[Python-Dev] PEP 557: Data Classes

Gustavo Carneiro gjcarneiro at gmail.com
Sat Sep 9 11:41:34 EDT 2017


Hi, it is not clear whether anything is done to total_cost:

    def total_cost(self) -> float:

Does this become a property automatically, or is it still a method call?
To that end, some examples of *using* a data class, not just defining one,
would be helpful.

If it remains a normal method, why put it in this example at all? Makes
little sense...

Otherwise I really like this idea, thanks!


On 8 September 2017 at 15:57, Eric V. Smith <eric at 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
>
> Will automatically add these methods:
>
>   def __init__(self, name: str, unit_price: float, quantity_on_hand: int =
> 0) -> None:
>       self.name = name
>       self.unit_price = unit_price
>       self.quantity_on_hand = quantity_on_hand
>   def __repr__(self):
>       return f'InventoryItem(name={self.name!r},unit_price={self.unit_pri
> ce!r},quantity_on_hand={self.quantity_on_hand!r})'
>   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
>   def __ne__(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
>   def __lt__(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
>   def __le__(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
>   def __gt__(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
>   def __ge__(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
>
> Data Classes saves you from writing and maintaining these functions.
>
> The PEP is largely complete, but could use some filling out in places.
> Comments welcome!
>
> Eric.
>
> P.S. I wrote this PEP when I was in my happy place.
>
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>



-- 
Gustavo J. A. M. Carneiro
Gambit Research
"The universe is always one step beyond logic." -- Frank Herbert
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