Check file size before upload process

[lesssugar]

I'm using IS_LENGTH in my model to validate file size of uploaded image:

IS_LENGTH(2097152, 1, error_message='Max image size: 2MB')

However, the size is being checked after the file's been uploaded, which is not very user friendly when it comes to files larger than 2MB. How can I first validate the file size and then - if it's lower than 2MB - proceed with upload? Can it be done with Python or do I need JS for this?

[Massimo Di Pierro]

You cannot check before it is uploaded. If you use apache or nginx, then can check and block large uploads, but you would get a web server error, not a web2py error.

[William Chen]

I believe you can use 'onvalidation' to check the file size before upload. 

http://web2py.com/books/default/chapter/29/07/forms-and-validators#onvalidation