How to know what to install (Ubuntu/Debian) for a given import?

[Chris Green]

I'm often hitting this problem, how does one find out what package to
install to provide what a give import needs?

Currently I'm modifying some code which has 'import gtk', I want to
migrate from Python 2 to Python 3 if I can but at the moment the
import fails in Python 3.

There are dozens of packages in the Ubuntu repositories which *might*
provide what I need I don't want to try them all!  So, is there an
easy way to find out?

... and while I'm here, can someone tell me what package I need?

-- 
Chris Green
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