document = [
  'look', 'into', 'my', 'eyes', 'look', 'into', 'my', 'eyes',
 'the', 'eyes', 'the', 'eyes', 'the', 'eyes', 'not', 'around', 'the',
 'eyes', "don't", 'look', 'around', 'the', 'eyes', 'look', 'into',
 'my', 'eyes', "you're", 'under']

word_count = {}
for word in document:
  if word in word_count:
    word_count[word] += 1
 else:
    word_count[word] = 1

#假如字典中不存在给定的键，则返回参数中提供的默认值；反之，则返回字典中保存的值。
for word in document:
  previous_count = word_count.get(word, 0)
  word_count[word] = previous_count + 1
#可以合并成一行
for word in document:
 word_count[word] = word_count.setdefault(word, 0) + 1

# 使用collections中的defalutdict来实现，defalutdict是一种值可以默认设置的dict
from collections import defaultdict
word_count = defaultdict(int)
for word in document:
  word_count[word] += 1

word_counter = Counter(document)

# 返回前n个最高词频，以字典的形式
word_counter.most_common(n)