move python example

demo/guide-python/predict_first_ntree.py

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+#!/usr/bin/python
+import sys
+import numpy as np
+sys.path.append('../../wrapper')
+import xgboost as xgb
+
+### load data in do training
+dtrain = xgb.DMatrix('../data/agaricus.txt.train')
+dtest = xgb.DMatrix('../data/agaricus.txt.test')
+param = {'max_depth':2, 'eta':1, 'silent':1, 'objective':'binary:logistic' }
+watchlist  = [(dtest,'eval'), (dtrain,'train')]
+num_round = 3
+bst = xgb.train(param, dtrain, num_round, watchlist)
+
+print ('start testing prediction from first n trees')
+### predict using first 1 tree
+label = dtest.get_label()
+ypred1 = bst.predict(dtest, ntree_limit=1)
+# by default, we predict using all the trees
+ypred2 = bst.predict(dtest)
+print ('error of ypred1=%f' % (np.sum((ypred1>0.5)!=label) /float(len(label))))
+print ('error of ypred2=%f' % (np.sum((ypred2>0.5)!=label) /float(len(label))))