title: How to read Scikit-Learn source code?
body: <p>I am learning to use scikit-learn to build a decision tree. However, when I go with the example code. I found the kernel code of the tree building is empty.</p>
  
  <p>I am using the following code:</p>
  
  <pre><code>from sklearn import tree
  X = [[0, 0], [1, 1]]
  Y = [0, 1]
  clf = tree.DecisionTreeClassifier()
  clf = clf.fit(X, Y)
  </code></pre>
  
  <p>I go to fit() method to see the details of the code. And I think the most important code for implementing decision tree is the following code at line 362 of the tree.py.</p>
  
  <pre><code> builder.build(self.tree_, X, y, sample_weight, X_idx_sorted)
  </code></pre>
  
  <p>However, when I go into the build method in _tree.py, I found that all method is empty which only contains 'pass' keyword, such as:</p>
  
  <pre><code>""" Build a decision tree in depth-first fashion. """
  def build(self, *args, **kwargs): # real signature unknown
      """ Build a decision tree from the training set (X, y). """
      pass
  </code></pre>
  
  <p>I am wondering about the strange code. I have no idea to figure it out. Am I wrong about the source code? How could this code run?</p>
  
  <p>I am using PyCharm as my IDE and using Anaconda3 as my environment....It was so strange</p>