Python set practice - Speaker Deck

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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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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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MOTIVATION Some common use cases for sets 3

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

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

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

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

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

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

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LOGIC AND SETS A close relationship 12

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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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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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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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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 16 x ∈ (A ∆ B) ⟺ (x ∈ A) ⊻ (x ∈ B)

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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) 17

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SETS IN SEVERAL LANGUAGES 18

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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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SETS IN PYTHON The built-in types 20

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BUILDING A SET FROM A SERIES OF NUMBERS Using a set comprehension: 21

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ANOTHER SET, FOR THE EXAMPLES 22

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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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ELEMENT CONTAINMENT: THE IN OPERATOR O(1) in sets, because they use a hash table to hold elements. Implemented by the __contains__ special method: 24

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FUNDAMENTAL SET OPERATIONS 25 Intersection Union Symmetric difference (a.k.a. XOR) Difference

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SET COMPARISONS Subset and superset testing (set length does not matter!). In math: ⊂, ⊆, ⊃, ⊇. 26

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DE MORGAN’S LAW: #1 27

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DE MORGAN’S LAW: #2 28

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SET METHODS Going beyond what operators can do. 29

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SET OPERATORS AND METHODS (1) 30

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SET OPERATORS AND METHODS (2) Diﬀerences: 31

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SET TESTS All of these return a bool: 32

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ADDITIONAL METHODS These have nothing to do with math, and all to do with practical computing: 33

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ABSTRACT SET INTERFACES These interfaces are all deﬁned in collections.abc. set and frozenset both implement Set set also implements MutableSet 34

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OPERATOR OVERLOADING Not as bad as they say 35

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A DESIGNER’S DILEMMA 36

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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: 37

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COMPARISON OPERATORS 38

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THE BEAUTY OF DOUBLE DISPATCH 40

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CONCLUSION 41

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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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Luciano Ramalho  @ramalhoorg | @standupdev  luciano.ramalho@thoughtworks.com THANK YOU!