title: What is python's equivalent of R's NA?
body: <p>What is python's equivalent of R's NA?</p>
  
  <p>To be more specific:
  R has NaN, NA, NULL, Inf and -Inf. NA is generally used when there is missing data. What is python's equivalent?</p>
  
  <p>How libraries such as numpy and pandas handle missing values?</p>
  
  <p>How does scikit-learn handle missing values?</p>
  
  <p>Is it different for python 2.7 and python 3?</p>