title: Error while passing parameters for DecisionTreeClassifier
body: <p>I am try DecisionTreeClassifier with parameters in a string.</p>
  
  <pre><code> print d    # d= 'max_depth=100'
   clf = DecisionTreeClassifier(d)
   clf.fit(X[:3000,], labels[:3000])
  </code></pre>
  
  <p>I am getting below error for this case. If I use <code>clf = DecisionTreeClassifier(max_depth=100)</code> it works fine.</p>
  
  <pre><code>Traceback (most recent call last):
    File "train.py", line 120, in &lt;module&gt;
      grid_search_generalized(X, labels, {"max_depth":[i for i in range(100, 200)]})
    File "train.py", line 51, in grid_search_generalized
      clf.fit(X[:3000,], labels[:3000])
    File "/usr/local/lib/python2.7/dist-packages/sklearn/tree/tree.py", line 790, in fit
      X_idx_sorted=X_idx_sorted)
    File "/usr/local/lib/python2.7/dist-packages/sklearn/tree/tree.py", line 326, in fit
      criterion = CRITERIA_CLF[self.criterion](self.n_outputs_,
  KeyError: 'max_depth=100'
  </code></pre>