What is the Difference between file_upload() and put_object() when uploading files to S3 using boto3

The upload_file method is handled by the S3 Transfer Manager, this means that it will automatically handle multipart uploads behind the scenes for you, if necessary.

The put_object method maps directly to the low-level S3 API request. It does not handle multipart uploads for you. It will attempt to send the entire body in one request.

One other difference I feel might be worth noticing is upload_file() API allows you to track upload using callback function. You can check about it here.

Also as already mentioned by boto's creater @garnaat that upload_file() uses multipart behind the scenes so its not straight forward to check end to end file integrity (there exists a way) but put_object() uploads whole file at one shot (capped at 5GB though) making it easier to check integrity by passing Content-MD5 which is already provided as a parameter in put_object() API.