Why can't you use mutable default values in constructors?

If you set a list or dictionary as the default value, the object is created once and then reused by all instances.

class User:
    def __init__(self, tags=[]):
        self.tags = tags

This results in a change in one instance affecting the others:

u1 = User(); u2 = User()
u1.tags.append("x"); print(u2.tags)

default_factory creates a new object each time the constructor is called, eliminating shared state:
field(default_factory=list)

Thus, each instance receives an independent data structure:
User().tags is User().tags

🔥 Using default_factory is an important practice when working with mutable types and prevents hard-to-detect state errors.

https://t.me/pythonRe 👍