Ch11 Understanding TF-IDF normalization

The explanation of tfidf shown at page326 as below.

`def tfidf(term, document, dataset):`
`term_freq = document.count(term)`
`doc_freq = math.log(sum(doc.count(term) for doc in dataset) + 1)`
`return term_freq / doc_freq`

Is it right? According to the formula, the total number of documents in the dataset is not shown in doc_freq.