title: Inner working of fillna() when used with groupby and transform
body: <p>I am trying to learn ML by solving the Titanic ML problem at kaggle and there are many <code>Age</code> values that are missing. I am following a tutorial to solve the problem.</p>
  
  <p>if I use </p>
  
  <pre><code>median_age = df.groupby('Title')['Age'].transform('median')
  
  </code></pre>
  
  <p>then it gives me something like </p>
  
  <pre><code>&gt;&gt;&gt;median_age.sample(12)
  
  PassengerId
  1053     4.0
  891     29.0
  524     35.0
  570     29.0
  1236     4.0
  463     29.0
  359     22.0
  403     22.0
  604     29.0
  791     29.0
  512     29.0
  1290    29.0
  Name: Age, dtype: float64
  
  </code></pre>
  
  <p>These are the <code>median ages</code> respective to each category of <code>Title</code></p>
  
  <p>My question is - </p>
  
  <p><strong>If I use</strong> <code>df['Age'].fillna(median_age, inplace=True)</code> <strong>, How is it filling the values? What is the inner working of this last line?</strong></p>