How to make a python dataclass field not allow None values

Question

After learning a little about dataclasses, it seems that a dataclass will always HAVE all the properties you defined. Adding the Optional only means that you don't have to pass a param into the constructor. I want to make a field required in the sense that it cannot be None.

So if I have

@dataclass
class SomeClass():
  a: int
  b: Optional[int] = None

test = SomeClass(a=None)

I want this to throw an error, but don't know the best way to achieve that result.

Two ideas I have are using the __post_init__ to run validation logic or maybe using pydantic.

Is there a generally accepted solution for this problem?

Answer 1

Both ways you mentioned are quite suitable, but pydantic.dataclasses will do automatic validation for you:

from typing import Optional
from pydantic.dataclasses import dataclass as pdataclass

@pdataclass
class SomeClass():
  a: int
  b: Optional[int] = None

test = SomeClass(a=None) 

---

Gives a verbose output/error:

    test = SomeClass(a=None)
           ^^^^^^^^^^^^^^^^^
  File "/data/.virtualenvs/test/lib/python3.11/site-packages/pydantic/_internal/_dataclasses.py", line 132, in __init__
    s.__pydantic_validator__.validate_python(ArgsKwargs(args, kwargs), self_instance=s)
pydantic_core._pydantic_core.ValidationError: 1 validation error for SomeClass
a
  Input should be a valid integer [type=int_type, input_value=None, input_type=NoneType]

---

With standard dataclasses you'd need to provide your own validation logic within __post_init__ method:

from dataclasses import dataclass
from typing import Optional

@dataclass
class SomeClass():
  a: int
  b: Optional[int] = None

  def __post_init__(self):
      if not isinstance(self.a, int):
          raise ValueError('Field `a` must be of integer type')

test = SomeClass(a=None) 

---

    test = SomeClass(a=None)
           ^^^^^^^^^^^^^^^^^
  File "<string>", line 5, in __init__
..., in __post_init__
    raise ValueError('Field `a` must be of integer type')
ValueError: Field `a` must be of integer type