Install the Transformers, Datasets, and Evaluate libraries to run this notebook.
!pip install datasets evaluate transformers[sentencepiece]
corpus = [
"This is the Hugging Face Course.",
"This chapter is about tokenization.",
"This section shows several tokenizer algorithms.",
"Hopefully, you will be able to understand how they are trained and generate tokens.",
]
from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained("bert-base-cased")
from collections import defaultdict
word_freqs = defaultdict(int)
for text in corpus:
words_with_offsets = tokenizer.backend_tokenizer.pre_tokenizer.pre_tokenize_str(text)
new_words = [word for word, offset in words_with_offsets]
for word in new_words:
word_freqs[word] += 1
word_freqs
defaultdict(
int, {'This': 3, 'is': 2, 'the': 1, 'Hugging': 1, 'Face': 1, 'Course': 1, '.': 4, 'chapter': 1, 'about': 1,
'tokenization': 1, 'section': 1, 'shows': 1, 'several': 1, 'tokenizer': 1, 'algorithms': 1, 'Hopefully': 1,
',': 1, 'you': 1, 'will': 1, 'be': 1, 'able': 1, 'to': 1, 'understand': 1, 'how': 1, 'they': 1, 'are': 1,
'trained': 1, 'and': 1, 'generate': 1, 'tokens': 1})
alphabet = []
for word in word_freqs.keys():
if word[0] not in alphabet:
alphabet.append(word[0])
for letter in word[1:]:
if f"##{letter}" not in alphabet:
alphabet.append(f"##{letter}")
alphabet.sort()
alphabet
print(alphabet)
['##a', '##b', '##c', '##d', '##e', '##f', '##g', '##h', '##i', '##k', '##l', '##m', '##n', '##o', '##p', '##r', '##s', '##t', '##u', '##v', '##w', '##y', '##z', ',', '.', 'C', 'F', 'H', 'T', 'a', 'b', 'c', 'g', 'h', 'i', 's', 't', 'u', 'w', 'y']
vocab = ["[PAD]", "[UNK]", "[CLS]", "[SEP]", "[MASK]"] + alphabet.copy()
splits = {
word: [c if i == 0 else f"##{c}" for i, c in enumerate(word)]
for word in word_freqs.keys()
}
def compute_pair_scores(splits):
letter_freqs = defaultdict(int)
pair_freqs = defaultdict(int)
for word, freq in word_freqs.items():
split = splits[word]
if len(split) == 1:
letter_freqs[split[0]] += freq
continue
for i in range(len(split) - 1):
pair = (split[i], split[i + 1])
letter_freqs[split[i]] += freq
pair_freqs[pair] += freq
letter_freqs[split[-1]] += freq
scores = {
pair: freq / (letter_freqs[pair[0]] * letter_freqs[pair[1]])
for pair, freq in pair_freqs.items()
}
return scores
pair_scores = compute_pair_scores(splits)
for i, key in enumerate(pair_scores.keys()):
print(f"{key}: {pair_scores[key]}")
if i >= 5:
break
('T', '##h'): 0.125
('##h', '##i'): 0.03409090909090909
('##i', '##s'): 0.02727272727272727
('i', '##s'): 0.1
('t', '##h'): 0.03571428571428571
('##h', '##e'): 0.011904761904761904
best_pair = ""
max_score = None
for pair, score in pair_scores.items():
if max_score is None or max_score < score:
best_pair = pair
max_score = score
print(best_pair, max_score)
('a', '##b') 0.2
vocab.append("ab")
def merge_pair(a, b, splits):
for word in word_freqs:
split = splits[word]
if len(split) == 1:
continue
i = 0
while i < len(split) - 1:
if split[i] == a and split[i + 1] == b:
merge = a + b[2:] if b.startswith("##") else a + b
split = split[:i] + [merge] + split[i + 2 :]
else:
i += 1
splits[word] = split
return splits
splits = merge_pair("a", "##b", splits)
splits["about"]
['ab', '##o', '##u', '##t']
vocab_size = 70
while len(vocab) < vocab_size:
scores = compute_pair_scores(splits)
best_pair, max_score = "", None
for pair, score in scores.items():
if max_score is None or max_score < score:
best_pair = pair
max_score = score
splits = merge_pair(*best_pair, splits)
new_token = (
best_pair[0] + best_pair[1][2:]
if best_pair[1].startswith("##")
else best_pair[0] + best_pair[1]
)
vocab.append(new_token)
print(vocab)
['[PAD]', '[UNK]', '[CLS]', '[SEP]', '[MASK]', '##a', '##b', '##c', '##d', '##e', '##f', '##g', '##h', '##i', '##k', '##l', '##m', '##n', '##o', '##p', '##r', '##s', '##t', '##u', '##v', '##w', '##y', '##z', ',', '.', 'C', 'F', 'H', 'T', 'a', 'b', 'c', 'g', 'h', 'i', 's', 't', 'u', 'w', 'y', 'ab', '##fu', 'Fa', 'Fac', '##ct', '##ful', '##full', '##fully', 'Th', 'ch', '##hm', 'cha', 'chap', 'chapt', '##thm', 'Hu', 'Hug', 'Hugg', 'sh', 'th', 'is', '##thms', '##za', '##zat', '##ut']
def encode_word(word):
tokens = []
while len(word) > 0:
i = len(word)
while i > 0 and word[:i] not in vocab:
i -= 1
if i == 0:
return ["[UNK]"]
tokens.append(word[:i])
word = word[i:]
if len(word) > 0:
word = f"##{word}"
return tokens
print(encode_word("Hugging"))
print(encode_word("HOgging"))
['Hugg', '##i', '##n', '##g'] ['[UNK]']
def tokenize(text):
pre_tokenize_result = tokenizer._tokenizer.pre_tokenizer.pre_tokenize_str(text)
pre_tokenized_text = [word for word, offset in pre_tokenize_result]
encoded_words = [encode_word(word) for word in pre_tokenized_text]
return sum(encoded_words, [])
tokenize("This is the Hugging Face course!")
['Th', '##i', '##s', 'is', 'th', '##e', 'Hugg', '##i', '##n', '##g', 'Fac', '##e', 'c', '##o', '##u', '##r', '##s', '##e', '[UNK]']