Reading UTF-8 strings from a binary file

Question

I have some files which contains a bunch of different kinds of binary data and I'm writing a module to deal with these files.

Amongst other, it contains UTF-8 encoded strings in the following format: 2 bytes big endian stringLength (which I parse using struct.unpack()) and then the string. Since it's UTF-8, the length in bytes of the string may be greater than stringLength and doing read(stringLength) will come up short if the string contains multi-byte characters (not to mention messing up all the other data in the file).

How do I read n UTF-8 characters (distinct from n bytes) from a file, being aware of the multi-byte properties of UTF-8? I've been googling for half an hour and all the results I've found are either not relevant or makes assumptions that I cannot make.

Answer 1

Given a file object, and a number of characters, you can use:

# build a table mapping lead byte to expected follow-byte count
# bytes 00-BF have 0 follow bytes, F5-FF is not legal UTF8
# C0-DF: 1, E0-EF: 2 and F0-F4: 3 follow bytes.
# leave F5-FF set to 0 to minimize reading broken data.
_lead_byte_to_count = []
for i in range(256):
    _lead_byte_to_count.append(
        1 + (i >= 0xe0) + (i >= 0xf0) if 0xbf < i < 0xf5 else 0)

def readUTF8(f, count):
    """Read `count` UTF-8 bytes from file `f`, return as unicode"""
    # Assumes UTF-8 data is valid; leaves it up to the `.decode()` call to validate
    res = []
    while count:
        count -= 1
        lead = f.read(1)
        res.append(lead)
        readcount = _lead_byte_to_count[ord(lead)]
        if readcount:
            res.append(f.read(readcount))
    return (''.join(res)).decode('utf8')

Result of a test:

>>> test = StringIO(u'This is a test containing Unicode data: \ua000'.encode('utf8'))
>>> readUTF8(test, 41)
u'This is a test containing Unicode data: \ua000'

Answer 2

One character in UTF-8 can be 1byte,2bytes,3byte3.

If you have to read your file byte by byte, you have to follow the UTF-8 encoding rules. http://en.wikipedia.org/wiki/UTF-8

Most the time, you can just set the encoding to utf-8, and read the input stream.

You do not need to care how much bytes you have read.