How to remove None from the result of a function?

Question

I'm sorry, but I still can not solve this None problem. I am using an NMF algorithm to have the themes of a corpus and then I try to retrieve the documents attached to each topic. But None prevents me! When I try to retrieve the documents, I have an error

Script:

import pandas
import numpy as np
import pandas as pd
from sklearn.decomposition import NMF
from sklearn.feature_extraction.text import TfidfVectorizer

def display_topics(model, feature_names, n_top_words):
    for topic_idx, topic in enumerate(model.components_):
        print "Topic %d:" % (topic_idx)
        print " ".join([feature_names[i] for i in topic.argsort()[:-n_top_words - 1:-1]])  

text = pandas.read_csv('pretraitement_virgile.csv', encoding = 'utf-8')
good_text = text['phrase']
bad_text = text['raw_phrase']
bad_text_list = bad_text.values.tolist()
good_text_list = good_text.values.tolist()

tfidf_vectorizer = TfidfVectorizer()
tfidf = tfidf_vectorizer.fit_transform(good_text_list)
tfidf_feature_names = tfidf_vectorizer.get_feature_names()

topics_number = 3

# Run NMF
nmf = NMF(n_components=topics_number, random_state=1, alpha=.1, l1_ratio=.5, init='nndsvd').fit(tfidf)
document_topics = nmf.fit_transform(tfidf)

n_top_words = 10
print 'NMF topics'
topics = display_topics(nmf, tfidf_feature_names, n_top_words)
print topics

print 

print 'Documents per topic'
for topic in range(len(topics)): 
    if topic == None:
        pass
    else:
        print("Topic {}:".format(topic))
        docs = np.argsort(document_topics[:, topic])[::-1]
        for mail in docs[:3]:
            bad_text_list_n = " ".join(bad_text_list[mail].split(",")[:2])
            print (" ".join(good_text_list[mail].split(",")[:2]) + ',' + bad_text_list_n)

I tried to put a condition to ignore the Name, but it does not work.I still have the same error.

Topic 0:

order cancel delivery date not want store always well ahead

Topic 1:

product not broken only package arrived received color color delivered

Topic 2:

product not return site store receipt order available in advance

None

Documents par topic

Traceback (most recent call last): File "NMF.py", line 49, in for topic in range(len(topics)):

TypeError: object of type 'NoneType' has no len()

I need this result:

Topic 0:

order cancel delivery date not want store always well ahead

Topic 1:

product not broken only package arrived received color color delivered

Topic 2:

product not return site store receipt order available in advance

Documents par topic

Topic 0:

text text text

text text text

text text text

Topic 1:

text text text

text text text

text text text

Topic 2:

text text text

text text text

text text text

Some (stupid) data exemple:

phrase,raw_phrase
delicious fruit mango, the mango is a delicious fruit
important object computer, the computer is an important object
popular banana fruit, banana is a popular fruit
pen important thing, pen is an important thing
purple grape, the grape is purple
phone world object, the phone is a worldwide object

Answer 1

As pointed out by the error message your error happens in this line:

for topic in range(len(topics)): 

because python tries to get the length of the object topics, being a None Type it has no length.

If you want to skip the whole loop when topics is Null you can use:

for topic in topics: 

and change all topics[topic] to just topic

Or if you want to catch that error you can write:

try:
    l = len(topics)
except TypeError:
    # do somthing about it like:
    l = 0

for topic in range(l):
   # go on in topic loop

Or you can check for the None after the creation of the topics object with:

if variable is None:
    topics = #something else or empty with ""

Answer 2

Your procedure display_topics doesn't return anything, yet you assign the result of it to the variable topics, which is then set to Null. And you can't iterate over a Null object.