🐛 should never use mutable defaults in class definitions

[ianardee]

🐛 mutable defaults in classes

We should never use mutable defaults in class definitions.

These work the same way as mutable defaults in function definitions, but are not checked by mypy it seems.

Consider using dataclasses in a future version.
https://docs.python.org/3/library/dataclasses.html#mutable-default-values

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)