Aaron Meurer <asmeu...@gmail.com> added the comment:

If it's of any interest to this discussion, for SymPy (for some time) we have 
used a custom subclass of DeprecationWarning that we enable by default 
https://github.com/sympy/sympy/blob/master/sympy/utilities/exceptions.py. I 
don't know if there are major libraries that do something similar. 

Our reasoning is that we really do want everybody to see the warnings. 
Obviously direct users of SymPy (both interactive users and library developers) 
need to see them so they can fix their code. But also if library X uses a 
deprecated behavior and a user of library X sees a deprecation warning for 
SymPy inside of library X, that incentivises them to bug the library X 
developers to fix the behavior (or PR it). The whole point of warnings as we 
see it is to be as loud as possible while still keeping things working, to 
avoid the situation where things stop working (when the deprecated behavior is 
removed). 

And +<however many points I'm allowed to have> to Nathaniel's point that 
DeprecationWarnings are about more than just the standard library. Tons of 
libraries use the built in warnings, and the default warnings behavior makes no 
distinction between warnings coming from the standard library and warnings 
coming from other places.

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<https://bugs.python.org/issue24294>
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