    <p>TypeError: 'NoneType' object is not subscriptable</p>
  </blockquote>