Terry Reedy writes:
This assumes the same range set (of addable items) also. If Python were to add d1 + d2 and d1 += d2, I think we should use this existing and most common definition and add values.
IMHO[1] that's way too special for the generic mapping types. If one wants such operations, she should define NumericValuedMapping and StringValuedMapping etc classes for each additive set of values.
For functions on different domains, I've seen it used to mean "apply the appropriate function on the disjoint union of the domains".
According to https://en.wikipedia.org/wiki/Disjoint_union, d_u has at least two meaning.
Either meaning will do here, with the distinction that the set- theoretic meaning (which I intended) applies to any two functions, while the alternate meaning imposes a restriction on the functions that can be added (and therefore is inappropriate for this discussion IMHO). Footnotes: [1] I mean the "H", I'm no authority.