Question

I have an image in blob store which is uploaded by users(their profile pic). I want to make a copy of the same and and re-size the copy so that it can be displayed as a thumbnail. I want to make a copy of the same instead of using the ImageService because this would be used more often compared to the profile image.

What I am doing here is this:

reader = profile_image.open() #get binary data from blob
data = reader.read()

file_name = files.blobstore.create(mime_type=profile_image.content_type)#file to write to

with files.open(file_name, 'a') as f:
   f.write(data)

files.finalize(file_name)


blob_key = files.blobstore.get_blob_key(file_name)

image = images.Image(blob_key = blob_key)
image.resize(width=32, height=32)

entity.small_profile_pic = <MyImageModel>(caption=<caption given by user>,
                                       picture=str(blob_key))

This is giving me error:

BadValueError: Image instance must have a complete key before it can be stored as a reference.

I think this is because the blob is not saved(put()) into the datastore, but how do i do it. Doed files.blobstore.get_blob_key(file_name) not do it ?

I would also like to ask: does the blobstore also cache the dynamically transformed images images served using get_serving_url() …

Answer 1

I would use the get_serving_url method. In the doc is stated that:

The get_serving_url() method allows you to generate a stable, dedicated URL for serving web-suitable image thumbnails. You simply store a single copy of your original image in Blobstore, and then request a high-performance per-image URL. This special URL can serve that image resized and/or cropped automatically, and serving from this URL does not incur any CPU or dynamic serving load on your application (though bandwidth is still charged as usual). Images are served with low latency from a highly optimized, cookieless infrastructure.

Also the code you posted doesn’t seem to follow the exampled posted in the docs. I would use something like this

img = images.Image(blob_key=original_image_key)
img.resize(width=32, height=32)
thumbnail = img.execute_transforms(output_encoding=images.JPEG)
file_name = files.blobstore.create(mime_type='image/jpeg')#file to write to
with files.open(file_name, 'a') as f: 
    f.write(thumbnail)
files.finalize(file_name)
blob_key = files.blobstore.get_blob_key(file_name)