title: How to make a cross-module variable?
body: <p>The <code>__debug__</code> variable is handy in part because it affects every module. If I want to create another variable that works the same way, how would I do it?</p>
  
  <p>The variable (let's be original and call it 'foo') doesn't have to be truly global, in the sense that if I change foo in one module, it is updated in others. I'd be fine if I could set foo before importing other modules and then they would see the same value for it.</p>