Python set practice

Let's talk about using sets in practice, and learn great API design ideas from Python's set types.

Transcript

1. u s i n g & b u i l d i n g
   PYTHON SET PRACTICE
   Learn great API design ideas from Python's set types.
   Luciano Ramalho
   @standupdev
   

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2. GOALS
   1

   Show why Python’s set
   types are a great example
   of API design.
   2

   Explain the __magic__
   behind the set types, and
   how to build your own.
   2
   

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3. MOTIVATION
   Some common use cases for sets
   3
   

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4. CASO DE USO #1
   4
   display product if
   all words in the
   query appear in
   the product
   description.
   

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5. SET-LESS SOLUTION #1
   I’ve written code like this in Go, which lacks built-in sets:
   5
   

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6. SET-LESS SOLUTION #2
   More readable, but still inefficient:
   6
   

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7. 7
   What if… Later! I am too busy
   coding nested loops!
   www.workcompass.com/
   

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8. CASO DE USO #1
   8
   www.workcompass.com/
   display product if
   all words in the
   query appear in
   the product
   description.
   

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9. CASO DE USO #1
   9
   Q ⊂ D
   www.workcompass.com/
   display product if
   all words in the
   query appear in
   the product
   description.
   

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10. CASO DE USO #2
    10
    Mark all products
    previously favorited,
    except those already
    in the shopping cart.
    

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11. CASO DE USO #2
    11
    F ∖ C
    Mark all products
    previously favorited,
    except those already
    in the shopping cart.
    

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12. LOGIC AND SETS
    A close relationship
    12
    

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13. Nobody has yet discovered a branch of
    mathematics that has successfully resisted
    formalization into set theory.
    Thomas Forster

    Logic Induction and Sets, p. 167
    13
    

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14. LOGIC CONJUNCTION IS INTERSECTION
    x belongs to the intersection of A
    with B.
    is the same as:
    x belongs to A and

    x also belongs to B.
    Math notation:
    x ∈ (A ∩ B) ⟺ (x ∈ A) ∧ (x ∈ B)
    In computing: AND
    14
    

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15. LOGIC DISJUNCTION: UNION
    x belongs to the union of A and B.
    is the same as:
    x belongs to A or

    x belongs to B.
    Math notation:
    x ∈ (A ∪ B) ⟺ (x ∈ A) ∨ (x ∈ B)
    In computing: OR
    15
    

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16. SYMMETRIC DIFFERENCE
    x belongs to A or

    x belongs to B but

    does not belong to both
    Is the same as:
    x belongs to the union of A with B
    less the intersection of A with B.
    Math notation:

    In computing: XOR
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    x ∈ (A ∆ B) ⟺ (x ∈ A) ⊻ (x ∈ B)
    

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17. DIFFERENCE
    x belongs to A but

    does not belong to B.
    is the same as:
    elements of A minus elements of B
    Math notation:
    x ∈ (A ∖ B) ⟺ (x ∈ A) ∧ (x ∉ B)
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18. SETS IN SEVERAL
    LANGUAGES
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19. SETS IN SEVERAL STANDARD LIBRARIES
    Some languages/platform APIs that implement sets in their
    standard libraries
    19
    Java Set interface: < 10 methods; 8 implementations
    Python set, frozenset: > 10 methods and operators
    .Net (C# etc.) ISet interface: > 10 methods; 2 implementations
    JavaScript (ES6) Set: < 10 methods
    Ruby Set: > 10 methods and operators
    Python, .Net and Ruby offer rich set APIs
    

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20. SETS IN PYTHON
    The built-in types
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21. BUILDING A SET FROM A SERIES OF NUMBERS
    Using a set comprehension:
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22. ANOTHER SET, FOR THE EXAMPLES
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23. STRING REPRESENTATION
    The __str__ and __repr__ methods:
    __str__ is used by str() and print().
    __repr__ is used by repr() and by the console, debugger etc.
    23
    

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24. ELEMENT CONTAINMENT: THE IN OPERATOR
    O(1) in sets, because they use a hash table to hold elements.
    Implemented by the __contains__ special method:
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25. FUNDAMENTAL SET OPERATIONS
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    Intersection
    Union
    Symmetric difference
    (a.k.a. XOR)
    Difference
    

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26. SET COMPARISONS
    Subset and superset testing (set length does not matter!).
    In math: ⊂, ⊆, ⊃, ⊇.
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27. DE MORGAN’S LAW: #1
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28. DE MORGAN’S LAW: #2
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29. SET METHODS
    Going beyond what operators can do.
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30. SET OPERATORS AND METHODS (1)
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31. SET OPERATORS AND METHODS (2)
    Differences:
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32. SET TESTS
    All of these return a bool:
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33. ADDITIONAL METHODS
    These have nothing to do with math, and all to do with practical
    computing:
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34. ABSTRACT SET INTERFACES
    These interfaces are all defined in collections.abc.
    set and frozenset both implement Set
    set also implements MutableSet
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35. OPERATOR
    OVERLOADING
    Not as bad as they say
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36. A DESIGNER’S DILEMMA
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37. DESIGN DECISIONS HAVE CONSEQUENCES
    Compound interest in Python (works for any numeric types
    that implement the needed operators):
    Compound interest in Java, if you need to use a non-primitive
    numeric type such as BigDecimal:
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38. COMPARISON OPERATORS
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40. THE BEAUTY OF DOUBLE DISPATCH
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41. CONCLUSION
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42. LEARNING FROM SETS
    Set operations can greatly simplify logic.
    Pythonic objects should implement __repr__, __eq__.
    Pythonic collections should implement __len__, __iter__,
    __contains__, and accept iterable arguments.
    Check out this example showing how to implement class
    designed for dense sets of small integers:
    Much more about these subjects in Fluent Python.
    42
    https://github.com/standupdev/uintset
    

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43. Luciano Ramalho

    @ramalhoorg | @standupdev

    luciano.ramalho@thoughtworks.com
    THANK YOU!
    

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