def detectBOM(self):
        """Attempts to detect at BOM at the start of the stream. If
        an encoding can be determined from the BOM return the name of the
        encoding otherwise return None"""
        bomDict = {
            codecs.BOM_UTF8: 'utf-8',
            codecs.BOM_UTF16_LE: 'utf-16-le', codecs.BOM_UTF16_BE: 'utf-16-be',
            codecs.BOM_UTF32_LE: 'utf-32-le', codecs.BOM_UTF32_BE: 'utf-32-be'
        }

        # Go to beginning of file and read in 4 bytes
        string = self.rawStream.read(4)

        # Try detecting the BOM using bytes from the string
        encoding = bomDict.get(string[:3])         # UTF-8
        seek = 3
        if not encoding:
            # Need to detect UTF-32 before UTF-16
            encoding = bomDict.get(string)         # UTF-32
            seek = 4
            if not encoding:
                encoding = bomDict.get(string[:2]) # UTF-16
                seek = 2

        # Set the read position past the BOM if one was found, otherwise
        # set it to the start of the stream
        self.rawStream.seek(encoding and seek or 0)

        return encoding
Attempts to detect at BOM at the start of the stream. If
        an encoding can be determined from the BOM return the name of the
        encoding otherwise return None
html5lib/inputstream.py