Allow mutable default values.

[kalekundert]

Currently, default values can't be mutable (e.g. []), because they're instantiated when the class is being setup, and are therefore shared between all instances of a class.

Some solutions:

Make a copy of the default for each object:

• Pros:
  • Mutable objects would work transparently.
• Cons:
  • Non-copyable object could not be used as defaults.
  • Experienced devs might be surprised by this.

Provide a way to specify a callable:

• Pros:
  • Would support everything.
  • This is how other similar libraries seem to approach this, e.g. dataclass, attrs
• Cons:
  • default itself could be a callable, so we would need a new attribute.

[kalekundert]

How other libraries do it:

• dataclass(default=SENTINEL, default_factory=SENTINEL)
• attr.ib(default=attr.Factory(...))
  • attr.ib(factory=...): sugar for above