<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _renderBlockDevice(self, block_device, build):
 """Render all of the block device's values."""

        rendered_block_device = yield build.render(block_device)
        if rendered_block_device['volume_size'] is None:
            source_type = rendered_block_device['source_type']
            source_uuid = rendered_block_device['uuid']
            volume_size = self._determineVolumeSize(source_type, source_uuid)
            rendered_block_device['volume_size'] = volume_size
        return rendered_block_device