[Python-Dev] PEP 557: Data Classes

[Jonathan Goble]

Interesting. I note that this under "Specification":

"""
field's may optionally specify a default value, using normal Python syntax:

@dataclass
class C:
    int a       # 'a' has no default value
    int b = 0   # assign a default value for 'b'
"""

...does not look like "normal Python syntax".

On Fri, Sep 8, 2017 at 11:44 AM Eric V. Smith <eric at trueblade.com> wrote:

> Oops, I forgot the link. It should show up shortly at
> https://www.python.org/dev/peps/pep-0557/.
>
> Eric.
>
> On 9/8/17 7:57 AM, Eric V. Smith 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_price!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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