def _create_default_transfer_manager(client, config, osutil):
    """Create the default TransferManager implementation for s3transfer."""
    executor_cls = None
    if not config.use_threads:
        executor_cls = NonThreadedExecutor
    return TransferManager(client, config, osutil, executor_cls)
Create the default TransferManager implementation for s3transfer.
boto3/s3/transfer.py