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
   

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2. LANGUAGES AND PROGRAMMING PARADIGMS
   2
   Functional
   Imperative
   
   
   (or is it procedural?)
   Object
   

   Oriented
   

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3. PARADIGMS
   Programming language categories
   3
   

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5. CALCULATOR PROGRAMMING WAS IMPERATIVE (1976)
   5
   HP-25 TI 58C
   

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6. HP-25 CALCULATOR PROGRAMMING LANGUAGE
   6
   

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7. A SURVEY-STYLE PROGRAMMING LANGUAGES BOOK
   Programming
   Language
   Pragmatics,
   

   4th edition (2015)
   
   
   Michael L. Scott
   
   
   7
   

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8. GCD ASM X86
   Greatest
   

   common divisor
   in x86 Assembly
   
   
   (Scott, 2015)
   8
   

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9. GCD IN C, OCAML AND PROLOG
   9
   

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10. GCD IN PYTHON
    Imperative style
    
    
    Functional style
    10
    

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11. GCD IN PYTHON
    Imperative style
    
    
    Functional style
    11
    Bad
    fi
    t for Python:
    no tail-call
    optimisation
    

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12. THE PROBLEM
    

    WITH CATEGORIES
    Ontologies are so 1900’s
    12
    

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13. DEWEY DECIMAL
    CLASSIFICATION
    DDC 23 (2013 edition)
    

    13
    

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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
    

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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
    

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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
    

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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
    

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18. CELESTIAL EMPORIUM
    By Jorge Luis Borges (1899-1986)
    18
    

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21. BETTER CATEGORIES?
    21
    

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22. A CLASSIFICATION BASED ON HARD FACTS
    22
    

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23. A CLASSIFICATION BASED ON HARD FACTS?
    23
    “Noble” gases!?
    

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24. LANGUAGE CATEGORIES (1)
    24
    

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25. LANGUAGE CATEGORIES (2)
    25
    

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26. LANGUAGE CATEGORIES (3)
    26
    

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27. LANGUAGE CATEGORIES (4)
    27
    

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28. ONE CLASSIFICATION
    28
    Programming
    Language
    Pragmatics,
    

    4th edition (2015)
    
    
    Michael L. Scott
    
    
    

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29. ONE CLASSIFICATION (ZOOM)
    29
    

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30. ONE CLASSIFICATION (ZOOM)
    30
    ???
    

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31. “Ontology is overrated.”
    
    
    Clay Shirky
    31
    

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32. DESIGN PATTERNS
    When languages fall short
    32
    

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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
    

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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
    

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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
    

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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
    

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41. THE ITERATOR
    PATTERN
    The classic recipe
    41
    

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42. THE ITERATOR FROM THE GANG OF FOUR
    Design Patterns
    

    Gamma, Helm, Johnson & Vlissides
    

    ©1994 Addison-Wesley
    42
    

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43. 43
    Head First Design
    Patterns Poster
    

    O'Reilly
    

    ISBN 0-596-10214-3
    

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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.
    

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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!
    

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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
    
    
    >>>
    

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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
    

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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
    

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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
    

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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)
    

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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
    

    
    

    >>> next(it), next(it), next(it)
    

    ('car #1', 'car #2', 'car #3')
    

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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
    

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56. FEATURES
    Core features, not mere syntax
    56
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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63. TYPE CHECKING & TYPE EXPRESSION
    63
    

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64. TYPE CHECKING & TYPE EXPRESSION IN PYTHON
    64
    

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65. TYPE CHECKING & TYPE EXPRESSION
    65
    

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66. STRATEGY IN PYTHON
    Leveraging Python’s fundamental features
    66
    

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67. CHOOSE AN ALGORITHM AT RUN TIME
    67
    

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68. 68
    Context
    Strategy
    Concrete strategies
    CHOOSE AN ALGORITHM AT RUN TIME
    

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69. DOCTESTS FOR CONTEXT AND ONE CONCRETE STRATEGY
    69
    Instance of Strategy
    

    is given to Order
    constructor
    

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70. DOCTESTS FOR TWO ADDITIONAL CONCRETE STRATEGIES
    70
    

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71. VARIATIONS OF STRATEGY IN PYTHON
    Classic implementation using ABC
    
    
    First-class function implementation
    
    
    Parameterised closure implementation
    
    
    Parameterised callable implementation
    
    
    71
    

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72. CLASSIC STRATEGY: THE CONTEXT CLASS
    72
    Strategy is given to
    constructor
    Strategy is used here
    

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73. STRATEGY ABC AND A CONCRETE STRATEGY
    73
    

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74. TWO CONCRETE STRATEGIES
    74
    

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75. FIRST-CLASS FUNCTION
    
    
    STRATEGY
    75
    

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76. CONTEXT: STRATEGY FUNCTION AS ARGUMENT
    76
    Strategy function is
    passed to Order
    constructor
    

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77. CONTEXT: INVOKING THE STRATEGY FUNCTION
    77
    

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78. CONCRETE STRATEGIES AS FUNCTIONS
    78
    

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79. CONCRETE STRATEGIES AS FUNCTIONS (2)
    79
    

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80. VECTORIZING
    One aspect of functional programming
    80
    

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88. WRAPPING UP
    Learn the fundamentals
    88
    

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89. INSTEAD OF PARADIGMS…
    89
    

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90. FOCUS ON FUNDAMENTAL FEATURES
    90
    

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91. FEATURES ARE THE BEST GUIDE…
    91
    …to decide whether a particular pattern or implementation
    makes the most of the language.
    

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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
    *
    

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93. Luciano Ramalho
    
    
    lramalho@thoughtworks.com
    

    Fediverse: @ramgarlic@fosstodon.org
    

    PFKAT: @ramalhoorg
    
    
    Repo: github.com/ramalho/beyond-paradigms
    
    
    Slides: speakerdeck.com/ramalho
    THANKS !
    

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