如何使用NLTK ne_chunk提取GPE(位置)？

import nltk
from nltk import ne_chunk
nltk.download('maxent_ne_chunker')
nltk.download('words')

def extract_gpe(text):
    sentences = nltk.sent_tokenize(text)
    tokenized_sentences = [nltk.word_tokenize(sentence) for sentence in sentences]
    tagged_sentences = [nltk.pos_tag(sentence) for sentence in tokenized_sentences]
    chunked_sentences = nltk.ne_chunk_sents(tagged_sentences, binary=False)
    
    gpe_list = []
    for tree in chunked_sentences:
        for chunk in tree:
            if hasattr(chunk, 'label') and chunk.label() == 'GPE':
                gpe_list.append(' '.join(c[0] for c in chunk.leaves()))
    
    return gpe_list

text = "I live in New York City and work in San Francisco."
gpe_entities = extract_gpe(text)
print(gpe_entities)