On 8 September 2017 at 07:57, Eric V. Smith <e...@trueblade.com> wrote:
> I've written a PEP for what might be thought of as "mutable namedtuples with
> defaults, but not inheriting tuple's behavior" (a mouthful, but it sounded
> simpler when I first thought of it). It's heavily influenced by the attrs
> project. It uses PEP 526 type annotations to define fields. From the
> overview section:
>
> @dataclass
> class InventoryItem:
> name: str
> unit_price: float
> quantity_on_hand: int = 0
>
> def total_cost(self) -> float:
> return self.unit_price * self.quantity_on_hand
Very nice!
> def __eq__(self, other):
> if other.__class__ is self.__class__:
> return (self.name, self.unit_price, self.quantity_on_hand) ==
> (other.name, other.unit_price, other.quantity_on_hand)
> return NotImplemented
My one technical question about the PEP relates to the use of an exact
type check in the comparison methods, rather than "isinstance(other,
self.__class__)".
I think I agree with that decision, but it isn't immediately obvious
that the class identity is considered part of the instance value for a
data class, so if you do:
@dataclass
class BaseItem:
value: Any
class DerivedItem:
pass
Then instances of DerivedItem *won't* be considered equivalent to
instances of BaseItem, and they also won't be orderable relative to
each other, even though "DerivedItem" doesn't actually add any new
data fields.
Cheers,
Nick.
--
Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia
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