ITP Programming Languages Chapter 2 Building Abstractions with Data 2.5 Object-Oriented Programming 1. Polymorphism repr, str Interface

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ITP 20005 Programming Languages Chapter 2 Building Abstractions with Data 2.5 Object-Oriented Programming 1. Polymorphism repr, str 2. Interface 3. @property 4.

1 ITP Programming Languages Chapter 2 Building Abstractions with Data 2.5 Object-Oriented Programming 1. Polymorphism repr, str 2. Interface 4. Example Complex number Major references: 1. Structure and Interpretation of Computer Programs(SICP) by Abelson and Sussman, MIT Composing Programs by John DeNero, Google 2. MITx OPENCOURSEWARE(OCW) 6.00 SC Lecture 3. UC Berkley CS61A Lecture 4. Python, Wikipedia and many on-line resources. Prof. Youngsup Kim, idebtor@handong.edu, 2014 Programming Languages, CSEE Dept., Handong Global University

2 may be thoroughly equipped for every good work.

3 All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, so that the man of God may be thoroughly equipped for every good work. (2Tim 3:16)

4 Object-Oriented Programming Object Abstraction String Representations 4

5 String Representations An object value should behave like the kind of data it is meant to represent. For instance, by producing a string representation of itself Strings are important: they represent language and programs In Python, all objects produce two string representations: The str is legible to humans. The repr is legible to the Python interpreter. The str and repr strings are often the same, but not always. 5

6 The repr String for an Object The repr function returns a Python expression (a string) that evaluates to an equal object. repr(object) -> string Return the canonical string representation of the object. For most object types, eval(repr(object)) == object. The result of calling repr on a value is what Python prints in an interactive session. >>> 12e >>> print(repr(12e12)) Some objects do not have a simple Python-readable string. >>> repr(min) '<built-in function min>' 6

7 The str String for an Object Human interpretable strings are useful as well: >>> import datetime >>> today = datetime.date(2014, 11, 20) >>> repr(today) 'datetime.date(2014, 11, 20)' >>> str(today) ' ' The result of calling str on the value of an expression is what Python prints using the print function: >>> print(today)

8 The str String for an Object (Demo) >>> from datetime import date >>> date >>> today = datetime.date(2014, 11, 20) >>> today datetime.date(2014, 11, 20) >>> repr(today) 'datetime.date(2014, 11, 20)' >>> repr(today) 'datetime.date(2014, 11, 20)' >>> print(today) >>> print(str(today)) >>> str(today) ' ' 8

9 Object-Oriented Programming Object Abstraction String Representations Polymorphic Functions 9

10 Polymorphic Functions A function that applies to many (poly) different forms (morph) of data. str and repr are both polymorphic; they apply to any object. repr invokes a zero-argument method repr on its argument. >>> today. repr () 'datetime.date(2014, 11, 20)' str invokes a zero-argument method str on its argument. >>> today. str () ' ' 10

11 Implementing repr and str The behavior of repr is slightly more complicated than invoking repr on its argument: An instance attribute called repr is ignored! Only class attributes are found. Question: How would we implement this behavior? The behavior of str is also complicated: An instance attribute called str is ignored. If no str attribute is found, uses repr string. Question: How would we implement this behavior? str is a class, not a function [Demo] 11

12 Demo class Bear: def repr (self): return 'Bear()' >>> Bear() Bear() >>> oski = Bear() >>> oski. repr () 'Bear()' >>> oski Bear() >>> 12

13 Demo class Bear: def repr (self): return 'Bear()' def print_bear(): oski = Bear() print(oski) print(str(oski)) print(repr(oski)) print(oski. repr ()) print(oski. str()) >>> print_bear() Bear() Bear() Bear() Bear() Bear() >>> 13

14 Demo class Bear: def repr (self): return 'Bear()' def str (self): return 'a bear' def print_bear(): oski = Bear() print(oski) print(str(oski)) print(repr(oski)) print(oski. repr ()) print(oski. str ()) >>> print_bear() a bear a bear Bear() Bear() a bear >>> 14

15 Demo class Bear: def init (self): self. repr =lambda:'oski' self. str =lambda:'this bear instance' def repr (self): return 'Bear()' def str (self): return 'a bear' def print_bear(): oski = Bear() print(oski) print(str(oski)) print(repr(oski)) print(oski. repr ()) print(oski. str ()) defining instance methods: repr, str >>> print_bear() a bear a bear Bear() oski this bear instance >>> There is a subtle difference between repr and repr, str and str. These search instance methods first, while others goes to class attributes first repr, str 15

16 Demo class Bear: def init (self): self. repr =lambda:'oski' self. str =lambda:'this bear' def repr (self): return 'Bear()' def str (self): return 'a bear' def print_bear(): oski = Bear() print(oski) print(str(oski)) print(repr(oski)) print(oski. repr ()) print(oski. str ()) def repr(x): type(x). repr (x) def str(x): t = type(x) if hasattr(t, ' str '): return t. str (x) else: return repr(x) >>> print_bear() a bear a bear Bear() oski this bear instance >>> This implementation works exactly how the Python interpreter evaluates. 16

17 Object-Oriented Programming Object Abstraction String Representations Polymorphic Functions Interfaces 17

18 Interfaces Message passing: Objects interact by looking up attributes on each other. (passing messages) The attribute look-up rules allow different data types to respond to the same message. A shared message (attribute name) that elicits similar behavior from different object classes is a powerful method of abstraction. An interface is a set of shared messages, along with a specification of what they mean. Example: Classes that implement repr and str methods that return Python- and human-readable strings implement an interface for producing string representations. 18

19 Object-Oriented Programming Object Abstraction String Representations Polymorphic Functions Property Methods 19

20 Property Methods Often, we want the value of instance attributes to stay in sync. >>> f = Rational(3, 5) >>> f.float_value >>> f.numer = 4 No method calls! >>> f.float_value

21 Property Methods Often, we want the value of instance attributes to stay in sync. >>> f = Rational(3, 5) >>> f.float_value 0.6 >>> f.numer = 4 >>> f.float_value 0.8 No method calls!

22 Property Methods Often, we want the value of instance attributes to stay in sync. >>> f = Rational(3, 5) >>> f.float_value 0.6 >>> f.numer = 4 >>> f.float_value 0.8 >>> f.denom -= 3 >>> f.float_value 2.0 No method calls!

23 Property Methods Often, we want the value of instance attributes to stay in sync. >>> f = Rational(3, 5) >>> f.float_value 0.6 >>> f.numer = 4 >>> f.float_value 0.8 >>> f.denom -= 3 >>> f.float_value 2.0 No method calls! decorator on a method designates that it will be called whenever it is looked up on an instance. 23

24 Property Methods Often, we want the value of instance attributes to stay in sync. >>> f = Rational(3, 5) >>> f.float_value 0.6 >>> f.numer = 4 No method calls! >>> f.float_value 0.8 >>> f.denom -= 3 >>> f.float_value 2.0 Attribute acts like a method! decorator on a method designates that it will be called whenever it is looked up on an instance. It allows zero-argument methods to be called without an explicit call expression. [Demo] 24

25 Property Methods [Demo] # Rational numbers class Rational: """A mutable fraction.""" def init (self, n, denom): self.numer = n self.denom = denom def repr (self): return 'Rational({0}, {1})'.format(self.numer, self.denom) 25

26 Property Methods [Demo] # Rational numbers class Rational: """A mutable fraction.""" def init (self, n, denom): self.numer = n self.denom = denom def repr (self): return 'Rational({0}, {1})'.format(self.numer, self.denom) >>> Rational(1, 2) Rational(1, 2) 26

27 Property Methods [Demo] # Rational numbers class Rational: """A mutable fraction.""" def init (self, n, denom): self.numer = n self.denom = denom def repr (self): return 'Rational({0}, {1})'.format(self.numer, self.denom) def str (self): return '{0}/{1}'.format(self.numer, self.denom) >>> Rational(1, 2) Rational(1, 2) >>> Rational(1, 2) 1/2 27

28 Property Methods [Demo] # Rational numbers class Rational: """A mutable fraction.""" def init (self, n, denom): self.numer = n self.denom = denom def repr (self): return 'Rational({0}, {1})'.format(self.numer, self.denom) def str (self): return '{0}/{1}'.format(self.numer, self.denom) def float_value(self): return self.numer/self.denom >>> Rational(3, 5).float_value <bound method Rational.float_val >>> Rational(3, 5).float_value()

29 Property Methods [Demo] # Rational numbers class Rational: """A mutable fraction.""" def init (self, n, denom): self.numer = n self.denom = denom def repr (self): return 'Rational({0}, {1})'.format(self.numer, self.denom) def str (self): return '{0}/{1}'.format(self.numer, def float_value(self): return self.numer/self.denom >>> x = Rational(3, 5) >>> x.float_value 0.6 >>> x.numer = 4 >>> x.float_value

30 Formatting Strings with the format() Method and %s In Python 3, you can include %s inside a string and follow it with a list of values for each %s such as: >>> # Python 2 and 3 >>> 'My name is %s and I am from %s.' % ('Al', 'Houston') 'My name is Al and I am from Houston.' >>> 'My name is {0} and I am from {1}.'.format('Al', 'Houston') 'My name is Al and I am from Houston.' >>> 'My name is {1} and I am from {0}.'.format('Al', 'Houston') 'My name is Houston and I am from Al.' 'My name is {0} and I like {thing} and I am from {hometown}.'.format('al', hometown='houston', thing='cats') 'My name is Al and I like cats and I am from Houston.' >>> 'My name is {0}, {0} and I'm from {1}.'.format('Al Gore', 'Houston') 'My name is Al Gore, Al Gore and I'm from Houston.' 30

31 Formatting Strings with the format() Method and %s The general syntax for a format placeholder is %[flags][width][.precision]type 31

32 Object-Oriented Programming Object Abstraction String Representations Polymorphic Functions Property Methods Example: Complex Number Property Methods Interfaces 32

33 Multiple Representations of Abstract Data Rectangular and polar representations for complex numbers Most programs don't care about the representation. Some arithmetic operations are easier using one representation than the other. 33

34 Implementing Complex Arithmetic Assume that there are two different classes that both represent Complex numbers. Number Rectangular representation Polar representation ComplexRI(1, 1) ComplexMA(sqrt(2), pi/4) Perform arithmetic using the most convenient representation, respectively. class Complex: def add(self, other): return ComplexRI(self.real + other.real, self.imag + other.imag) def mul(self, other): return ComplexMA(self.magnitude * other.magnitude, self.angle + other.angle) 34

35 Complex Arithmetic Abstraction Barriers Parts of the program that... Treat complex numbers as... Using... Use complex numbers to perform computation whole data values x.add(y), x.mul(y) Add complex numbers real and imaginary parts real, imag, ComplexRI Multiply complex numbers magnitudes and angles magnitude, angle, ComplexMA Implementation of the Python object system 35

36 Object-Oriented Programming Object Abstraction String Representations Polymorphic Functions Property Methods Example: Complex Number Implementation 36

37 An Interface for Complex Numbers All complex numbers should have real and imag components. All complex numbers should have a magnitude and angle. All complex numbers should share an implementation of add and mul. Complex ComplexRI ComplexMA [Demo] 37

38 An Interface for Complex Numbers All complex numbers should have real and imag components. All complex numbers should have a magnitude and angle. All complex numbers should share an implementation of add and mul. Complex What is called in OOP? ComplexRI ComplexMA [Demo] 38

39 An Interface for Complex Numbers All complex numbers should have real and imag components. All complex numbers should have a magnitude and angle. All complex numbers should share an implementation of add and mul. Complex ComplexRI ComplexMA What is called in OOP? [Demo] 39

40 Complex numbers from math import atan2, sin, cos, pi class Complex: def add(self, other): return ComplexRI(self.real + other.real, self.imag + other.imag) def mul(self, other): return ComplexMA(self.magnitude * other. magnitude, self.angle + other.angle) 40

41 The Rectangular Representation class ComplexRI(Complex): """A rectangluar representation of a complex number.""" def init (self, real, imag): self.real = real self.imag = def magnitude(self): Property decorator: "Call this function on attribute look-up" return (self.real ** 2 + self.imag ** 2) ** 0.5 math.atan2(y,x): Angle x-axis and the point(x,y) def angle(self): return atan2(self.imag, self.real) def repr (self): return 'ComplexRI({0}, {1})'.format(self.real, self.imag) decorator allows zero-argument methods to be called without the standard call expression syntax, so that they appear to be simple attributes. 41

42 Complex numbers from math import atan2, sin, cos, pi class Complex: def add(self, other): return ComplexRI(self.real + other.real, self.imag + other.imag) def mul(self, other): return ComplexMA(self.magnitude * other. magnitude, self.angle + other.angle) >>> ComplexRI(1, 1) ComplexRI(1, 1) >>> x = ComplexRI(1, 1) >>> x.add(x) ComplexRI(2, 2) >>> x.add(complexri(2, 4)) ComplexRI(3, 5) 42

43 The Polar Representation class ComplexMA(Complex): """A polar represenation of a complex number""" def init (self, magnitude, angle): self.magnitude = magnitude self.angle = def real(self): return self.magnitude * def imag(self): return self.magnitude * sin(self.angle) def repr (self): return 'ComplexMA({0}, {1})'.format(self.magnitude, self.angle) 43

44 Complex numbers from math import atan2, sin, cos, pi class Complex: def add(self, other): return ComplexRI(self.real + other.real, self.imag + other.imag) def mul(self, other): return ComplexMA(self.magnitude * other. magnitude, self.angle + other.angle) >>> ComplexMA(1, 0) ComplexMA(1, 0) >>> i = ComplexRI(0, 1) >>> i.mul(i) ComplexMA(1.0, >>> x = i.mul(i) >>> x.real -1.0 >>> x.imag e-16 >>> round(x.imag) 0 44

45 Using Complex Numbers Either type of complex number can be either argument to add or mul: class Complex: def add(self, other): return ComplexRI(self.real + other.real, self.imag + other.imag) def mul(self, other): return ComplexMA(self.magnitude * other.magnitude, self.angle + other.angle) >>> from math import pi >>> ComplexRI(1, 2).add(ComplexMA(2, pi/2)) ComplexRI( , 4.0) >>> ComplexRI(0, 1).mul(ComplexRI(0, 1)) ComplexRI(0, 2) ComplexMA(0, pi/2) 45

46 Using Complex Numbers Either type of complex number can be either argument to add or mul: class Complex: def add(self, other): return ComplexRI(self.real + other.real, self.imag + other.imag) def mul(self, other): return ComplexMA(self.magnitude * other.magnitude, self.angle + other.angle) >>> from math import pi >>> ComplexRI(1, 2).add(ComplexMA(2, pi/2)) ComplexRI( , 4.0) >>> ComplexRI(0, 1).mul(ComplexRI(0, 1)) ComplexRI(0, 2) ComplexMA(0, pi/2) 46

47 Using Complex Numbers Either type of complex number can be either argument to add or mul: class Complex: def add(self, other): return ComplexRI(self.real + other.real, self.imag + other.imag) def mul(self, other): return ComplexMA(self.magnitude * other.magnitude, self.angle + other.angle) >>> from math import pi >>> ComplexRI(1, 2).add(ComplexMA(2, pi/2)) ComplexRI( , 4.0) >>> ComplexRI(0, 1).mul(ComplexRI(0, 1)) ComplexMA(1.0, ) ComplexRI(0, 2) ComplexMA(0, pi/2) 47

48 ITP Programming Languages Chapter 2 Building Abstractions with Data 2.5 Object-Oriented Programming 1. Polymorphism repr, str 2. Interface 4. Example Complex number 3.0 The Scheme Programming Language Major references: 1. Structure and Interpretation of Computer Programs(SICP) by Abelson and Sussman, MIT Composing Programs by John DeNero, Google 2. MITx OPENCOURSEWARE(OCW) 6.00 SC Lecture 3. UC Berkley CS61A Lecture 4. Python, Wikipedia and many on-line resources. Prof. Youngsup Kim, idebtor@handong.edu, 2014 Programming Languages, CSEE Dept., Handong Global University

String Representations

61A Lecture 16 Announcements String Representations String Representations An object value should behave like the kind of data it is meant to represent For instance, by producing a string representation