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This is deliberate.  If, e.g., you don't override the 
slicing operator in Test, then t[:] is handled by the base 
class's slicing operator.  The base class can't know what 
invariants Test needs to preserve, so does the best it can 
by constructing a list.  If you want operators that return 
Test instances instead, you have to supply them.

Note that base-class operators *sometimes* returned 
instances of subclasses in earlier 2.2 alphas, but 
unpredictably (depending on internal optimizations).  This 
was properly reported as a bug, and was fixed for 2.2a4; 
see bug 460020 for details.