python - 在 Python 中将字符串截断为字节长度

Question

我这里有一个函数可以将给定的字符串截断为给定的字节长度:

LENGTH_BY_PREFIX = [
  (0xC0, 2), # first byte mask, total codepoint length
  (0xE0, 3), 
  (0xF0, 4),
  (0xF8, 5),
  (0xFC, 6),
]

def codepoint_length(first_byte):
    if first_byte < 128:
        return 1 # ASCII
    for mask, length in LENGTH_BY_PREFIX:
        if first_byte & mask == mask:
            return length
    assert False, 'Invalid byte %r' % first_byte

def cut_string_to_bytes_length(unicode_text, byte_limit):
    utf8_bytes = unicode_text.encode('UTF-8')
    cut_index = 0
    while cut_index < len(utf8_bytes):
        step = codepoint_length(ord(utf8_bytes[cut_index]))
        if cut_index + step > byte_limit:
            # can't go a whole codepoint further, time to cut
            return utf8_bytes[:cut_index]
        else:
            cut_index += step
    # length limit is longer than our bytes strung, so no cutting
    return utf8_bytes

在引入表情符号问题之前，这似乎工作正常:

string = u"\ud83d\ude14"
trunc = cut_string_to_bytes_length(string, 100)

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "<console>", line 5, in cut_string_to_bytes_length
  File "<console>", line 7, in codepoint_length
AssertionError: Invalid byte 152

谁能准确解释这里发生了什么，以及可能的解决方案是什么？

编辑:我这里有另一个代码片段，它不会抛出异常，但有时会出现奇怪的行为:

import encodings
_incr_encoder = encodings.search_function('utf8').incrementalencoder()

def utf8_byte_truncate(text, max_bytes):
    """ truncate utf-8 text string to no more than max_bytes long """
    byte_len = 0
    _incr_encoder.reset()
    for index,ch in enumerate(text):
        byte_len += len(_incr_encoder.encode(ch))
        if byte_len > max_bytes:
            break
    else:
        return text
    return text[:index]

>>> string = u"\ud83d\ude14\ud83d\ude14\ud83d\ude14\ud83d\ude14\ud83d\ude14" 
>>> print string
(prints a set of 5 Apple Emoji...)😔😔😔😔😔
>>> len(string)
10
>>> trunc = utf8_byte_truncate(string, 4)
>>> print trunc
???
>>> len(trunc)
1

因此对于第二个示例，我有一个 10 字节的字符串，将其截断为 4，但奇怪的事情发生了，结果是一个大小为 1 字节的字符串。

Answer 1

正如@jwpat7 指出的那样，算法是错误的。下面是一种更简单的算法，但请注意一些可感知的单个字符(称为字素)由多个 Unicode 代码点组成，例如👨‍👩‍👧‍👦。这不会尝试维护字素。

# NOTE: This is Python 2 to match OP's code

# s = u'\ud83d\ude14\ud83d\ude14\ud83d\ude14\ud83d\ude14\ud83d\ude14'
# Same as above
s = u'\U0001f614' * 5   # Unicode character U+1F614

def utf8_lead_byte(b):
    '''A UTF-8 intermediate byte starts with the bits 10xxxxxx.'''

    # (b & 0xC0) != 0x80 # Python 3 no need for ord()
    return (ord(b) & 0xC0) != 0x80

def utf8_byte_truncate(text, max_bytes):
    '''If text[max_bytes] is not a lead byte, back up until a lead byte is
    found and truncate before that character.'''
    utf8 = text.encode('utf8')
    if len(utf8) <= max_bytes:
        return utf8
    i = max_bytes
    while i > 0 and not utf8_lead_byte(utf8[i]):
        i -= 1
    return utf8[:i]

# test for various max_bytes:
for m in range(len(s.encode('utf8'))+1):
    b = utf8_byte_truncate(s,m)
    print m,len(b),b.decode('utf8')

###输出

0 0 
1 0 
2 0 
3 0 
4 4 😔
5 4 😔
6 4 😔
7 4 😔
8 8 😔😔
9 8 😔😔
10 8 😔😔
11 8 😔😔
12 12 😔😔😔
13 12 😔😔😔
14 12 😔😔😔
15 12 😔😔😔
16 16 😔😔😔😔
17 16 😔😔😔😔
18 16 😔😔😔😔
19 16 😔😔😔😔
20 20 😔😔😔😔😔