NP-chunker value error (Python nltk) - python-3.x

Question

I am building an NLP-pipeline based on the Python NLTK book (chapter 7). The first segment of codes correctly preprocesses the data, but I am unable to run its output through my NP-chunker:
import nltk, re, pprint
#Import Data
data = 'This is a test sentence to check if preprocessing works'
#Preprocessing
def preprocess(document):
sentences = nltk.sent_tokenize(document)
sentences = [nltk.word_tokenize(sent) for sent in sentences]
sentences = [nltk.pos_tag(sent) for sent in sentences]
return(sentences)
tagged = preprocess(data)
print(tagged)
#regular expression-based NP chunker
grammar = "NP: {<DT>?<JJ>*<NN>}"
cp = nltk.RegexpParser(grammar) #chunk parser
chunked = []
for s in tagged:
chunked.append(cp.parse(tagged))
print(chunked)
This is the traceback I get:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Users\u0084411\AppData\Local\Continuum\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 866, in runfile
execfile(filename, namespace)
File "C:\Users\u0084411\AppData\Local\Continuum\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 102, in execfile
exec(compile(f.read(), filename, 'exec'), namespace)
File "C:/Users/u0084411/Box Sync/Procesmanager DH/Text Mining/Tools/NLP_pipeline.py", line 24, in <module>
chunked.append(cp.parse(tagged))
File "C:\Users\u0084411\AppData\Local\Continuum\Anaconda3\lib\site-packages\nltk\chunk\regexp.py", line 1202, in parse
chunk_struct = parser.parse(chunk_struct, trace=trace)
File "C:\Users\u0084411\AppData\Local\Continuum\Anaconda3\lib\site-packages\nltk\chunk\regexp.py", line 1017, in parse
chunkstr = ChunkString(chunk_struct)
File "C:\Users\u0084411\AppData\Local\Continuum\Anaconda3\lib\site-packages\nltk\chunk\regexp.py", line 95, in __init__
tags = [self._tag(tok) for tok in self._pieces]
File "C:\Users\u0084411\AppData\Local\Continuum\Anaconda3\lib\site-packages\nltk\chunk\regexp.py", line 95, in <listcomp>
tags = [self._tag(tok) for tok in self._pieces]
File "C:\Users\u0084411\AppData\Local\Continuum\Anaconda3\lib\site-packages\nltk\chunk\regexp.py", line 105, in _tag
raise ValueError('chunk structures must contain tagged '
ValueError: chunk structures must contain tagged tokens or trees
>>>
What is my mistake here? 'Tagged' is tokenized, so why does the program not recognize this?
Many thanks!
Tom

Answer 1

You'll slap your forehead when you see this. Instead of this
for s in tagged:
chunked.append(cp.parse(tagged))
it should have been this:
for s in tagged:
chunked.append(cp.parse(s))
You were getting the error because you were not passing cp.parse() a tagged sentence, but a list of them.