All 15 Dataclasses & NamedTuples questions
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What does the @dataclass decorator automatically generate for a class? How do you create a simple immutable record using collections.namedtuple? How do you specify default values for dataclass fields, and what ordering restriction applies? How do you make a dataclass immutable (read-only after creation) using frozen=True? How does typing.NamedTuple differ from collections.namedtuple in terms of syntax and type annotation support? How do you add validation logic to a dataclass field using __post_init__? How do you use dataclasses.field(compare=False) or field(repr=False) to exclude a specific field from the auto-generated __eq__ or __repr__? How do you implement custom ordering (<, <=, >, >=) for a dataclass using order=True, and what constraint does it place on field comparison? How do you convert a dataclass instance to and from a dictionary using dataclasses.asdict() and manual reconstruction? How would you implement inheritance between dataclasses, and what field-ordering rule must be respected across the hierarchy?
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Python › Dataclasses & NamedTuples

Why can't you use a mutable default value (like an empty list) directly as a dataclass field default, and how do you use field(default_factory=...) to fix it?

Intermediate

Answer

Using a mutable object (list, dict, set) as a default value would share the SAME object across every instance (the classic Python mutable-default-argument trap), so @dataclass explicitly raises a ValueError at class definition time if you try; field(default_factory=callable) instead calls the given zero-argument function to produce a FRESH object for each new instance.
from dataclasses import dataclass, field

@dataclass
class ShoppingCart:
    items: list = field(default_factory=list)  # a NEW list for each instance

cart1 = ShoppingCart()
cart1.items.append("apple")
cart2 = ShoppingCart()
print(cart2.items)  # [] -- independent from cart1's list, thanks to default_factory
Real-world example Avoiding the classic mutable-default-argument bug when a dataclass field should default to an empty list or dict.

Common follow-ups: What would happen if you simply wrote 'items: list = []' directly, without using field(default_factory=list)?

Functions & Scope
All 15 Dataclasses & NamedTuples questions