bla.py

from sklearn.preprocessing import LabelEncoder
import pandas as pd
import bisect

le = LabelEncoder()
train_c = le.fit_transform(['a', 'b', 'c', 'a'])
test_c = pd.Series(['a', 'b', 'c', 'd']).map(lambda s: '<unk>' if s not in le.classes_ else s)

print(test_c.tolist()) # ['a', 'b', 'c', '<unk>']

# very clunky!
le_classes = le.classes_.tolist()
bisect.insort_left(le_classes, '<unk>')
le.classes_ = le_classes

print(le.transform(test_c)) # [1, 2, 3, 0]