In [1]: 
from nltk.corpus.reader import TaggedCorpusReader
from nltk.tag import AffixTagger
import pickle
In [2]: 
#trainer
reader = TaggedCorpusReader('.', 'greek_training_set.pos')
train_sents = reader.tagged_sents()

2-char prefix tagger

In [3]: 
two_prefix_tagger = AffixTagger(train_sents, affix_length=2)
In [4]: 
two_prefix_tagger.evaluate(train_sents)
Out[4]:
0.1399196687464037

3-char prefix tagger

In [5]: 
three_prefix_tagger = AffixTagger(train_sents, affix_length=3)
In [6]: 
three_prefix_tagger.evaluate(train_sents)
Out[6]:
0.166630938815114

4-char prefix tagger

In [7]: 
four_prefix_tagger = AffixTagger(train_sents, affix_length=4)
In [8]: 
four_prefix_tagger.evaluate(train_sents)
Out[8]:
0.16541243103584444