Why is my NLTK function slow when processing the DataFrame?

Your original nlkt() loops through each row 3 times.

def nlkt(val):
    val=repr(val)
    clean_txt = [word for word in val.split() if word.lower() not in stopwords.words('english')]
    nopunc = [char for char in str(clean_txt) if char not in string.punctuation]
    nonum = [char for char in nopunc if not char.isdigit()]
    words_string = ''.join(nonum)
    return words_string

Also, each time you’re calling nlkt(), you’re re-initializing these again and again.

• stopwords.words('english')
• string.punctuation

These should be global.

stoplist = stopwords.words('english') + list(string.punctuation)

Going through things line by line:

val=repr(val)

I’m not sure why you need to do this. But you could easy cast a column to a str type. This should be done outside of your preprocessing function.

Hopefully this is self-explanatory:

>>> import pandas as pd
>>> df = pd.DataFrame([[0, 1, 2], [2, 'xyz', 4], [5, 'abc', 'def']])
>>> df
   0    1    2
0  0    1    2
1  2  xyz    4
2  5  abc  def
>>> df[1]
0      1
1    xyz
2    abc
Name: 1, dtype: object
>>> df[1].astype(str)
0      1
1    xyz
2    abc
Name: 1, dtype: object
>>> list(df[1])
[1, 'xyz', 'abc']
>>> list(df[1].astype(str))
['1', 'xyz', 'abc']

Now going to the next line:

clean_txt = [word for word in val.split() if word.lower() not in stopwords.words('english')]

Using str.split() is awkward, you should use a proper tokenizer. Otherwise, your punctuations might be stuck with the preceding word, e.g.

>>> from nltk.corpus import stopwords
>>> from nltk import word_tokenize
>>> import string
>>> stoplist = stopwords.words('english') + list(string.punctuation)
>>> stoplist = set(stoplist)

>>> text="This is foo, bar and doh."

>>> [word for word in text.split() if word.lower() not in stoplist]
['foo,', 'bar', 'doh.']

>>> [word for word in word_tokenize(text) if word.lower() not in stoplist]
['foo', 'bar', 'doh']

Also checking for .isdigit() should be checked together:

>>> text="This is foo, bar, 234, 567 and doh."
>>> [word for word in word_tokenize(text) if word.lower() not in stoplist and not word.isdigit()]
['foo', 'bar', 'doh']

Putting it all together your nlkt() should look like this:

def preprocess(text):
    return [word for word in word_tokenize(text) if word.lower() not in stoplist and not word.isdigit()]

And you can use the DataFrame.apply:

data['Anylize_Text'].apply(preprocess)