[Python-ideas] Enhancing dict.values

Steven D'Aprano steve at pearwood.info
Fri May 27 12:17:40 EDT 2016

On Fri, May 27, 2016 at 06:43:35AM -0700, Steve Dower wrote:
> Neat!
> My bikeshed is coloured `dict.getmany(*keys, default=None)`, 

"getmany" doesn't tell you, many of what?

> and while 
> returning a namedtuple might be cool, an iterator is probably the way 
> to go.

The disadvantage of a namedtuple is that every invocation would create a 
new class, which is then used once and once only for a singleton 
instance. Could get very expensive.

I don't think an iterator would be needed. The motivating use-case is 
for sequence unpacking:

# apologies for breaking my own rule about realistic names
fee, fi, fo, fum = mydict.getmany('fee', 'fi', 'fo', 'fum')

so I don't think the lazy aspect of an iterator is useful. It's going to 
be consumed eagerly, and immediately. I think a regular tuple is better. 
That also matches the behaviour of itemgetter:

py> d = {'fe': 1, 'fi': 2, 'fo': 3, 'fum': 4}
py> from operator import itemgetter
py> f = itemgetter('fe', 'fi', 'fo', 'fum')
py> f(d)
(1, 2, 3, 4)

So the core functionality already exists, it's just hidden away in the 
operator module. (Guido's time machine strikes again.)

Open questions:

- Is it worth making this a dict method? +1 from me.

- Name? "getvalues"?

- Any other functionality? 

Possibly a keyword-only "default" argument:

  mydict.getvalues(*keys, default=None)

On more shaky ground, how about a "pop" argument?

  mydict.getvalues(*keys, pop=True)
will delete the keys as well as return the values. Use-case: methods 
(particularly __init__ or __new__) which take extra keyword args which 
need to be popped before calling super.

    def __init__(self, **kwargs):
        colour, height = kwargs.getvalues('colour', 'height', pop=True)
        self.process(colour, height)


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