The problem is that what you wrote isn't what most people want. Here's your example without dataclasses. I've added an "append_to_x" method, which does the obvious thing:


>>> class C:
...   def __init__(self, x=[]):
...      self.x = x
...  
...   def append_to_x(self, val):
...     self.x.append(val)
... 

Now create two objects, and inspect their "x" properties:
>>> a = C()
>>> b = C()
>>> a.x
[]
>>> b.x
[]

So far so good. Now append something to "a.x":
>>> a.append_to_x(10)
>>> a.x
[10]

And notice that "b.x" changes, too:
>>> b.x
[10]

So the naive behavior isn't what you want. dataclasses is trying to prevent you from doing this.

You should look at "mutable defaults", perhaps starting here (from a random Google search): https://blog.florimond.dev/python-mutable-defaults-are-the-source-of-all-evil