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Date: Tue, 25 Feb 2014 20:21:08 -0600
From: Tim Chase <python.list@tim.thechases.com>
To: python-list@python.org
Subject: Re: intersection, union, difference, symmetric difference for dictionaries
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On 2014-02-25 23:10, Steven D'Aprano wrote:
> On Tue, 25 Feb 2014 15:03:51 -0600, Tim Chase wrote:
> 
> > On 2014-02-25 14:40, Skip Montanaro wrote:
> >> What's the correct result of evaluating this expression?
> >> 
> >> {'A': 1} | {'A': 2}
> >> 
> >> I can see (at least) two possible "correct" answers.
> > 
> > I would propose at least four:
> > 
> >   {'A': 1}   # choose the LHS
> >   {'A': 2}   # choose the RHS
> >   {'A': (1,2)} # a resulting pair of both 
> 
> Should that value be a tuple, a list or a set?

I'd say a tuple: it has order (thus not a set), and it's a fixed
record of (LHS, RHS), not mutable as a list would suggest.

> Option 5: raise an exception if the values are different.

Inconvenient in many use cases, but certainly another possibility

> Option 6: "or" the values, so that the LHS value is used only if it
> is truthy, otherwise the RHS value is used. That is:
> 
>     {'A': leftdict['A'] or rightdict['A']}
> 
> 
> I don't really understand the use-case behind Option 6

I can imagine a use case for it, but certainly not the *default*
behavior.  Yick.

> > If dicts were to support set ops, the last one would be my
> > preferred result.
> 
> What, getting a set back?
> 
> No, I disagree. If you want a set, it's easy to do:
> 
> dicta.keys() | dictb.keys()
> 
> (In Python 2, use viewkeys instead.)

Now that I'm aware of .viewkeys()/.keys() (herein "vk/k" for brevity)
thanks to Peter Otten's post, that does what I most commonly want for
set-ops-on-dicts.  I'd just consider this a promotion, since

  for k in my_dict:
    pass

is the same as

  for k in my_dict.keys(): # or .iterkeys() in 2.x
    pass

I'd find it intuitive to have set-ops behave similarly, defaulting
to vk/k.

> All of these operations are quite simple to write, so personally I
> would recommend people just add their own helper function with the
> behaviour they prefer.

And with vk/k, I now fall pretty firmly in agreement with you (except
for the aforementioned possibility of having a dict's currently-unused
set-ops use the results of vk/k as well).

-tkc