Beyond Paradigms (with Python examples)

Java is object oriented and Haskell is functional. How about Python?
Is it really OO with free-standing functions and porous encapsulation?
Python has lambdas and closures, but is it functional? Are these
useful questions?

Updated talk presented at North Bay Python, July 29th, 2022.

Transcript

1. T h e o r y f o r p r a c t i c e BEYOND PARADIGMS Understand language features, 
 use the right patterns. Luciano Ramalho @ramalhoorg@fosstodon.org North Bay Python 
 July 29th, 2023
2. LANGUAGES AND PROGRAMMING PARADIGMS 2 Functional Imperative (or is it procedural?) Object 
 Oriented
3. PARADIGMS Programming language categories 3
5. CALCULATOR PROGRAMMING WAS IMPERATIVE (1976) 5 HP-25 TI 58C
6. HP-25 CALCULATOR PROGRAMMING LANGUAGE 6
7. A SURVEY-STYLE PROGRAMMING LANGUAGES BOOK Programming Language Pragmatics, 
 4th edition (2015) Michael L. Scott 7
8. GCD ASM X86 Greatest 
 common divisor in x86 Assembly (Scott, 2015) 8
9. GCD IN C, OCAML AND PROLOG 9
10. GCD IN PYTHON Imperative style Functional style 10
11. GCD IN PYTHON Imperative style Functional style 11 Bad fi t for Python: no tail-call optimisation
12. THE PROBLEM 
 WITH CATEGORIES Ontologies are so 1900’s 12
13. DEWEY DECIMAL CLASSIFICATION DDC 23 (2013 edition) 
 13
14. DEWEY'S CLASSIFICATION: A TIDY HIERARCHY 000 Computer science, information and general works 100 Philosophy and psychology 200 Religion 300 Social sciences 400 Language 500 Pure science 600 Technology 700 Arts and recreation 800 Literature 900 History and geography 14
15. A TIDY HIERARCHY 500 Natural sciences and mathematics 510 Mathematics 516 Geometry 516.3 Analytic geometries 516.37 Metric differential geometries 516.375 Finsler geometry 15
16. A TIDY HIERARCHY? 200 Religion 210 Philosophy and theory of religion 220 The Bible 230 Christianity 240 Christian practice and observance 250 Christian orders and local church 260 Social and ecclesiastical theology 270 History of Christianity 280 Christian denominations 290 Other religions 16
17. A TIDY HIERARCHY? 200 Religion ... 290 Other religions 291 No longer used — formerly "Comparative religion" 292 Classical religion (Greek and Roman religion) 293 Germanic religion 294 Religions of Indic origin 295 Zoroastrianism (Mazdaism, Parseeism) 296 Judaism 297 Islam, Bábism and Baháʼí Faith 298 No longer used — formerly "Mormonism" 299 Religions not provided for elsewhere 300 Social Sciences 17
18. CELESTIAL EMPORIUM By Jorge Luis Borges (1899-1986) 18
21. BETTER CATEGORIES? 21
22. A CLASSIFICATION BASED ON HARD FACTS 22
23. A CLASSIFICATION BASED ON HARD FACTS? 23 “Noble” gases!?
24. LANGUAGE CATEGORIES (1) 24
25. LANGUAGE CATEGORIES (2) 25
26. LANGUAGE CATEGORIES (3) 26
27. LANGUAGE CATEGORIES (4) 27
28. ONE CLASSIFICATION 28 Programming Language Pragmatics, 
 4th edition (2015) Michael L. Scott
29. ONE CLASSIFICATION (ZOOM) 29
30. ONE CLASSIFICATION (ZOOM) 30 ???
31. “Ontology is overrated.” Clay Shirky 31
32. DESIGN PATTERNS When languages fall short 32
33. GOF: CLASSIC BOOK BY THE “GANG OF FOUR” Design Patterns: Elements of Reusable Object-Oriented Software (1995) Erich Gamma 
 Richard Helm 
 Ralph Johnson 
 John Vlissides 33
34. NOT EVERY PATTERN IS UNIVERSAL Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software (Addison-Wesley, 1995), p. 4. 34
35. NOT EVERY PATTERN IS UNIVERSAL Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software (Addison-Wesley, 1995), p. 4. 35
36. NOT EVERY PATTERN IS UNIVERSAL Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software (Addison-Wesley, 1995), p. 4. 36
41. THE ITERATOR PATTERN The classic recipe 41
42. THE ITERATOR FROM THE GANG OF FOUR Design Patterns 
 Gamma, Helm, Johnson & Vlissides 
 ©1994 Addison-Wesley 42
43. 43 Head First Design Patterns Poster 
 O'Reilly 
 ISBN 0-596-10214-3
44. THE FOR LOOP MACHINERY • In Python, the for loop, automatically: •Obtains an iterator from the iterable •Repeatedly invokes next() on the iterator, 
 retrieving one item at a time •Assigns the item to the loop variable(s) 44 for item in an_iterable: process(item) for item in an_iterable: process(item) •Terminates when a call to next() raises StopIteration.
45. ITERABLE VERSUS ITERATOR 45 • iterable: implements Iterable interface (__iter__ method) •__iter__ method returns an Iterator 
 • iterator: implements Iterator interface (__next__ method) •__next__ method returns next item in series and •raises StopIteration to signal end of the series Python iterators are also iterable!
46. AN ITERABLE TRAIN An instance of Train can be iterated, car by car 46 >>> t = Train(3) >>> for car in t: ... print(car) car #1 car #2 car #3 >>>
47. CLASSIC ITERATOR IMPLEMENTATION The pattern as described by Gamma et. al. 47 class Train: def __init__(self, cars): self.cars = cars def __iter__(self): return TrainIterator(self.cars) class TrainIterator: def __init__(self, cars): self.next = 0 self.last = cars - 1 def __next__(self): if self.next <= self.last: self.next += 1 return 'car #%s' % (self.next) else: raise StopIteration() >>> t = Train(4) >>> for car in t: ... print(car) car #1 car #2 car #3 car #4
48. BARBARA LISKOV'S CLU LANGUAGE 48 CLU Reference Manual — B. Liskov et. al. — © 1981 Springer-Verlag — also available online from MIT: http://publications.csail.mit.edu/lcs/pubs/pdf/MIT-LCS-TR-225.pdf © 2010 Kenneth C. Zirkel — CC-BY-SA
52. ITERATION IN CLU CLU also introduced true iterators with yield and a generic for/ in statement. 52 year: 1975 CLU Reference Manual, p. 2 B. Liskov et. al. — © 1981 Springer-Verlag
53. ITERABLE OBJECTS: THE KEY TO FOREACH • Python, Java & CLU let programmers de fi ne iterable objects 
 
 
 
 
 
 
 • Some languages don't o ff er this fl exibility • C has no concept of iterables • In Go, only some built-in types are iterable and can be used with foreach (written as the for … range special form) 53 for item in an_iterable: process(item) for item in an_iterable: process(item)
54. ITERABLE TRAIN WITH A GENERATOR METHOD The Iterator pattern as a language feature: 54 class Train: def __init__(self, cars): self.cars = cars def __iter__(self): for i in range(self.cars): yield 'car #%s' % (i+1) Train is now iterable because __iter__ returns a generator! >>> t = Train(3) >>> it = iter(t) >>> it 
 <generator object __iter__ at 0x…> 
 >>> next(it), next(it), next(it) 
 ('car #1', 'car #2', 'car #3')
55. COMPARE: CLASSIC ITERATOR × GENERATOR METHOD The classic Iterator recipe is obsolete in Python since v.2.2 (2001) 55 class Train: def __init__(self, cars): self.cars = cars def __iter__(self): for i in range(self.cars): yield 'car #%s' % (i+1) class Train: def __init__(self, cars): self.cars = cars def __iter__(self): return IteratorTrem(self.cars) class TrainIterator: def __init__(self, cars): self.next = 0 self.last = cars - 1 def __next__(self): if self.next <= self.last: self.next += 1 return 'car #%s' % (self.next) else: raise StopIteration() Generator function handles the state of the iteration
56. FEATURES Core features, not mere syntax 56
57. SAMPLE FEATURES ✖ LANGUAGES 57 Common Lisp C Java Python Go First-class functions ✔ ∗ ✔ ✔ ✔ First-class types ✔ ✔ Iterators ∗ ✔ ✔ ∗ Variable model reference value* value and reference reference value* and reference Type checking dynamic static static dynamic static Type expression structural nominal nominal structural structural
58. SAMPLE FEATURES ✖ LANGUAGES 58 Common Lisp C Java Python Go First-class functions ✔ ∗ ✔ ✔ ✔ First class types ✔ ✔ Iterators ∗ ✔ ✔ ∗ Variable model reference value* value and reference reference value* and reference Type checking dynamic static static dynamic static Type expression structural nominal nominal structural structural Functions as objects Classes as objects
59. SAMPLE FEATURES ✖ LANGUAGES 59 Common Lisp C Java Python Go First-class functions ✔ ∗ ✔ ✔ ✔ First-class types ✔ ✔ Iterators ∗ ✔ ✔ ∗ Variable model reference value* value and reference reference value* and reference Type checking dynamic static static dynamic static Type expression structural nominal nominal structural structural
60. SAMPLE FEATURES ✖ LANGUAGES 60 Common Lisp C Java Python Go First-class functions ✔ ∗ ✔ ✔ ✔ First-class types ✔ ✔ Iterators ∗ ✔ ✔ ∗ Variable model reference value* value and reference reference value* and reference Type checking dynamic static static dynamic static Type expression structural nominal nominal structural structural
61. SAMPLE FEATURES ✖ LANGUAGES 61 Common Lisp C Java Python Go First-class functions ✔ ∗ ✔ ✔ ✔ First-class types ✔ ✔ Iterators ∗ ✔ ✔ ∗ Variable model reference value* value and reference reference value* and reference Type checking dynamic static static dynamic static Type expression structural nominal nominal structural structural
62. SAMPLE FEATURES ✖ LANGUAGES 62 Common Lisp C Java Python Go First-class functions ✔ ∗ ✔ ✔ ✔ First-class types ✔ ✔ Iterators ∗ ✔ ✔ * Variable model reference value* value and reference reference value* and reference* Type checking dynamic static static dynamic static Type expression structural nominal nominal structural structural
63. TYPE CHECKING & TYPE EXPRESSION 63
64. TYPE CHECKING & TYPE EXPRESSION IN PYTHON 64
65. TYPE CHECKING & TYPE EXPRESSION 65
66. STRATEGY IN PYTHON Leveraging Python’s fundamental features 66
67. CHOOSE AN ALGORITHM AT RUN TIME 67
68. 68 Context Strategy Concrete strategies CHOOSE AN ALGORITHM AT RUN TIME
69. DOCTESTS FOR CONTEXT AND ONE CONCRETE STRATEGY 69 Instance of Strategy 
 is given to Order constructor
70. DOCTESTS FOR TWO ADDITIONAL CONCRETE STRATEGIES 70
71. VARIATIONS OF STRATEGY IN PYTHON Classic implementation using ABC First-class function implementation Parameterised closure implementation Parameterised callable implementation 71
72. CLASSIC STRATEGY: THE CONTEXT CLASS 72 Strategy is given to constructor Strategy is used here
73. STRATEGY ABC AND A CONCRETE STRATEGY 73
74. TWO CONCRETE STRATEGIES 74
75. FIRST-CLASS FUNCTION STRATEGY 75
76. CONTEXT: STRATEGY FUNCTION AS ARGUMENT 76 Strategy function is passed to Order constructor
77. CONTEXT: INVOKING THE STRATEGY FUNCTION 77
78. CONCRETE STRATEGIES AS FUNCTIONS 78
79. CONCRETE STRATEGIES AS FUNCTIONS (2) 79
80. VECTORIZING One aspect of functional programming 80
88. WRAPPING UP Learn the fundamentals 88
89. INSTEAD OF PARADIGMS… 89
90. FOCUS ON FUNDAMENTAL FEATURES 90
91. FEATURES ARE THE BEST GUIDE… 91 …to decide whether a particular pattern or implementation makes the most of the language.
92. WHY LEARN THE FUNDAMENTALS* Learn new languages faster Leverage language features Choose among alternative implementations Make sensible use of design patterns Debug hard problems Emulate missing features when they are helpful 92 Inspired by Programming Language Pragmatics Michael L. Scott *
93. Luciano Ramalho lramalho@thoughtworks.com 
 Fediverse: @ramgarlic@fosstodon.org 
 PFKAT: @ramalhoorg Repo: github.com/ramalho/beyond-paradigms Slides: speakerdeck.com/ramalho THANKS !