Summary

Conclusions

A special method is part of the language contract. It lets you use a class as a value, container, or resource if its behavior meets expectations. Dataclass removes repetition from code, and an enumeration makes valid states explicit. Immutability, equality, and hashing must be designed together.

Self-check questions

1. Where does Python look for the special method for len?
2. How do repr, str, and format differ?
3. Why does an arithmetic method return NotImplemented?
4. When can the right type's reflected method take priority?
5. Does += always preserve object identity?
6. What is the relationship between equality and hashing?
7. Why is a mutable key dangerous for a dictionary?
8. What does __getitem__ receive for a slice?
9. How can you provide two independent traversals of a container?
10. What should __exit__ return to avoid hiding a failure?
11. Which methods does dataclass generate by default?
12. Why is default_factory better than a shared list?
13. Does frozen make nested fields immutable?
14. When do you need Enum, StrEnum, and Flag?
15. How does __match_args__ affect positional patterns?

Useful links

• https://docs.python.org/3.14/reference/datamodel.html.
• https://docs.python.org/3.14/library/dataclasses.html.
• https://docs.python.org/3.14/library/enum.html.
• https://docs.python.org/3.14/library/contextlib.html.