Python dataclasses: field(default_factory) in depth
Python dataclasses automatically generate __init__, __repr__, and __eq__ from type annotations. The moment an attribute needs a mutable default value (list, dict, set), you hit a fundamental Python pitfall. field(default_factory=...) is the solution, and understanding why it is necessary changes how you reason about initialization in Python. The mutable default value trap In Python, default parameter values are evaluated once at function definition time, not at each call. This is a language property, not a bug. ...