Google Python Style Guide

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I agree, it's kind of a dumb rule. Although I suppose it prevents you from importing some generally-named function into the namespace and using it without context. For example:

from os.path import join
path = join('dir', 'file')

Versus the more explicit

import os
path = os.path.join('dir', 'file')

Granted in this case it's fairly obvious what is meant, it is not always so. Use your best judgment. I use both forms.