Reverse math operators (__rsub_, __radd__ etc.) interfere with non reverse operators of custom classes

[PlasmaHH]

I have a class (lets call it MyClass) that wraps a pint.Quantity object and wants to provide some maths on it via sub etc.

Now I have something like

m = MyClass(...) 
u = pint.Quantity(...)
x = m - u

Now what I would like to happen is that my classes sub handles this, however due to the python rules ( both are not the same type ) it first tries u.rsub(m) which will ultimately fail in quantity.py PlainQuantity._add_sub() because of the first checks there and a throw of DimensionalityError.

Now a common good practice is to handle the rsub case only for when you really know how to handle the other case, and otherwise return NotImplemented from within u.__rsub__(m) to then have python try m.__sub__(u) which would be the proper implementation in my case.

This might work if all the __r math functions would catch the DimensionalityError and return NotImplemented instead, because in most cases that error simply means that PlainQuantity got a type there it can't work with.

[andrewgsavage]

Try adding MyClass to pint.compat.upcast_type_map

see also #1888