jupyter中命名错误:n_features

tf_vectorizer = CountVectorizer(strip_accents = 'unicode',
                                max_features=n_features,
                                stop_words='english',
                                max_df = 0.5,
                                min_df = 10)
tf = tf_vectorizer.fit_transform(df.content_cutted)

运行结果
NameError                                 Traceback (most recent call last)
~\AppData\Local\Temp/ipykernel_8172/3651868830.py in <module>
      1 tf_vectorizer = CountVectorizer(strip_accents = 'unicode',
----> 2                                 max_features=n_features,
      3                                 stop_words='english',
      4                                 max_df = 0.5,
      5                                 min_df = 10)

NameError: name 'n_features' is not defined