title: How to fill null value in object attributes in feature engineering?
body: <p>I have look into the fill null method on Kaggle in feature engineering.
  Some players fill the NA with another object value. </p>
  
  <p>For example, there are 'Male', 'Female' and NA values in sex column. The method is fill NA with another object value, like, 'Middle'. And after that, it treats the sex attribute without any null and pandas will not find null.</p>
  
  <p>I want to know the method has really good impact on machine learning model's performance or a good feature engineering?
  Besides that, is there any other good way to fill NA after no knowledgeable discovery in the data set?</p>