1 programming for engineers in python autumn 2011-12 lecture 6: more object oriented programming
TRANSCRIPT
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Programming for Engineers in
Python
Autumn 2011-12
Lecture 6: More Object Oriented Programming
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Lecture 5 Highlights
• Functions review
• Object Oriented Programming
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is and== is will return True if two variables point to the same object, == if the objects referred to by the variables are equal
>>> a = [1, 2, 3]>>> b = a>>> b is a True>>> b == aTrue>>> b = a[:]>>> b is aFalse>>> b == aTrue
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Object-Oriented Programming (OOP)
• Represent problem-domain entities using a computer language
• When building a software in a specific domain, describe the different components of the domain as types and variables
• Thus we can take another step up in abstraction
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Class as a BlueprintA class is a blueprint of objects
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Car Example
• Members: 4 wheels, steering wheel, horn, color,…
• Every car instance has its own
• Methods: drive, turn left, honk, repaint,…
• Constructors: by color (only), by 4 wheels, engine,…
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Shapes – 2D Point, Circle• __init__• self• Attributes• Instances and memory• Copy (shallow / deep)• Methods
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Code – Define Classes
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Code – Using Classes
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Today• Continue with 2D Shapes• Rational numbers implementation
• It should feel like native language support
• Inspired by chapter 6 from the book Programming in Scala
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A Rectangle (design options)
• It is not always obvious what the attributes of an object should be
• How would you represent a rectangle?• (for simplicity ignore angle, assume the rectangle is
vertical or horizontal)• There are several possibilities:
• One corner / center point + width and height• Two opposing corners
• We shall select the width, height, lower-left corner
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A Rectangle - ImplementationIn class Rectangle:
Shell:
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Rectangle in Memory
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Find Center
In class Rectangle:
Shell:
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Grow Rectangle
In class Rectangle:
Shell:
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Has Attributes?
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Print All Attributes
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Inheritance (briefly)
• The general idea• class Point(object) – what does object stands for?• Example: Animals• Polymorphism
• We have seen that already!• Histogram example
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Histogram (polymorphism)
Source: Think Python
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Rational Numbers
• A rational number is a number that can be expressed as a ratio n/d (n, d integers, d not 0)
• Examples: 1/2, 2/3, 112/239, 2/1
• Not an approximation!
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Specification• print should work smoothly
• Add, subtract, multiply, divide
• Immutable
• It should feel like native language support
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Constructing a Rational• What are the attributes?• How a client programmer will create a new Rational
object?
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Constructing a Rational
?
Shell:
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Reimplementing __str__• __str__ method return a string representation of an object• A more useful implementation of __str__ would print out
the values of the Rational’s numerator and denominator• override the default implementation
In class Rational:
Shell:
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__repr__• __repr__ method returns the “official” string
representation of an object
In class Rational:
Shell:
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Checking Preconditions
• Ensure the data is valid when the object is constructed
In class Rational:
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Checking Preconditions
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Defining Operators• Why not use natural arithmetic operators?
• Operator precedence will be kept
• All operations are method calls
From the book Programming in Scala
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Operator Overloading
• By defining other special methods, you can specify the behavior of operators on user defined types
• +, -, *, /, <, >,…
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Adding Rational Numbers
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Define __add__ Method
• Immutable
In class Rational:
Shell:
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Other Arithmetic Operations
In class Rational:
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Other Arithmetic Operations
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<, >, max
INCORRECT!
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<, >, max
In class Rational:
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<, >, max
How come max works?
• Constructors other then the primary? • Example: a rational number with a denominator
of 1 (e.g., 5/1 5)• We would like to do: Rational(5)
• Default arguments• Useful not solely for constructors
• Remember sorted (reverse, key are default arguments)?37
Default Arguments to Constructor
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Revised Rational
In class Rational:
Shell:
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Greatest Common Divisor (gcd)
• 66/42 = 11/7• To normalize divide the numerator and
denominator by their greatest common divisor (gcd)• gcd(66,42) = 6 (66/6)/(42/6) = 11/7• No need for Rational clients to be aware of this• Encapsulation
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Off Topic: Calculate gcd(Only if time allows)
• gcd(a,b) = g• a = n * g• b = m * g• gcd(n,m)=1(otherwise g is not the gcd)• a = t * b + r = t * m * g + r g is a divisor of r
• gcd(a,b) = gcd(b,a%b)• The Euclidean algorithm: repeat iteratively:
if (b == 0) return aelse repeat using a b, b a%b
• http://en.wikipedia.org/wiki/Euclidean_algorithm
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Correctness• Example:
gcd(40,24) gcd(24,16) gcd(16,8) gcd(8,0) 8
• Prove: g = gcd(a,b) = gcd(b,a%b)= g1• g1 is a divisor of a ( g1 ≤ g)• There is no larger divisor of a ( g1 ≥ g)
• ≤ : a = t * b + r a = t * h * g1 + v * g1 g1 is a divisor of a
• ≥ : assume g > g1 a = t * b + r g is a divisor of b and r contradiction
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gcd Implementation
• Let’s leave it for next lesson (Recursion)• Actually, we can use the implementation in the
module fractions.gcd
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Revised Rational
In class Rational:
Shell:
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Mixed Arithmetic's• Now we can add and multiply rational numbers!• What about mixed arithmetic?
• r + 2 won’t work
• r + Rational(2) is not nice
• Add new methods for mixed addition and multiplication
• Will work thanks to polymorphism
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Usage
• The + method invoked is determined in each case by the type of the right operand
• In our code it is implemented only for the operator + on integers (in “real” life it should have been implemented for every operator and every data type that is supported)
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Revised __add__• Isinstance takes a value and a class object, and returns
True if the vlaue is an instance of the class• Handles addition of integers correctly• Type based dispatch – dispatches the computation to
different executions based on the types of the arguments
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Implicit Conversions
• 2 + r 2.+(r) method call on 2 (int) int class contains no __add__ method that takes a Rational argument
• The problem: Python is asking an integer to add a Rational object, and it doesn’t know how to do that
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__radd__ - Right Side Add
In class Rational:
Shell:
• __radd__ invoked when a Rational object appears on the right side of the + operator
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Summary• Customize classes so that they are natural
to use• Attributes, methods, constructor• Method overriding• Encapsulation• Define operators as method• Method overloading
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Rational Numbers in Python
• Actually, there is a Python implementation of Rational numbers
• It is called fractions http://docs.python.org/library/fractions.html
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Next Week
• No class (tirgulim as usual)• Next topic: Recursion