Better error message when using sum() with plots

I think it’d be nice to make holoviews work with Python sum().

sum([holoviews.Area(data=(x[num], y[num])) for num in range(8)]).cols(2)

This is currently raising an exception, since sum([area1, area2, area3, ...]) is equivalent to 0 + area1 + area2 + area3 (the 0 is the start in sum(iterable, /, start=0)). And Area (or other elements) __radd__ is not implemented for 0.

Feels like using sum() would make cases like the above much more readable than alternatives using functools.reduce or loops. I think it’d be worth implementing it, even if Area() + 0 may not make so much sense per se.

[datapythonista]

You’ve got pretty amazing error messages suggesting the right thing to do for some mistakes (like a misspelled opt). Not sure how many users would think of sum, but maybe raising an error message suggesting to use Layout when _radd_ is called for 0 could be useful. Something like:

def __radd__(self, other):
    if isinstance(other, int) and other == 0:
        raise TypeError("unsupported operand type(s) for +: 'int' and 'Overlay'. "
                        "If you are using `sum(elements)` to combine a list of elements, "
                        "we recommend you to use `holoviews.Layout(elements)` instead.")
    ...

[jlstevens]

Addressed in the PR referenced above.