There are TF-IDF implementations in scikit-learn
and gensim
.
There are simple implementations Simple implementation of N-Gram, tf-idf and
I guess, there are enough evidences to conclude non-existence of TF-IDF in NLTK:
Unfortunately, calculating tf-idf is not available in NLTK so we'll use another data analysis library, scikit-learn
from COMPSCI 290-01 Spring 2014 lab
More important, source code contains nothing related to tfidf (or tf-idf). Exceptions are NLTK-contrib, which contains map-reduce implementation for TF-IDF.
There are several libs for tf-idf mentioned in related question.
Upd: search by tf idf or tf_idf lets to find the function already found by @yvespeirsman
The NLTK TextCollection class has a method for computing the tf-idf of terms. The documentation is here, and the source is here. However, it says "may be slow to load", so using scikit-learn may be preferable.