Django File Upload Handler Errors

Question

In Django, I want to stop any file uploads that don't end in my designated extension as soon as they're received. To do this, I define a custom Upload Handler and write new_file to look something like this:

def new_file(self, field_name, file_name, content_type, content_length, charset=None):
    basename, extension = os.path.splitext(file_name)
    if extension != ".txt":
        raise StopUpload(True) # Error: Only .txt files are accepted

This works fine for stopping the upload but it also clears request.FILES. Now, when my view receives the request, it has no way to telling whether the upload handler caused the file to be missing and I can't display a useful message to the user.

Is there any way to propagate messages from the Upload Handler to the corresponding view, such that I can display the error to the user? I've tried using the request object, but it is immutable.

Answer 1

I've found a reason for why I couldn't get it working and a solution as well.

It would appear that Django uses lazy evaluation for request.FILES to determine when the upload handler is called. Therefore, the upload handler is only evoked when and if you attempt to access request.FILES. Additionally, the request object I am using (WSGIRequest in my case) has made GET and POST immutable dictionaries, so we can't pass information through there. However, META is still available to add information to.

My combined solution has the line "request.FILES" in the view that handles uploads, which forces the upload handler to begin. When the error is captured in new_files, I set self.request.META['error'] to the error message and raise StopUpload, which pushes us back into the view without a file. Finally, I check for request.META['error'] in the view and display that message when there is a problem.

I hope this helps!