Better error message when _cext is missing

Addresses #96

setup.py

@@ -14,7 +14,7 @@ def run_setup(with_binary):
 
     setup(
         name='shap',
-        version='0.16.1',
+        version='0.16.2',
         description='A unified approach to explain the output of any machine learning model.',
         url='http://github.com/slundberg/shap',
         author='Scott Lundberg',

shap/explainers/tree.py

@@ -2,8 +2,10 @@
 import multiprocessing
 import sys
 
+have_cext = False
 try:
     from .. import _cext
+    have_cext = True
 except ImportError:
     pass
 except:
@@ -167,8 +169,8 @@ def _tree_shap_ind(self, i):
         return phi
 
     def tree_shap(self, tree, x, x_missing, phi, condition=0, condition_feature=0):
-
         # start the recursive algorithm
+        assert have_cext, "C extension was not built during install!"
         _cext.tree_shap(
             tree.max_depth, tree.children_left, tree.children_right, tree.children_default, tree.features,
             tree.thresholds, tree.values, tree.node_sample_weight,
@@ -187,6 +189,7 @@ def __init__(self, children_left, children_right, children_default, feature, thr
         self.node_sample_weight = node_sample_weight
 
         # we compute the expectations to make sure they follow the SHAP logic
+        assert have_cext, "C extension was not built during install!"
         self.max_depth = _cext.compute_expectations(
             self.children_left, self.children_right, self.node_sample_weight,
             self.values