TfidfVectorizer in scikit-learn : ValueError: np.nan is an invalid document

I was getting MemoryError even after using .values.astype('U') for the reviews in my dataset.

So i tried .astype('U').values and it worked.

This is a answer from: Python: how to avoid MemoryError when transform text data into Unicode using astype('U')

You need to convert the dtype object to unicode string as is clearly mentioned in the traceback.

x = v.fit_transform(df['Review'].values.astype('U'))  ## Even astype(str) would work

From the Doc page of TFIDF Vectorizer:

fit_transform(raw_documents, y=None)

Parameters: raw_documents : iterable
an iterable which yields either str, unicode or file objects

I find a more efficient way to solve this problem.

x = v.fit_transform(df['Review'].apply(lambda x: np.str_(x)))

Of course you can use df['Review'].values.astype('U') to convert the entire Series. But I found using this function will consume much more memory if the Series you want to convert is really big. (I test this with a Series with 800k rows of data, and doing this astype('U') will consume about 96GB of memory)

Instead, if you use the lambda expression to only convert the data in the Series from str to numpy.str_, which the result will also be accepted by the fit_transform function, this will be faster and will not increase the memory usage.

I'm not sure why this will work because in the Doc page of TFIDF Vectorizer:

fit_transform(raw_documents, y=None)

Parameters: raw_documents : iterable

an iterable which yields either str, unicode or file objects

But actually this iterable must yields np.str_ instead of str.