[Python-ideas] Adding dictionary merge operator(s) [was: Allow using ** twice]

[Stephen J. Turnbull]

Updating the topic.

Masklinn writes:

 > One of the rather annoying things in dict.update is that it alters the
 > caller in place,

If it didn't, it wouldn't be "update", it would be "merge".

 > so it can't be used to merge multiple dicts in an expression.

The fundamental problem is that "dict.merge" is not a well-defined
operation.  Instead, it's more like image composition, which is a
family of trinary operators (something like 26 of them), combining a
target, source, and alpha mask in various ways.

"Update" presumably is the most often used operator, but once you get
into *any* variations from that, there are lots of plausible ones.

 > > The "first found wins" interpretation could use a different method:
 > > 
 > >    {}.extend(d1, d2, …)
 > 
 > IIRC in underscore.js this is called "defaults", d1, d2, … are a
 > "stack" of applicable defaults, and thus are merged into the
 > subject if and only if the corresponding keys are missing from
 > the subject.

The "stack of defaults" interpretation of the list seems natural to
me, but it's not obvious to me whether the list *constructs* the stack
by pushing each argument in turn (ie, di is consulted in R2L order),
or *is* the stack (so each di is consulted in L2R order).  I guess I
would use "fallback", so that

    d0.fallback(d1, d2, ...)

reads "start with d0, fall back to d1 for missing keys, fall back to
d2 for still missing keys, ...".  YMMV.

Then-again-maybe-I've-just-been-programming-Lisp-for-too-long-ly y'rs,