Let's implement useless Python objects (revised)

PythonAsia 2026 at DE LA SALLE UNIVERSITY (DLSU), MALATE, MANILA

1. Let’s implement useless Python objects Hayao Suzuki PythonAsia 2026 at DE LA SALLE UNIVERSITY, MALATE, MANILA March 21, 2026
2. Share it GitHub › https://github.com/HayaoSuzuki/pythonasia2026 Hashtag › #PythonAsia2026 2 / 30
3. Who am I ? About me Name Hayao Suzukiʢླ໦ɹॣʣ ///////// Twitter X @CardinalXaro Work Software Engineer at Tokyo Gas CO.,LTD Fun Playing tuba in the Tokyo Gas Wind Orchestra About Tokyo Gas › The largest natural gas utility in Japan › Production and sale of city gas, LNG sales, etc. › Gas Pipeline Service BusinessɺSupply of city gas › Production, supply and sale of electricity 3 / 30
4. Who am I ? Presentations of PyCon › Tomorrow graphlib, Let Us Use Everybody(PyCon JP 2025) › Let’s Implement Useless Python Objects(PyCon APAC 2023) › Unknown Evolution of the Built-in Function pow(PyCon JP 2021) › How to Use In-Memory Streams(PyCon JP 2020) › Do You Know cmath Module?(PyCon mini Shizuoka 2020) › Elementary Number Theory with Python(PyCon mini Hiroshima 2019) › Symbolic Mathematics Using SymPy(PyCon JP 2018) Listed at https://xaro.hatenablog.jp/ 4 / 30
5. Today’s Theme Let’s implement useless Python objects 5 / 30
6. What is it mean useless? From Longman Dictionary of Contemporary English 1 not useful or effective in any way 2 (informal) unable or unwilling to do anything properly 6 / 30
7. Is the useless object really useless? From Zhuangzi "The Human World"ʢ૳ࢠ ਓؒੈรʣ ਓօ஌༗༻೭༻ ࣕല஌ແ༻೭༻໵ Everyone knows the usefulness of the useful, but no one knows the usefulness of the useless. 7 / 30
8. Today’s Theme Let’s implement useless Python objects The useless objects are useless, but how to make a useless object is very useful. 8 / 30
9. What is a useless Python object? Example: LiarContainer >>> c = LiarContainer(["spam", "egg", "bacon"]) >>> "spam" in c False >>> "tomato" in c True 9 / 30
10. What is a useless Python object? Example: FibonacciSized >>> s = FibonacciSized(range(50)) >>> len(s) 12586269025 10 / 30
11. What is a useless Python object? Example: ShuffledIterable >>> it = ShuffledIterable([1, 2, 3, 4, 5]) >>> for _ in range(3): ... for v in it: ... print(v, end=" ") ... print() ... 5 3 4 2 1 4 1 2 3 5 2 5 3 1 4 11 / 30
12. What is a useless Python object? Definition of a useless Python object in this talk A useless Python object behave Pythonic, but does not work as expected. 12 / 30
13. Data Structures and Operations Basic Data Structures of Python List [1, 4, 9, 16, 25, 36] Tuple ("pen", "pineapple", "apple", "pen") Dictionary {"Answer": 42} Set {41, 43, 47, 53, 57, 59} Common Operations of Data Structure len() Length of object in Membership test for Iteration 13 / 30
14. in and Container object.__contains__() Called to implement membership test operators. Example: LiarContainer class LiarContainer(Container): def __contains__(self, item) -> bool: return item not in self._data 14 / 30
15. len() and Sized object.__len__() Called to implement the built-in function len(). Example: FibonacciSized class FibonacciSized(Sized): PHI: Final[float] = (1 + math.sqrt(5)) / 2 def __len__(self) -> int: return round( (1 / math.sqrt(5)) * pow(self.PHI, len(self._data)) ) 15 / 30
16. for and Iterable object.__iter__() Called when an iterator is required for a container. Example: ShuffledIterable class ShuffledIterable(Iterable): def __iter__(self) -> Iterator: return iter( random.sample(self._data, k=len(self._data)) ) 16 / 30
17. Object Protocols How to implement Pythonic Python objects We need to understand object protocols. Ref: https://docs.python.org/3/reference/datamodel.html 17 / 30
18. collections.abc collections.abc This module provides abstract base classes that can be used to test whether a class provides a particular interface. From https://docs.python.org/3/library/collections.abc.html 18 / 30
19. collections.abc collections.abc and Interface ABC Interface Sized __len__() Container __contains__() Iterable __iter__() Collection Sized, Container, Iterable 19 / 30
20. Collection collections.abc.Collection Sized and Container and Iterable Example: Collection class UselessCollection( FibonacciSized, LiarContainer, ShuffledIterable ): pass 20 / 30
21. collections.abc collections.abc and Built-in Objects ABC built-in objects Sequence tuple MutableSequence list MutableSet set MutableMapping dict 21 / 30
22. Sequence Example: ModularSequence class ModularSequence(Sequence): def __getitem__(self, key): match key: case int(): return self._data[key % len(self._data)] case slice(): n = len(self._data) s = slice( key.start % n, key.stop % n, key.step ) return self._data[s] case _: raise TypeError 22 / 30
23. Sequence Example: ModularSequence >>> seq = ModularSequence(range(20)) >>> print(seq[21:44]) [1, 2, 3] >>> print(seq[65543]) 3 >>> seq.count(13) # It does not stop. 23 / 30
24. Sequence Example: CompetitionSequence class CompetitionSequence(Sequence): def __getitem__(self, index): return self._data[index] def __iter__(self): return iter(reversed(self._data)) def __len__(self): return len(self._data) 24 / 30
25. Sequence Example: CompetitionSequence >>> s = CompetitionSequence("abcdefg") >>> s[0] 'a' >>> list(s) ['g', 'f', 'e', 'd', 'c', 'b', 'a'] >>> len(s) 7 25 / 30
26. Mapping Example: MisprintedDictionary class MisprintedDictionary(Mapping): def __init__(self, _dict: dict): values = _dict.values() rotated_values = chain( islice(values, 1, None), islice(values, 1), ) self._data = dict( zip(_dict, rotated_values) ) 26 / 30
27. Mapping Example: MisprintedDictionary >>> d = MisprintedDictionary({"a": 1, "b": 2, "c": 3}) >>> d["a"], d["b"], d["c"] (2, 3, 1) >>> list(d.keys()) ['a', 'b', 'c'] >>> len(d) 2 27 / 30
28. Set Example: CrowdSet @functools.total_ordering class CrowdSet(Set): def __lt__(self, other): return self._data >= other._data def __and__(self, other): return CrowdSet(self._data | other._data) def __or__(self, other): return CrowdSet(self._data & other._data) 28 / 30
29. Set Example: CrowdSet >>> s = CrowdSet(("egg", "bacon", "spam")) >>> t = CrowdSet(("egg", "egg", "spam", "spam")) >>> s > t True >>> sorted(s & t) ['bacon', 'egg', 'spam'] >>> sorted(s | t) ['egg', 'spam'] 29 / 30
30. Conclusion Let’s implement useless Python objects › Useless Python objects are useful › collections.abc module is very useful › Once you understand object protocol, you can do anything 30 / 30