title: How to detect and mask missing data on imported csv files in Python?
body: <p>I am very new to Python and I have been trying to detect missing data in lists created from data in imported csv files so that I can plot the series using matplotlib without getting an error.</p>
  
  <p>I show you what I have:</p>
  
  <pre><code>import numpy as np
  # import matplotlib.pyplot as plt
  import csv
  from pylab import *
  
  res = csv.reader(open('cvs_file_with_data.csv'), delimiter=',')
  res.next() # do not read header
  
  ColOneData = []
  ColTwoData = []
  ColThreeData = []
  
  for col in res:
      ColOneData.append(col[0])
      ColTwoData.append(col[1])
      ColThreeData.append(col[2])
  
  print ColOneData # I got here the following ['1', '2', '3', '4', '5'] 
  
  print ColTwoData # I got here the following ['1', '2', '', '', '5']
  
  print ColThreeData # I got here the following ['', '', '3', '4', '']
  
  ColTwoData_M = np.ma.masked_where(ColTwoData == '', ColTwoData) # This does not work
  </code></pre>
  
  <p>I need to mask the empty values e.g. '' so that I can plot the series without errors. Any suggestion to solve this problem?</p>
  
  <p>Regards...</p>