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Uploading files to the Google Blobstore

Uploading files to the Google Blobstore is far from easy.  The main difficulty is that the simulator behaves differently from the online appengine service.  Here is an example of how to get it all working. 

First, let us define a minimal table:

    Field('content', 'upload'),

db.mytable.blob_key.readable = db.mytable.blob_key.writable = False
db.mytable.original_filename.readable = db.mytable.original_filename.writable = False

Here is the main controller code:

from google.appengine.ext import blobstore

def upload():
    # As an example ONLY, we always read the item of the table with id 1.
    item = db.mytable(request.args(0))
    upload_url = URL('download', args=request.args(0))
    if item is None:
        old_blob_key = None
        form = SQLFORM(db.mytable, upload=upload_url, deletable=False)
        old_blob_key = item.blob_key
        form = SQLFORM(db.mytable, record=item, upload=upload_url, deletable=False)
    # If this is a POST, stores the upload information.
    content_ok = True
    if request.env.request_method == 'POST': 
        blob_info = None
        # Checks if there is new content. 
        if request.vars.content is not None and request.vars.content != '':
            # Decodes the new content.
                blob_info = blobstore.parse_blob_info(request.vars.content)
            except Exception, e:
                # If the file is missing, in appengine (as opposed to the development
                # environment) we get a request.vars.content with a special structure.
                # Tries to detect it.
                if ('Content-Length: 0\r\n' in request.vars.content and
                    'filename=""\r\n' in request.vars.content):
                    # Nothing was uploaded.
                    blob_info = None
                    # We really don't know how to make sense of this.
                    blob_info = None
                    content_ok = False
            if blob_info != None:
                # Checks that the upload succeeded.
                if blob_info.size == 0:
                    content_ok = False
                    form.vars.blob_key = blob_info.key()
                        form.vars.original_filename = request.vars.content.filename
                    except Exception, e:
                        content_ok = False
        # Checks if a deletion has been requested without new content.
        if blob_info is None and request.vars.content__delete == 'on':
            form.vars.original_filename = None
            form.vars.blob_key = None
            form.vars.content = None

    # Form processing.
    if form.process(onvalidation=process_upload_form(content_ok)).accepted:

        # Deletes the content field, in case the attachment has been deleted.
        if (request.vars.content__delete == 'on' and blob_info is None):
            db( ==
                content = None,
                blob_key = None, 
                original_filename = None)

        if request.vars.content__delete == 'on' and blob_info is None:
            session.flash = T('The attached file has been deleted.')
            if form.vars.blob_key == old_blob_key:
                if old_blob_key is None:
                    session.flash = T('No file has been uploaded.')
                    session.flash = T('Your attached file has been left unchanged.')
                if form.vars.blob_key is None:
                    session.flash = T('Your attached file has been left unchanged.')
                    session.flash = T('Attached file updated.')
        # For demo only, we go back to where we were.
        raise HTTP(303, Location=URL('upload', args=[]))
    elif form.errors:
        # There was an error, let's delete the newly uploaded content.
        if blob_info != None:
    # Produces an upload URL.
    upload_url = blobstore.create_upload_url(URL('upload', args=request.args))
    form['_action'] = upload_url
    return dict(form=form)

def process_upload_form(content_ok):
    def f(form):
        if not content_ok:
            form.errors.content = T('File upload error')
    return f

def download():
    item = db.mytable(request.args(0))
    if item is None:
        raise HTTP(404, "Not found")
    filename = item.original_filename or 'file'
    if item.blob_key is None:
        response.headers['Content-Disposition'] = 'attachment; filename="none"'
        import StringIO
        return''), chunk_size=1024)
    blob_info = blobstore.get(item.blob_key)
    if blob_info is None:
        #This being appengine, we need to try again.
        blob_info = blobstore.get(item.blob_key)
        if blob_info is None:
            raise HTTP(503, "Temporarily unavailable; try again later.")
    response.headers['X-AppEngine-BlobKey'] = item.blob_key;
    response.headers['Content-Type'] = blob_info.content_type;
    response.headers['Content-Disposition'] = 'attachment; filename="%s"' % filename.replace('"', '\"').replace(' ', '_')
    return response.body.getvalue()