cpython/Lib/encodings/utf_8_sig.py

""" Python 'utf-8-sig' Codec
This work similar to UTF-8 with the following changes:

* On encoding/writing a UTF-8 encoded BOM will be prepended/written as the
  first three bytes.

* On decoding/reading if the first three bytes are a UTF-8 encoded BOM, these
  bytes will be skipped.
"""
import codecs

### Codec APIs

def encode(input, errors='strict'):
    return (codecs.BOM_UTF8 + codecs.utf_8_encode(input, errors)[0], len(input))

def decode(input, errors='strict'):
    prefix = 0
    if input.startswith(codecs.BOM_UTF8):
        input = input[3:]
        prefix = 3
    (output, consumed) = codecs.utf_8_decode(input, errors, True)
    return (output, consumed+prefix)

class IncrementalEncoder(codecs.IncrementalEncoder):
    def __init__(self, errors='strict'):
        codecs.IncrementalEncoder.__init__(self, errors)
        self.first = True

    def encode(self, input, final=False):
        if self.first:
            self.first = False
            return codecs.BOM_UTF8 + codecs.utf_8_encode(input, self.errors)[0]
        else:
            return codecs.utf_8_encode(input, self.errors)[0]

    def reset(self):
        codecs.IncrementalEncoder.reset(self)
        self.first = True

class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
    def __init__(self, errors='strict'):
        codecs.BufferedIncrementalDecoder.__init__(self, errors)
        self.first = True

    def _buffer_decode(self, input, errors, final):
        if self.first and codecs.BOM_UTF8.startswith(input): # might be a BOM
            if len(input) < 3:
                # not enough data to decide if this really is a BOM
                # => try again on the next call
                return (u"", 0)
            (output, consumed) = codecs.utf_8_decode(input[3:], errors, final)
            self.first = False
            return (output, consumed+3)
        return codecs.utf_8_decode(input, errors, final)

    def reset(self):
        codecs.BufferedIncrementalDecoder.reset(self)
        self.first = True

class StreamWriter(codecs.StreamWriter):
    def reset(self):
        codecs.StreamWriter.reset(self)
        try:
            del self.encode
        except AttributeError:
            pass

    def encode(self, input, errors='strict'):
        self.encode = codecs.utf_8_encode
        return encode(input, errors)

class StreamReader(codecs.StreamReader):
    def reset(self):
        codecs.StreamReader.reset(self)
        try:
            del self.decode
        except AttributeError:
            pass

    def decode(self, input, errors='strict'):
        if len(input) < 3 and codecs.BOM_UTF8.startswith(input):
            # not enough data to decide if this is a BOM
            # => try again on the next call
            return (u"", 0)
        self.decode = codecs.utf_8_decode
        return decode(input, errors)

### encodings module API

def getregentry():
    return codecs.CodecInfo(
        name='utf-8-sig',
        encode=encode,
        decode=decode,
        incrementalencoder=IncrementalEncoder,
        incrementaldecoder=IncrementalDecoder,
        streamreader=StreamReader,
        streamwriter=StreamWriter,
    )