Beautiful Python - Speaker Deck

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BEAUTIFUL PYTHON Great ideas of language design that make Python enjoyable and useful to so many people. E l e g a n c e b e g e t s s i m p l i c i t y

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FLUENT PYTHON Published in 9 languages so far: •Chinese (simpliﬁed) •Chinese (traditional) •English •French •Japanese •Korean •Polish •Portuguese •Russian  2nd edition: Q3 of 2020 3 Book signing at 12:00 today on 3rd ﬂoor

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INTRODUCTION 4

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To Love and create beauty are conditions to happiness. — Walter Gropius (1883–1969), founder of Bauhaus School

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SIMPLE SYNTAX 8

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A SIMPLE PROGRAM IN C 9 #include int main(int argc, char *argv[]) { for(int i = 0; i < argc; i++) printf("%s\n", argv[i]); return 0; }

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A SIMPLE PROGRAM IN C 10 #include int main(int argc, char *argv[]) { for(int i = 0; i < argc; i++) printf("%s\n", argv[i]); return 0; } $ ./args alpha bravo charlie ./args alpha bravo charlie

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A TRADITIONAL CHESS SET 11

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A BAUHAUS CHESS SET BY JOSEF HARTWIG 12

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A SIMPLE PROGRAM IN PYTHON 13 import sys for arg in sys.argv: print arg

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PYTHON: A MODERNIST LANGUAGE 14 #include int main(int argc, char *argv[]) { for(int i = 0; i < argc; i++) printf("%s\n", argv[i]); return 0; } import sys for arg in sys.argv: print arg

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PYTHON: A MODERNIST LANGUAGE 15

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PYTHON: A MODERNIST LANGUAGE 16

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PYTHON: A MODERNIST LANGUAGE 17

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MODERNIST 18

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ANCIENT AND MODERNIST 20

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ENHANCED FOR 22

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THAT SAME SIMPLE PROGRAM IN JAVA < 5 23 class Arguments { public static void main(String[] args) { for (int i=0; i < args.length; i++) System.out.println(args[i]); } }

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Specialists are people who always repeat the same mistakes.   — Walter Gropius

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class Arguments2 { public static void main(String[] args) { for (String arg : args) System.out.println(arg); } } FOREACH: SINCE JAVA 5 25 The oﬃcial name of the foreach syntax in Java is "enhanced for"

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class Arguments2 { public static void main(String[] args) { for (String arg : args) System.out.println(arg); } } FOREACH: SINCE JAVA 5 26 The oﬃcial name of the foreach syntax in Java is "enhanced for" import sys for arg in sys.argv: print arg

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class Arguments2 { public static void main(String[] args) { for (String arg : args) System.out.println(arg); } } FOREACH: SINCE JAVA 5 27 The oﬃcial name of the foreach syntax in Java is "enhanced for" year: 2004 import sys for arg in sys.argv: print arg year: 1991

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FOREACH IN BARBARA LISKOV'S CLU 28 Barbara Liskov, Institute Professor at MIT

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FOREACH IN BARBARA LISKOV'S CLU 29 Barbara Liskov, Institute Professor at MIT Prof. Liskov and her students

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ITERABLES 31

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CLU, Python, Java, etc. let we create iterable objects        ITERABLE OBJECTS: THE KEY TO FOREACH 32 for item in an_iterable: process(item) an_iterable

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CLU, Python, Java, etc. let we create iterable objects              Some languages don't oﬀer this ﬂexibility 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 syntax) ITERABLE OBJECTS: THE KEY TO FOREACH 33 for item in an_iterable: process(item) an_iterable

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avoidable belieavable extensible ﬁxable iterable movable readable playable washable iterable, adj. — Capable of being iterated.

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THE ITERATOR PATTERN The classic recipe 35

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37 Head First Design Patterns Poster  O'Reilly  ISBN 0-596-10214-3

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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) 38 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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AN ITERABLE TRAIN An instance of Train can be iterated, car by car 39 >>> t = Train(3) >>> for car in t: ... print(car) car #1 car #2 car #3 >>>

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CLASSIC ITERATOR IMPLEMENTATION The pattern as described by Gamma et. al. 40 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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COMPARE: CLASSIC ITERATOR × GENERATOR METHOD The classic Iterator recipe is obsolete in Python since v.2.2 (2001) 41 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 keeps the state of the iteration in its own local scope

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SPECIAL METHODS 43

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THE PYTHONIC DEFINITION OF ITERABLE Returns iterator for iterable by: • invoking __iter__ (if available)  or • building an iterator to fetch items via __getitem__ with   0-based indices (seq[0], seq[1], etc…) 44 iterable, adj. — (Python) An object from which the iter() function can build an iterator. iter(iterable):

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SPECIAL METHODS 45

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THE ZEN OF LEN 46

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LEN FUNCTION AND __LEN__ METHOD   Why do we call len(x) and not x.len()? For performance, the length of a sequence must be fast to get. For built-in collections (str, list, tuple, array, set, etc.) and extensions (Numpy arrays), the interpreter uses the C API to get the length from the ob_size field in the PyVarObject struct in memory. For collections defined in Python code, the interpreter calls the user- defined __len__ special method. 47

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OPERATOR OVERLOADING 49

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EXAMPLE: SET OPERATORS 51

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SETS IN A FEW LANGUAGES AND PLATFORMS Some languages/platform APIs that implement sets in their standard libraries 52 Python set, frozenset: > 10 methods and operators Ruby Set: > 10 methods and operators Elixir MapSet: > 10 methods .Net (C# etc.) ISet interface: > 10 methods; 2 implementations JavaScript (ES6) Set: < 10 methods Java Set interface: < 10 methods; 8 implementations Go Do it yourself, or pick one of several packages…

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SETS IN A FEW LANGUAGES AND PLATFORMS Some languages/platform APIs that implement sets in their standard libraries 53 Python set, frozenset: > 10 methods and operators Ruby Set: > 10 methods and operators Elixir MapSet: > 10 methods .Net (C# etc.) ISet interface: > 10 methods; 2 implementations JavaScript (ES6) Set: < 10 methods Java Set interface: < 10 methods; 8 implementations Go Do it yourself, or pick one of several packages…

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SETS IN A FEW LANGUAGES AND PLATFORMS Some languages/platform APIs that implement sets in their standard libraries 54 Python set, frozenset: > 10 methods and operators Ruby Set: > 10 methods and operators Elixir MapSet: > 10 methods .Net (C# etc.) ISet interface: > 10 methods; 2 implementations JavaScript (ES6) Set: < 10 methods Java Set interface: < 10 methods; 8 implementations Go Do it yourself, or pick one of several packages…

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SETS IN A FEW LANGUAGES AND PLATFORMS Some languages/platform APIs that implement sets in their standard libraries 55 Python set, frozenset: > 10 methods and operators Ruby Set: > 10 methods and operators Elixir MapSet: > 10 methods .Net (C# etc.) ISet interface: > 10 methods; 2 implementations JavaScript (ES6) Set: < 10 methods Java Set interface: < 10 methods; 8 implementations Go Do it yourself, or pick one of several packages…

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FULL SET OPERATORS IN PYTHON 56 Intersection Union Symmetric difference (a.k.a. XOR) Difference

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USING INFIX OPERATORS: DE MORGAN’S LAW 57

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SET OPERATORS AND METHODS (1) 58

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SET OPERATORS AND METHODS (2) Diﬀerences: 59

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SET TESTS All of these return a bool: 60

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CODE: UINTSET CLASS 61

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UINTSET: A SET CLASS FOR NON-NEGATIVE INTEGERS Inspired by the intset example in chapter 6 of The Go Programming Language by A. Donovan and B. Kernighan An empty set is represented by zero.  A set of integers {a, b, c} is represented by on bits in an integer at oﬀsets a, b, and c. Source code: 62 https://github.com/standupdev/uintset

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REPRESENTING SETS OF INTEGERS AS BIT PATTERNS This set: UintSet({13, 14, 22, 28, 38, 53, 64, 76, 94, 102, 107, 121, 136, 143, 150, 157, 169, 173, 187, 201, 213, 216, 234, 247, 257, 268, 283, 288, 290}) Is represented by this integer 2502158007702946921897431281681230116680925854234644385938703363396454971897652 283727872  Which has this bit pattern: 1010000100000000000000100000000001000000000100000000000010000000000000000010010 0000000000100000000000001000000000000010001000000000001000000100000010000001000 0000000000010000000000000100001000000010000000000000000010000000000010000000000 100000000000000100000000010000010000000110000000000000 65

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REPRESENTING SETS OF INTEGERS AS BIT PATTERNS This set: UintSet({290}) 66 Is represented by this integer 1989292945639146568621528992587283360401824603189390869761855907572637988050133 502132224  Which has this bit pattern: 1000000000000000000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000000000000000000000 000000000000000000000000000000000000000000000000000000

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REPRESENTING SETS OF INTEGERS AS BIT PATTERNS (2) UintSet() → 0 │0│ └─┘ UintSet({0}) → 1 │1│ └─┘ UintSet({1}) → 2 │1│0│ └─┴─┘ UintSet({0, 1, 2, 4, 8}) → 279 │1│0│0│0│1│0│1│1│1│ └─┴─┴─┴─┴─┴─┴─┴─┴─┘ UintSet({0, 1, 2, 3, 4, 5, 6, 7, 8, 9}) → 1023 │1│1│1│1│1│1│1│1│1│1│ └─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘ UintSet({10}) → 1024 │1│0│0│0│0│0│0│0│0│0│0│ └─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘ UintSet({0, 2, 4, 6, 8, 10, 12, 14, 16, 18}) → 349525  │1│0│1│0│1│0│1│0│1│0│1│0│1│0│1│0│1│0│1│ └─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘ UintSet({1, 3, 5, 7, 9, 11, 13, 15, 17, 19}) → 699050    │1│0│1│0│1│0│1│0│1│0│1│0│1│0│1│0│1│0│1│0│ └─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘ 67

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SAMPLE METHOD: INTERSECTION OPERATOR & 68 https://github.com/standupdev/uintset

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69 https://github.com/standupdev/uintset

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THE BEAUTY OF DOUBLE DISPATCH 70

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FAIL FAST 71

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NO "UNDEFINED" VALUES 74

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NO SURPRISING NULL VALUES FROM DICTS 75

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NO SURPRISING NULL VALUES FROM DICTS 77

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NO SURPRISING NULL VALUES WHEN UNPACKING 78

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NO SURPRISING NULL VALUES WHEN UNPACKING 79

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NO SURPRISING RESULTS FOR "2" + 8 81

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THE LAST PARAGRAPH OF FLUENT PYTHON 82

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COMMUNITY 83

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PYTHON BRASIL 2018 BY JULIO MELANDA (INSTAGRAM @JULIOMELANDA) 85

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ELEGANCE 94

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FIRST QUOTATION IN CHAPTER 1 OF FLUENT PYTHON 95

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BRUCE ECKEL Author of: • On Java 8 • Thinking in Java • Atomic Scala • Atomic Kotlin • and many other books. Loves Python . Designed t-shirt for PyCon US 2009. 96

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99 易易經 Yìjīng

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Elegance Begets Simplicity

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import sys for arg in sys.argv: print arg

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e-mail: [email protected] Twitter: @ramalhoorg WeChat: Luciano_Ramalho 谢谢

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