def read(self, size=-1):
        """Read at most `size` bytes from the file (less if there
        isn't enough data).

        The bytes are returned as an instance of :class:`str` (:class:`bytes`
        in python 3). If `size` is negative or omitted all data is read.

        :Parameters:
          - `size` (optional): the number of bytes to read
        """
        self._ensure_file()

        if size == 0:
            return EMPTY

        remainder = int(self.length) - self.__position
        if size < 0 or size > remainder:
            size = remainder

        received = 0
        data = StringIO()
        while received < size:
            chunk_data = self.readchunk()
            received += len(chunk_data)
            data.write(chunk_data)

        # Detect extra chunks.
        max_chunk_n = math.ceil(self.length / float(self.chunk_size))
        chunk = self.__chunks.find_one({"files_id": self._id,
                                        "n": {"$gte": max_chunk_n}})
        # According to spec, ignore extra chunks if they are empty.
        if chunk is not None and len(chunk['data']):
            raise CorruptGridFile(
                "Extra chunk found: expected %i chunks but found "
                "chunk with n=%i" % (max_chunk_n, chunk['n']))

        self.__position -= received - size

        # Return 'size' bytes and store the rest.
        data.seek(size)
        self.__buffer = data.read()
        data.seek(0)
        return data.read(size)
Read at most `size` bytes from the file (less if there
        isn't enough data).

        The bytes are returned as an instance of :class:`str` (:class:`bytes`
        in python 3). If `size` is negative or omitted all data is read.

        :Parameters:
          - `size` (optional): the number of bytes to read