Python3's '/' operator is different from Python2's '/' operator. This is clear when you try to overload them. Python2's is called '__div__', while Python3's is '__truediv__'.

The Python3's operator '/' (on numbers) is defined to return a float, and it'll convert any integer parameters as required. The same is true for other operators, like 'True + True' being 2.

Because of these definitions, you should consider the operators as explicit conversions. 'total / sum' is less explicit than 'total / float(sum)', but still clear enough.

And about guidelines, there are others that fit this case:

    Simple is better than complex.
    Flat is better than nested.
    Readability counts.
    [...] practicality beats purity.
    

PS: I think you made an important argument and I'm glad you did it, even though I disagree with it. I fail to see why you are getting downvotes.

I think it's better to think of it as division being an operation that returns the appropriate type for the operation, i.e., a float where necessary. The type of the operands remain unchanged so "upcasting" is not applicable if you ask me.