[Tutor] question about metaclasses

[Albert-Jan Roskam]

On Jan 10, 2018 18:57, Steven D'Aprano <steve at pearwood.info> wrote:
>
> On Wed, Jan 10, 2018 at 04:08:04PM +0000, Albert-Jan Roskam wrote:
>
> > In another thread on this list I was reminded of
> > types.SimpleNamespace. This is nice, but I wanted to create a bag
> > class with constants that are read-only.
>
> If you expect to specify the names of the constants ahead of time, the
> best solution is (I think) a namedtuple.

Aaah *slaps forehead*, for some reason I didn't think about this, though I use namedtuples quite often. Using a metaclass for the very first time was great fun though :-)

> from collections import namedtuple
> Bag = namedtuple('Bag', 'yes no dunno')
> a = Bag(yes=1, no=0, dunno=42)
> b = Bag(yes='okay', no='no way', dunno='not a clue')
>
> ought to do what you want.
>
> Don't make the mistake of doing this:
>
> from collections import namedtuple
> a = namedtuple('Bag', 'yes no dunno')(yes=1, no=0, dunno=42)
> b = namedtuple('Bag', 'yes no dunno')(yes='okay', no='no way', dunno='not a clue')

But if I do:
Bag = namedtuple('Bag', 'yes no dunno')
... and then I create hundreds of Bag instances, this doesn't have a large memory footprint, right? (Because of __slots__) Or is a regular tuple still (much) less wasteful?

> because that's quite wasteful of memory: each of a and b belong to a
> separate hidden class, and classes are rather largish objects.
>
>
> If you expect to be able to add new items on the fly, but have them
> read-only once set, that's a different story.
>
>
> --
> Steve
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