The design goal for my implementation was compatibility.  My version can be used as a drop in replacement for the existing urllib's HTTPHandler.  The only thing that need to be changed in the calling code is that it must call build_opener() to select the chunked handler in the place of the default HTTPHandler.  After this, the calling code can use the returned opener in the very same way as usual.

I guess, switching to a push interface would require major changes in the calling code.  

In principle, you could certainly support both schemes at the same time: you could change the internal design to a push interface and than wrap this by a pull interface for the compatibility with existing code.  But I'm not sure whether this would be worth the effort.  If, as Piotr suggests, the current urllib is going to be replaced by urllib3, then I guess, its questionable if it makes sense to add  major design changes that are incompatible with existing code to the current standard lib.