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The difference between the two used to be clearer: prior to Python 2.7, PendingDeprecationWarning was hidden by default (and thus mainly only visible to folks testing with -Wall), while DeprecationWarning was visible by default.
We blurred the line between the two thoroughly when DeprecationWarning also became hidden by default, giving the status quo:
Test frameworks: both visible by default
Interactive REPL: both hidden by default
Non-interactive execution: both hidden by default
Splitting them in the interactive REPL case would restore a meaningful behavioural difference that can help pragmatically guide decisions as to which is more appropriate to use:
Test frameworks: both visible by default
Interactive REPL: DW visible by default, PendingDW hidden by default
Non-interactive execution: both hidden by default
The current unanswerable "How do my users interpret the difference between DW and PendingDW?" question would be replaced by the much simpler "Do I want folks using the interactive REPL to see this warning or not?"