# train and test are pandas.DataFrame's and c is whatever column
le = LabelEncoder()
le.fit(train[c])
test[c] = test[c].map(lambda s: '' if s not in le.classes_ else s)
le.classes_ = np.append(le.classes_, '')
train[c] = le.transform(train[c])
test[c] = le.transform(test[c])