Storing a file in the blobstore from a URL in app engine?

I want to retrieve a file form a URL like http://www.abc.com/files/file.pdf and store in the blobstore... how 开发者_如何学编程do I do this?

Right now it only allows from form posts ...

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http://code.google.com/appengine/docs/python/urlfetch/fetchfunction.html and then POST it yourself?

Chances are Google won't allow you to just pass it arbitrary URLs anytime soon to avoid using it as a DDOS.