Protocol: keystone of Python type hints

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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

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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

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LEARNING ABOUT PROTOCOLS ...and contributing to typeshed 3

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PRINCIPLES 4

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STATIC V. DYNAMIC TYPING 5 dynamic typing static typing

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STATIC V. DYNAMIC TYPING 6 RUN TIME CHECKING STATIC  CHECKING dynamic typing static typing

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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

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TWO DIMENSIONS OF TYPING 8 STRUCTURAL TYPES NOMINAL TYPES RUN TIME  CHECKING STATIC  CHECKING duck  typing static  typing

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TWO DIMENSIONS OF TYPING 9 STRUCTURAL TYPES NOMINAL TYPES RUN TIME  CHECKING STATIC  CHECKING duck  typing static  typing Java Land Python Land

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TWO DIMENSIONS OF TYPING 10 STRUCTURAL TYPES NOMINAL TYPES RUN TIME  CHECKING STATIC  CHECKING duck  typing static  typing goose typing supported by ABCs

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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

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THE POWER OF MAX 13

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EXAMPLES WITH 2-ARGUMENT MAX 15

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No content

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MAX WITH NUMBERS 18

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EXAMPLES WITH 2-ARGUMENT MAX 19

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STEP 0: FLOATS 20

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0. IMPLEMENTATION & TESTS 21

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0. TESTS PASSING 22

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0. PENDING ISSUE 23

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1ST TRY: NUMBER (ABC) (MISSTEP) 25

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1. IMPLEMENTATION 26

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ISSUE 1.1: UNSUPPORTED OPERAND >= 27

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From the standard library: cpython/Lib/numbers.py CAUSE 1.1: NUMBER ABC HAS NO METHODS ! ! 28

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2ND TRY: NUMERIC UNION    (MISSTEP) 30

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2. IMPLEMENTATION 31

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ISSUE 2.1: NUMERIC UNION IS INCOMPATIBLE WITH FLOAT 32

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ISSUE 2.2: CAN'T USE A FRACTION RETURNED AS NUMERIC 33

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STEP 3: RESTRICTED TYPEVAR 34

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3. IMPLEMENTATION 35

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3. TESTS PASSING 36

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3. ISSUE 2.1 FIXED: RETURN TYPE REFLECTS ARGUMENT TYPES 37

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DUCK TYPED MAX 38

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MORE EXAMPLES WITH 2-ARGUMENT MAX 39

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MORE EXAMPLES WITH 2-ARGUMENT MAX 40

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MORE EXAMPLES WITH 2-ARGUMENT MAX 41

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4TH TRY: PROTOCOL RESULT (MISSTEP) 42

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4. IMPLEMENTATION 43

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4. TESTS PASSING, MYPY HAPPY (SO FAR) 44

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4. MYPY UNHAPPY 45

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STEP 5: PROTOCOL + BOUND TYPEVAR 46

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5. IMPLEMENTATION 47

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5. TESTS PASSING, MYPY HAPPY 48

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5. RESULTS FIXED 49

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MAX OVERLOAD 50

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SIX OVERLOADS FOR DIFFERENT ARGUMENT COMBINATIONS Code: https://github.com/python/typeshed/…/stdlib/builtins.pyi 51

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OTHER USES 52

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OTHER USES IN THE STANDARD LIBRARY _typeshed._SupportsLessThan is used in these packages and functions of the Python 3.9 standard library: 53

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WRAPPING UP 54

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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

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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

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CELEBRATE AND EMBRACE GRADUAL TYPING 57 RUN TIME CHECKING STATIC  CHECKING gradual typing dynamic typing static typing

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e-mail: luciano.ramalho@thoughtworks.com @ramalhoorg THANK YOU!