Can I format dataclass attributes in python?

Question

I am trying to creating a python dataclass where fields can have formatted values. If I have the following

@dataclass(kw_only=True)
class childtestProduct(testProduct):

  t1:float 
  t2:str

how can I have for example t1 be like 99.90€ and t2 be like 10/8/2002 ?

What are you trying to achieve? Say t1 is 99.9 (float), when do you want it to become "99.90€" (string)? — fsimonjetz
– fsimonjetz, Commented Aug 10, 2022 at 19:14
so for ex would a user would pass in 99.9€ or a float 7.893217854? also would a string be pass in to date value so ilke 02/08/10 for example? — Wizard.Ritvik
– Wizard.Ritvik, Commented Aug 10, 2022 at 19:19
a float of 7.8909080 and for t2 maybe the value is initialy 1/10/22 and become 1/10/2022 — N Miaoulis
– N Miaoulis, Commented Aug 10, 2022 at 19:20
You could probably do this because Python will let you do almost anything. However, for your case, you might want to consider a regular class and use the @property decorator so that setting the instance attributes programmatically defines your attributes the way you want. With properties, you can even store a true value and return a formatted value with the attribute getter. — philosofool
– philosofool, Commented Aug 10, 2022 at 19:24

Wizard.Ritvik · Accepted Answer · 2022-08-10 19:25:27Z

5

You could use an approach with @property decorator, as mentioned, and turn all dataclass fields into field properties:

from dataclasses import dataclass
from datetime import datetime, date


@dataclass(kw_only=True)
class ChildTestProduct:
    t1: float
    t2: datetime | str

    @property
    def t1(self) -> str:
        return f'{self._t1:.2f}€'

    @t1.setter
    def t1(self, value: float):
        self._t1 = value

    @property
    def t2(self) -> str:
        return self._t2.strftime('%m/%d/%Y')

    @t2.setter
    def t2(self, value: datetime | str):
        if isinstance(value, str):
            self._t2 = datetime.strptime(value, '%m/%d/%y').date()
        else:
            self._t2 = value


p = ChildTestProduct(t1=99.9, t2='10/8/22')
print(p)  # ChildTestProduct(t1='99.90€', t2='10/08/2022')

assert isinstance(p._t1, float)
assert isinstance(p._t2, date)

answered Aug 10, 2022 at 19:25

Wizard.Ritvik

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2 Comments

fsimonjetz Over a year ago

Maybe this is irrelevant in this case, but apparently implementing custom getters/setters with the same name as a field causes a bug that when you don't pass a value, it won't complain and set the value to the property object, see stackoverflow.com/questions/69845745/…

Wizard.Ritvik Over a year ago

@fsimonjetz Yes, this is entirely accurate, if in the above example you would try to create an object like ChildTestProduct(), the getters/setters would fail because dataclasses would pass in the @property object as the default value. The most straightforward resolution I think would be to use a metaclass or similar solution such as one I added in this gist, that will correctly set the __post_init__() so that default values are then passed in as expected.

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