Python Type Hints: Take It Easy

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1 0 0 % o p t i o n a l TYPE HINTS: TAKE IT EASY Introduction, pros and cons of  Python's gradual type system Luciano Ramalho @ramalhoorg

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2 https://twitter.com/raymondh/status/1255580152706158592

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TYPE HINTS? A brief introduction 3

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STATIC TYPE DECLARATIONS Formalized in PEP 484 — Type Hints (and other PEPs) Supported by the typing module in the standard library Mostly for the use of external type checkers, not runtime 4 def parse_flag(text: str) -> bool: return text.upper() in {'Y', 'YES', 'T', 'TRUE'}

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ABOUT GRADUAL TYPING Dynamic typing: type checking at runtime only Ex. Lisp, Smalltalk, Python, JavaScript, Ruby, Clojure, Elixir, etc.  Static typing: type declarations in source code, type checked before runtime by static analysis of the code Ex. C, C++, Pascal, Java, C#, Haskell, Rust, Kotlin, Swift, Elm, etc.  Gradual typing: hybrid approach—optional static type declaration, no eﬀect during runtime Ex. ActionScript, Hack, Dart, TypeScript, Python with type hints... 5

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EXAMPLES 6

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THE FIRST BUG MYPY FOUND FOR ME 7

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FIXED! random_queens_and_guard.py: https://tgo.li/rnd_queens 8

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"SUBPASCAL" INTERPRETER Example from Programming Languages Workshop  at Garoa Hacker Clube https://tgo.li/OfLiPr2018 9

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SUBPASCAL PARSER 10

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IMPORTING TYPES FROM TYPING 11

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DATA CLASSES PEP 557, Python 3.7 12

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NAMED TUPLE (CLASSIC) 13

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TYPED NAMED TUPLE 14

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DATA CLASS 15

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DATA CLASS WITH MORE FIELDS 16

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OVERLOADS 17

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OVERLOAD EXAMPLE #1 18

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MAX OVERLOAD 19

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MAX: CURRENT TYPE HINTS ON TYPESHED 20

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RECENT DEVELOPMENTS 21

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USABILITY IMPROVEMENTS PEP 585 — Type Hinting Generics In Standard Collections Starting with Python 3.9 (backported to 3.7, 3.8)   22 from __future__ import annotations def loads(input: Union[str, bytes], *, encoding: Optional[str] = None) -> dict[str, Any]: ...

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STATIC DUCK TYPING PEP 544 — Protocols: Structural subtyping (static duck typing) 23

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STATIC DUCK TYPING PEP 544 — Protocols: Structural subtyping (static duck typing) 24

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MORE REALISTIC EXAMPLE 25

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MAX: PROPOSED TYPE HINTS FOR TYPESHED 26

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MAX: PROPOSED TYPE HINTS FOR TYPESHED 27

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BENEFITS 28

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BENEFITS OF TYPE HINTS Document function signatures Support the IDE for better code completion and refactorings Finding bugs before running the program Automating object serialization/desseralization They are optional, avoiding the problem of diminishing returns. 29

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COST There is no free lunch 30

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DAVID BEAZLEY IS NOT ON BOARD... https://twitter.com/dabeaz/status/981699763996450817 31

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A TSUNAMI OF PEPS (OUTATED! 17 PEPS AS OF JUN/2020) 32 Ano PEP Título Python 2006 3107 Function Annotations 3.0 2014 484 Type Hints 3.5P 483 The Theory of Type Hints N/A 2015 482 Literature Overview for Type Hints N/A 2016 526 Syntax for Variable Annotations 3.6 2017 544 Protocols: Structural subtyping (static duck typing) 3.8 557 Data Classes 3.7 560 Core support for typing module and generic types 3.7 561 Distributing and Packaging Type Information 3.7 563 Postponed Evaluation of Annotations 3.7 2018 586 Literal Types 3.8 2019 585 Type Hinting Usability Conventions 3.8D 589 TypedDict: Type Hints for Dicts with a Fixed Set of Keys 3.8 591 Adding a ﬁnal qualiﬁer to typing 3.8 3.8

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DOWNSIDES Eﬀort to learn a whole new set of abstractions Generics, variance, structural typing, type variables etc. Eﬀort to write type hints Most of the time is not hard, but it may be hard or even impossible Better support in recent versions of Python But also: possible to use in Python 2.7, easing transition Slight delay in program startup Not important for long running processes 33

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COST X BENEFIT Beneﬁts grow when... The codebase is large The team is made of professional software developers Bernat Gabor* gives a good rule:  if you invest in automated tests,  you should invest in type hints   * link at the end 34

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FAQ Some answers 35

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WILL PYTHON BECOME A STATICALLY TYPED LANGUAGE? No. Section Non-goals of PEP 484: 36

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WHAT IS THE RUNTIME EFFECT OF TYPE HINTS? None. The Python interpreter and standard library ignore type hints* Also from PEP 484: 37 * a few exceptions, ex.: dataclasses and functools.singledistpatch at import time

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SHOULD EVERYONE USE TYPE HINTS? No. Python users are very diverse: engineers, financial analysts, students, scientists and researches from almost every field (physics, biology, sociology, artificial intelligence). The cost-benefit analysis is not the same for all of these groups. To learn how to program or do exploratory programming, type annotations introduce complexity, reduce the flexibility of the code, and do not bring much value. Type hints is most useful in teams of professional developers, with large code bases, used to a more complex tooling (IDEs, linters, automated tests, continuous integration etc.) 38

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REFERENCES Before reading all those PEPs 39

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BERNAT GABOR: THE STATE OF TYPE HINTS IN PYTHON https://www.bernat.tech/the-state-of-type-hints-in-python/ 40

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REAL PYTHON: PYTHON TYPE CHECKING GUIDE https://realpython.com/python-type-checking/ 41

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Let's keep this conversation going! Twitter: @ramalhoorg E-mail: lramalho@thoughtworks.com DANKE SCHÖN