Protocol: keystone of Python type hints

The static type system supporting type hints in Python is becoming more expressive with each new PEP, but PEP 544—Protocols: Structural subtyping (static duck typing) is the most important enhancement since type hints were first introduced. The typing.Protocol special class lets you define types in terms of the interface implemented by objects, regardless of type hierarchies, in the spirit of duck typing—but in a way that can be verified by static type checkers and IDEs.

Transcript

1. TYPING . PROTOCOL The keystone of Python's static type system d u c k t y p i n g f t w 🦆 Luciano Ramalho luciano.ramalho@thoughtworks.com @ramalhoorg
2. THE LIZARD BOOK, SECOND EDITION •1st ed: published in 9 languages •Now with: 100+ pages on type hints, many annotated examples •New async/await content, examples with FastAPI, asyncio & Curio •Draft available on early release at O'Reilly Learning (oreilly.com) •In print: Q4, 2021 (estimated) 2
3. LEARNING ABOUT PROTOCOLS ...and contributing to typeshed 3
4. PRINCIPLES 4
5. STATIC V. DYNAMIC TYPING 5 dynamic typing static typing
6. STATIC V. DYNAMIC TYPING 6 RUN TIME CHECKING STATIC
 CHECKING dynamic typing static typing
7. Don’t check whether it is-a duck:
 check whether it quacks-like-a duck,
 walks-like-a duck, etc, etc,
 depending on exactly what subset
 of duck-like behavior you need [...] — Alex Martelli in comp-lang-python, 2000-07-26
 "polymorphism (was Re: Type checking in python?)" 
 source: https://mail.python.org/pipermail/python-list/2000-July/046184.html Duck typing defined
8. TWO DIMENSIONS OF TYPING 8 STRUCTURAL TYPES NOMINAL TYPES RUN TIME
 CHECKING STATIC
 CHECKING duck
 typing static
 typing
9. TWO DIMENSIONS OF TYPING 9 STRUCTURAL TYPES NOMINAL TYPES RUN TIME
 CHECKING STATIC
 CHECKING duck
 typing static
 typing Java Land Python Land
10. TWO DIMENSIONS OF TYPING 10 STRUCTURAL TYPES NOMINAL TYPES RUN TIME
 CHECKING STATIC
 CHECKING duck
 typing static
 typing goose typing supported by ABCs
11. TWO DIMENSIONS OF TYPING 11 STRUCTURAL TYPES NOMINAL TYPES RUN TIME
 CHECKING STATIC
 CHECKING duck
 typing static
 typing goose typing static duck typing focus of this talk
13. THE POWER OF MAX 13
15. EXAMPLES WITH 2-ARGUMENT MAX 15
18. MAX WITH NUMBERS 18
19. EXAMPLES WITH 2-ARGUMENT MAX 19
20. STEP 0: FLOATS 20
21. 0. IMPLEMENTATION & TESTS 21
22. 0. TESTS PASSING 22
23. 0. PENDING ISSUE 23
25. 1ST TRY: NUMBER (ABC) (MISSTEP) 25
26. 1. IMPLEMENTATION 26
27. ISSUE 1.1: UNSUPPORTED OPERAND >= 27
28. From the standard library: cpython/Lib/numbers.py CAUSE 1.1: NUMBER ABC HAS NO METHODS ! ! 28
30. 2ND TRY: NUMERIC UNION
 
 (MISSTEP) 30
31. 2. IMPLEMENTATION 31
32. ISSUE 2.1: NUMERIC UNION IS INCOMPATIBLE WITH FLOAT 32
33. ISSUE 2.2: CAN'T USE A FRACTION RETURNED AS NUMERIC 33
34. STEP 3: RESTRICTED TYPEVAR 34
35. 3. IMPLEMENTATION 35
36. 3. TESTS PASSING 36
37. 3. ISSUE 2.1 FIXED: RETURN TYPE REFLECTS ARGUMENT TYPES 37
38. DUCK TYPED MAX 38
39. MORE EXAMPLES WITH 2-ARGUMENT MAX 39
40. MORE EXAMPLES WITH 2-ARGUMENT MAX 40
41. MORE EXAMPLES WITH 2-ARGUMENT MAX 41
42. 4TH TRY: PROTOCOL RESULT (MISSTEP) 42
43. 4. IMPLEMENTATION 43
44. 4. TESTS PASSING, MYPY HAPPY (SO FAR) 44
45. 4. MYPY UNHAPPY 45
46. STEP 5: PROTOCOL + BOUND TYPEVAR 46
47. 5. IMPLEMENTATION 47
48. 5. TESTS PASSING, MYPY HAPPY 48
49. 5. RESULTS FIXED 49
50. MAX OVERLOAD 50
51. SIX OVERLOADS FOR DIFFERENT ARGUMENT COMBINATIONS Code: https://github.com/python/typeshed/…/stdlib/builtins.pyi 51
52. OTHER USES 52
53. OTHER USES IN THE STANDARD LIBRARY _typeshed._SupportsLessThan is used in these packages and functions of the Python 3.9 standard library: 53
54. WRAPPING UP 54
55. PROTOCOLS: THE KEYSTONE OF PYTHON'S TYPE HINTS PEP 484—Type Hints not possible to correctly annotate max etc. from the standard library and from important packages (e.g. Requests)
 PEP 544—Protocols: Structural subtyping problem solved with the introduction of static duck typing 55
56. I'm not against types, but I don't know of any type systems that aren't a complete pain, so I still like dynamic typing. — Alan Kay, lead developer of Smalltalk
 2003 ACM Turing Award Laureate source: http://www.purl.org/stefan_ram/pub/doc_kay_oop_en
57. CELEBRATE AND EMBRACE GRADUAL TYPING 57 RUN TIME CHECKING STATIC
 CHECKING gradual typing dynamic typing static typing
58. e-mail: luciano.ramalho@thoughtworks.com @ramalhoorg THANK YOU!