python-3.x - NLTKを使用した文レベルのブルースコア

Question

このリンクをたどって文レベルを見つけていますbleu score

NLTK: コーパス レベルの BLEU とセンテンス レベルの BLEU スコア

この例を実行すると:

import nltk
hypothesis = ['This', 'is', 'cat'] 
reference = ['This', 'is', 'a', 'cat']
references = [reference]
nltk.translate.bleu_score.sentence_bleu(references, hypothesis)

出力：

8.987727354491445e-155

出力は0

警告：

/home/mac/.local/lib/python3.6/site-packages/nltk/translate/bleu_score.py:516: UserWarning: 
The hypothesis contains 0 counts of 3-gram overlaps.
Therefore the BLEU score evaluates to 0, independently of
how many N-gram overlaps of lower order it contains.
Consider using lower n-gram order or use SmoothingFunction()
  warnings.warn(_msg)
/home/mac/.local/lib/python3.6/site-packages/nltk/translate/bleu_score.py:516: UserWarning: 
The hypothesis contains 0 counts of 4-gram overlaps.
Therefore the BLEU score evaluates to 0, independently of
how many N-gram overlaps of lower order it contains.
Consider using lower n-gram order or use SmoothingFunction()
  warnings.warn(_msg)

を持つ文に対して同じ関数を実行するにはどうすればよいn-gram for n<4ですか?

また、内部で渡すことができるすべてのパラメーターnltk.translate.bleu_score.sentence_bleu()は何ですか?