Python's default values are class attributes... which makes them behave totally weird!

PEP 0557.md

Python implementation issue.

Default values are class attributes, which are shared among all instances!

Reference: https://peps.python.org/pep-0557/#mutable-default-values

So dataclasses tries to fix this...

class X:
   i : int = 0
   s : str = str()
   l : list = []

No problem.

from dataclasses import dataclass
@dataclass
class Y:
   i: int = 0
   s: str = str()
   l: list = []

Output: Initializing with an empty list fails for dataclass:

Traceback (most recent call last):
  File "/tmp/tt", line 8, in <module>
    class Y:
[...]
    raise ValueError(f'mutable default {type(f.default)} for field '
ValueError: mutable default <class 'list'> for field l is not allowed: use default_factory

---

For normal classes you get no error, but unexpected behavior:

class X:
   l : list = []

instance1 = X()
instance2 = X()
instance1.l.append("Added to instance 1 only.")

print(f'''
      instance1.l: {instance1.l}
      instance2.l: {instance2.l}
      ''')

Output

      instance1.l: ['Added to instance 1 only.']
      instance2.l: ['Added to instance 1 only.']

😦

For more details PEP-0557